Comets survive plunge through Sun's atmosphere
NASA NEWS RELEASE
Posted: June 12, 2003

A run through the jungle is too easy; for the ultimate reality show contest, try a race through the Sun's atmosphere, where two comets recently lost their heads. The tails from a pair of comets survived a close encounter with the Sun, even after the Sun's intense heat and radiation vaporized their heads (nuclei and coma), an extremely rare event photographed by the Solar and Heliospheric Observatory (SOHO) spacecraft.

Credit: SOHO, NASA/ESA

On May 24, 2003, a pair of comets arced in tandem towards the Sun, their paths taking them to just 0.1 solar radii above the Sun's surface, deep within the searing multimillion-degree solar atmosphere (corona).

They belong to the Kreutz family of sun-grazing comets, often seen by the SOHO spacecraft while diving towards their final rendezvous with the Sun. But as in humans, twins are rare. Even more so, this pair showed another very unusual trait: What looks like a faint tail (or "puff of smoke") can be seen moving away from the Sun, seemingly emanating from a point in the orbit beyond the comet's closest approach. Normally, sungrazers simply fade and disappear at an earlier stage, obliterated by the Sun's intense heat and radiation pressure.

Another pair of Kreutz sungrazers with such a "headless tail" was observed in June 1998, when the observing geometry was very similar. But out of more than 600 sungrazing comets observed during more than six years by SOHO, this is only the third showing any signs of such behavior. However, this seems now likely to confirm the existence of such comets.

"Everyone who's seen this agrees it's a very interesting observation," said Dr. Douglas Biesecker, a solar researcher at the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder, Colorado, and the head of SOHO's comet discovery program. SOHO has become the most prolific comet finder in history.

The tail is most likely the dusty remains of the comet's nucleus, being pushed out by sunlight (radiation pressure) after all the ice in the nucleus has evaporated, thus eliminating the processes maintaining a bright coma surrounding the nucleus. Studies of the dust cloud may reveal clues to the size distribution of the dust grains.

"The fact that the tail 'holds together' so well probably means that the dust is mostly the same size," said Biesecker.

Comets are chunks of ice and dust that zoom around the solar system in elongated orbits. This "dirty snowball" is the nucleus of the comet; it ranges in size from a large boulder to a large city. As the comet gets close to the Sun, solar heat and light liberate gas and dust from the nucleus, forming the coma, which is an extensive, bright cloud around the nucleus, and one or more tails. A comet's dust tail can be millions of miles (kilometers) long and is pushed away from the Sun by sunlight. Comets also have a tail of electrically charged particles (ions) that is usually fainter and is pushed away from the Sun by the solar wind, a thin stream of electrified gas that blows constantly from the Sun. Both tails point away from the Sun, even for comets that are traveling back outwards in the solar system. Studies of the tails can reveal changes in solar wind structure and radiance of the Sun.

SOHO is a project of international cooperation between the European Space Agency and NASA.

A New Neighbor Star
By Alan M. MacRobert

The size of our newly discovered neighbor dwarf compared to the Sun. It probably has only 1/7 the Sun's diameter, 1/14 of the Sun's mass, and 1/300,000 of the Sun's luminosity. Sky & Telescope diagram

June 13, 2003 | A faint red dwarf star newly discovered in Aries may turn out to be the third-closest star system to the Sun. Or maybe not; its distance is still uncertain, and it might not even make the list of the closest two dozen. But in any case it's a rare find.

Known as SO 025300.5+165258, the star glows at a very dim visual magnitude of 15.4, which is why it went unnoticed for so long. Like other dim red and brown dwarfs that astronomers have recently discovered nearby, it was found by its fast proper motion (motion across the sky). A team led by Bonnard J. Teegarden (NASA/Goddard Space Flight Center) is searching for high-proper-motion stars in the online image database created by the NEAT asteroid-hunting program. Their new catch is speeding southeast at slightly more than 5 arcseconds per year. Only seven star systems in the sky are known to have proper motions that fast.

Using the NEAT images, the team also was able to measure a rough parallax for the star. This indicates that it lies between 6 and 11 light-years away, with 7.8 light-years being the likeliest value. Only the Alpha Centauri system and Barnard's Star are closer than that.

The team also observed the star's spectrum and found a spectral type of M6.5, placing the star toward the cool, dim end of the red-dwarf sequence. If it is like other M6.5 dwarfs, its apparent brightness would put it about 12 light-years away. High-quality parallax measurements now under way by the U.S. Naval Observatory should give a very accurate distance by the end of the year. Details of the discovery are in a NASA press release and the astronomers' preprint.

The star is within reach of amateur CCD imagers, who might be able to measure its parallax themselves over the course of the next year. It is currently at right ascension 2h 53m 01.7s, declination 16° 52' 40" (J2000, epoch

Fonte: http://skyandtelescope.com/news/article_976_1.asp

 

Céu claro para todos!

José Geraldo Mattos - Moderador

 

"Não se pode ensinar tudo a alguém, pode-se apenas ajudá-lo a encontrar por si mesmo"

(Galileu Galilei )

 

Leia Astronomynews na WEB: http://br.groups.yahoo.com/group/Astronomynews/messages

Hot news for cold dark matter from Chandra
CHANDRA X-RAY CENTER NEWS RELEASE
Posted: June 14, 2003

Astronomers have used NASA's Chandra X-ray Observatory to make the most detailed probe yet of the distribution of dark matter in a massive cluster of galaxies. Their results indicate that about 80 percent of the matter in the universe consists of cold dark matter -- mysterious subatomic particles left over from the dense early universe.

The galaxy cluster Abell 2029 is composed of thousands of galaxies enveloped in a gigantic cloud of hot gas, and an amount of dark matter equivalent to more than a hundred trillion Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies. Credit: NASA/CXC/UCI/A.Lewis et al.

Chandra observed a cluster of galaxies called Abell 2029 located about a billion light years from Earth. The cluster is composed of thousands of galaxies enveloped in a gigantic cloud of hot gas, and an amount of dark matter equivalent to more than a hundred trillion Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies.

The X-ray data show that the density of dark matter increases smoothly all the way into the central galaxy of the cluster. This discovery agrees with the predictions of cold dark matter models, and is contrary to other dark matter models that predict a leveling off of the amount of dark matter in the center of the cluster.

"I was really surprised at how well we could measure the dark matter so deep into the core of a rich cluster," said Aaron Lewis of the University of California, Irvine, lead author of a paper describing the results in a recent issue of The Astrophysical Journal. "We still have very little idea as to the exact nature of these particles, but our results show that they must behave like cold dark matter."

Cold dark matter gets its name from the assumption that the dark matter particles were moving slowly when galaxies and galaxy clusters began to form. Dark matter particles interact with each other and "normal" matter only through gravity.

The astronomers' success in placing such tight constraints on the dark matter distribution was partly due to Chandra's ability to make a high resolution intensity and temperature map, and partly due to their choice of a target. The cluster and central galaxy are unusually regular, with little or no sign of disturbance.

This optical image shows thousands of galaxies in the cluster Abell 2029. The image was taken with the Palomar 48-inch Schmidt telescope. Credit: Pal.Obs. DSS

The hot gas in a cluster is held in the cluster primarily by the gravity of the dark matter, so the distribution of the hot gas is determined by that of the dark matter. By precisely measuring the distribution of X-rays from the hot gas, the astronomers were able to make the best measurement yet of the distribution of dark matter in the inner region of a galaxy cluster.

"While Abell 2029 might be boring for the average person to look at," said David Buote, a coauthor of the paper, "it is a pure delight for astrophysicists to study, because it allows for a very straightforward and accurate comparison of theory and observation."

As a case in point, earlier observations of the Hydra A galaxy cluster by Larry David of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. and colleagues found a similar result but the evidence of explosive activity in the central galaxy made it difficult to draw definite conclusions about the nature of the dark matter. The dark matter profile deduced for Abell 2029 provides evidence that the Hydra results are reliable and is an important independent confirmation of cold dark matter predictions.

John Stocke of the University of Colorado, Boulder was also involved in this research. Chandra observed Abell 2029 with the ACIS detector for 5.6 hours on April 12, 2000. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the Office of Space Science, NASA Headquarters, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Shuttle Creates Clouds that Only Come Out at Night
Space shuttle and other launches may help produce night-shining clouds.
by Matt Quandt

These noctilucent ("night-shining" ) clouds were photographed in Norway. Pekka Parviainen A recently released study by the Naval Research Laboratory has shed some light on the formation of night-shining "noctilucent" clouds. In a paper for the Geophysical Research Letters, researchers assert that some of the exhaust released from the main engines of NASA's space shuttles and other launch vehicles eventually become gleaming clouds above the Arctic. "This study is important because it shows that there is a new source of water ice for the polar upper atmosphere," says lead author Michael Stevens. "Our results indicate that the water vapor released by launch vehicles can end up in the Arctic mesosphere." Water vapor comprises 97% of a space shuttle's main engine exhaust. As a shuttle shoots through the atmosphere, its trail of steam gathers in the highest layer of Earth's atmosphere, the thermosphere, and drifts toward the north pole. "Drifts" may be the wrong term, however, because Stevens and his colleagues found that this vapor can reach the Arctic in just one day. Such a speedy trip to the north pole is too fast to be carried by atmospheric winds, according to current models. There is currently no conclusive explanation for how the vapor travels so quickly. As it nears the Arctic, the vapor continues to settle and freezes in the cold (–220° F) air of the mesosphere 51 miles above the Arctic surface, forming distinct ice clouds.

Space Shuttle Discovery leaves a trail as it launches on STS-92. NASA Sometimes called polar mesospheric clouds when viewed from space, these thin clouds are invisible to the naked eye during the day. At night, the sun's rays illuminate these thin clouds from below the horizon, causing them to shine behind the dark lower atmosphere. During STS-85 in August 1997, Space Shuttle Discovery astronauts used an instrument called the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) to witness the formation of these brilliant clouds. For eight days, MAHRSI allowed Stevens and other scientists to monitor the plume's journey to the north pole. By the end of the mission scientists had mapped the vapor's trail to the Arctic, witnessed its condensing into ice particles, and finally the evolution into a Noctilucent Cloud. A ground-based experiment in Norway also observed the Arctic-bound water vapor as it passed overhead a day after the STS-85 launch. "The amount of water found here is tiny compared to the amount in the lower atmosphere," Stevens says. "But the long-term effects in the upper atmosphere have yet to be studied."

Almost as Flat as a Pancake
The bright southern star Achernar is the flattest star ever measured.
by Vanessa Thomas

The member units of the Very Large Telescope can work together as an interferometer. ESO Just like planets, moons, asteroids, and galaxies, stars spin. And just like those other citizens of the cosmos, some stars rotate faster than others. Stellar spins cause these huge balls of gas to bulge out at their middles, and the faster they spin, the wider they are. The sun, for example, is 42 kilometers larger along its equator than it is across its poles. However, many stars spin much faster than our middle-aged sun and are much more oblate. Astronomers using the Very Large Telescope Interferometer (VLTI) in Chile have now identified the flattest star ever measured: Achernar in the southern constellation Eridanus. Its equatorial radius is more than 50 percent greater than its polar radius.

Achernar is the brightest star in Eridanus and is the ninth-brightest star in the sky. DSS Also known as Alpha Eridani, Achernar shines at magnitude 0.5 and is the ninth-brightest star in the sky (ahead of Betelgeuse in Orion and behind Procyon in Canis Minor). Located 145 light-years away, it's a hot B-type star six times as massive as the sun. Achernar was the target when two of the four 8.2-meter telescopes of the VLTI were combined for the first time in October 2001. But from September 11 to November 12 of 2002, astronomers had some real science in mind when they focused on the star. A team headed by French astronomer Armando Domiciano de Souza used two of the VLTI's 40-centimeter testing telescopes to take periodic measurements of Achernar's angular diameter. As Earth itself turned, the astronomers were able to measure the star's angular size in different directions.

This profile shows the measured shape of the star Achernar. ESO The measurements revealed that Achernar's greatest diameter is approximately 0.00253 arcsecond wide and that its shortest diameter is no more than 0.00162 arcsecond across. At Achernar's distance, these angular diameters correspond to radii of about 8.4 million kilometers for its major axis and 5.4 million kilometers for its minor axis, or about 12 and 7.7 solar radii, respectively. This means Achernar is at least 56 percent larger at its equator than along its poles — it may be even flatter. Because we don't know exactly which way the star's rotational axis is pointing, the estimate of Achernar's polar axis is an upper limit. The true polar axis may be even smaller. The incredible flatness of Achernar presents a conundrum for astrophysicists. It is flatter than current models predict. Scientists will have to adapt their models to explain the new observations.

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Cosmic Sketch
The Hubble Space Telescope takes a closer look at the Pencil Nebula.
by Vanessa Thomas

Part of the Pencil Nebula (NGC 2736) was imaged by the Hubble Space Telescope in 2002. NASA / Hubble Heritage Team (STScI / AURA) Out of tragedy comes art. This is especially true for the cosmos. The collision of two galaxies provides a dramatic snapshot of the evolution of our universe. A black hole consumes stars and material that have wandered too close and dispels titanic jets of material we see extending beyond its home galaxy. The explosive death of a massive star produces a beautiful supernova remnant that helps us appreciate stellar life cycles. An example of the latter affair is the subject of the most recent portrait released by the Hubble Heritage Team, which produces many of the stunning images we enjoy from the Hubble Space Telescope. A star roughly 815 light-years away in the southern constellation Vela exploded thousands of years ago, and its remains form the huge, glowing Vela supernova remnant. One long and thin piece of this remnant has earned a separate distinction among astronomers as the Pencil Nebula. Catalogued as NGC 2736, the Pencil Nebula was first recorded by John Herschel in the 1840s.

This full-field image of the Pencil Nebula shows the smaller field imaged by the Hubble Space Telescope. AAO / David Malin (DMI) The new Hubble image, taken by the Advanced Camera for Surveys in October 2002, shows just three-quarters of the Pencil Nebula. In all, the Vela supernova remnant is 114 light-years across. Astronomers believe the Pencil Nebula outlines a supernova shock wave that has slammed into a pocket of dense gas. This collision heated the gas by millions of degrees, and the material is now emitting light as it cools. Colors in the Hubble image map out the temperatures of the various regions, where blue-colored areas are filled with hot, ionized oxygen and red regions contain cooler hydrogen. The corpse of the star that, in death, gave birth to the Vela remnant is now a pulsar that spins 11 times each second. From this rate, astronomers calculate that the explosion happened 11,000 years ago and that the event blasted material away from the dying star at a rate of 22 million miles per hour. These fragments have been slowing ever since, but the Pencil Nebula is still moving at a pretty good clip at 400,000 miles per hour.

Infrared Vision Sees into Mars's Past
A comprehensive look at observations by Mars Odyssey's THEMIS instrument uncovers details about the Mars of today and yesterday.
by Vanessa Thomas

Mars Odyssey is using a suite of instruments to map Mars. NASA / JPL Since Mars Odyssey's science mission began in February 2002, its Thermal Emission Imaging System (THEMIS) has been studying the martian surface like no other instrument has before. After looking over a year's worth of THEMIS data, scientists have found some new surprises about Mars and are realizing how much the Red Planet has changed. In an upcoming Science report (already published online by Science Express), THEMIS principal investigator Phil Christensen of Arizona State University and colleagues describe their findings from the first overall analysis of THEMIS data so far. Among the most significant results are observed layers with "dramatically different physical properties," Christensen says. They have also found unexpected formations and discovered water-soluble olivine inside a canyon. "It's very difficult to say exactly what happened in any particular place, but what we've found is that in many places on Mars it hasn't just been the same old thing happening for year after year for billions of years," Christensen summarizes. With a resolution 300 times better than Mars Global Surveyor's Thermal Emission Spectrometer, THEMIS uses visible and infrared cameras to observe the temperatures of surface features during the day and night.

Infrared observations of Terra Meridiani detail layering at the surface. NASA / JPL / ASU While THEMIS's skills may not seem very exciting at first, this temperature data can reveal what certain features are made of. For example, if you've ever walked barefoot on a beach on a hot summer day, you know sand heats up quickly. But at night, sand and other fine-grained materials lose that heat just as fast, while solid rock stays warm longer. THEMIS uses such properties to study the composition of the martian surface. "The camera on Mars Global Surveyor takes exquisite images that show layers," Christensen says, "but it doesn't tell me anything about composition — is it a layer of boulders with a layer of sand on top? I have no way of knowing. With the THEMIS temperature data, I can actually get an idea because the layers vary — and each layer has remarkably different physical properties." According to Christensen, the physical properties of these layers change "because the environment in which those rocks were deposited changed." Other findings show that Mars remains dynamic today. Kilometer-sized patches of bare bedrock discovered by THEMIS are surprising, considering how dusty Mars is. The fact they're exposed and not buryed in layers of dust show environmental forces are clearing these areas of such sediment.

This immense landslide lies along the southern wall of Ganges Chasma. NASA / JPL / ASU Christensen and his colleagues draw a similar conclusion from piles of loose rock THEMIS has found on martian hillsides. "If those rocks had been made a billion years ago, they'd be covered with dust," Christensen explains. "This shows a dynamic Mars — it's an active place." Another interesting find is the strong presence of olivine near the bottom of Ganges Chasma, a canyon 4.5 kilometers (2.8 miles) deep within the huge Valles Marineris system. The mineral quickly decomposes with water, so its existence suggests there may be little or no history of water in the region. "There can't have been much water — ever — in this place," Christensen comments. "If there was groundwater present when it was deep within the surface, the olivine would have disappeared. And since the canyon has opened up, if there had ever been water at the surface it would be gone too. This is a very dry place."

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First measurement made of a dead star's magnetism
EUROPEAN SPACE AGENCY NEWS RELEASE
Posted: June 11, 2003

Using the superior sensitivity of ESA's X-ray observatory, XMM-Newton, a team of European astronomers has made the first direct measurement of a neutron star's magnetic field. The results provide deep insights into the extreme physics of neutron stars and reveal a new mystery yet to be solved about the end of this star's life.

The neutron star 1E1207.4-5209 is seen as the bright yellow object in the centre of this image, which was taken with the European Photon Imaging Camera aboard ESA's X-ray observatory, XMM-Newton. Credit: ESA/CESR

A neutron star is very dense celestial object that usually has something like the mass of our Sun packed into a tiny sphere only 20-30 km across. It is the product of a stellar explosion, known as a supernova, in which most of the star is blasted into space, but its collapsed heart remains in the form of a super-dense, hot ball of neutrons that spins at a incredible rate.

Despite being a familiar class of object, individual neutron stars themselves remain mysterious. Neutron stars are extremely hot when they are born, but cool down very rapidly. Therefore, only few of them emit highly energetic radiation, such as X-rays. This is why they are traditionally studied via their radio emissions, which are less energetic than X-rays and which usually appear to pulse on and off. Therefore, the few neutron stars which are hot enough to emit X-rays can be seen by X-ray telescopes, such as ESA's XMM-Newton.

One such neutron star is 1E1207.4-5209. Using the longest ever XMM-Newton observation of a galactic source (72 hours), Professor Giovanni Bignami of the Centre d'Etude Spatiale des Rayonnements (CESR) and his team have directly measured the strength of its magnetic field. This makes it the first ever isolated neutron star where this could be achieved. All previous values of neutron star magnetic fields could only be estimated indirectly. This is done by theoretical assumptions based on models that describe the gravitational collapse of massive stars, like those which lead to the formation of neutron stars. A second indirect method is to estimate the magnetic field by studying how the neutron star's rotation slows down, using radio astronomy data.

In the case of 1E1207.4-5209, this direct measurement using XMM-Newton reveals that the neutron star's magnetic field is 30 times weaker than predictions based on the indirect methods.

How can this be explained? Astronomers can measure the rate at which individual neutron stars decelerate. They have always assumed that 'friction' between its magnetic field and its surroundings was the cause. In this case, the only conclusion is that something else is pulling on the neutron star, but what? We can speculate that it may be a small disc of supernova debris surrounding the neutron star, creating an additional drag factor.

The result raises the question of whether 1E1207.4-5209 is unique among neutron stars, or it is the first of its kind. The astronomers hope to target other neutron stars with XMM-Newton to find out.

X-rays emitted by a neutron star like 1E1207.4-5209, have to pass through the neutron star's magnetic field before escaping into space. En route, particles in the star's magnetic field can steal some of the outgoing X-rays, imparting on their spectrum tell-tale marks, known as 'cyclotron resonance absorption lines'. It is this fingerprint that allowed Prof. Bignami and his team to measure the strength of the neutron star's magnetic field.

These results are being published in this week's issue of Nature

Grupo quer adiar aposentadoria do telescópio Hubble

da Folha de S.Paulo

Um grupo de pesquisadores liderado por John Bahcall, da Universidade Princeton, está estudando meios de manter o telescópio espacial Hubble em operação depois de 2010, informou ontem o boletim da Associação Americana para o Avanço da Ciência "Science Now"
(sciencenow.sciencemag.org).

Originalmente, os planos da Nasa (agência espacial americana) eram conduzir mais uma missão de reforma do sistema orbital em 2004 e então mantê-lo em operação pelos seis anos seguintes, antes da desativação.

Os astrônomos temem que em 2010, data em que o Hubble deveria deixar a cena, seu sucessor, o telescópio espacial James Webb, ainda não esteja pronto. Atualmente o sistema, com lançamento marcado para 2011, ainda está em estágio inicial de desenvolvimento

Scientists image 3-D surface of the sun
LOCKHEED MARTIN NEWS RELEASE
Posted: June 17, 2003

Solar physicists from Lockheed Martin, the National Center for Atmospheric Research, The Institute of Theoretical Astrophysics of the University of Oslo, and the Institute for Solar Physics of the Royal Swedish Academy of Sciences have analyzed the highest resolution images ever taken near the solar limb (or visible edge of the sun), and found a surprising variety of structure. Their results, which are being reported today at the American Astronomical Society's Solar Physics Division meeting in Laurel, Maryland, address long-standing theories on how the brightness of the Sun varies over the course of its magnetic cycle. Such changes may influence the Earth's climate on long timescales.

Image of a solar active region taken on July 24, 2002 near the eastern limb of the Sun at heliographic coordinates S15 E53 degrees. The limb is towards the top of the image. The tick marks are 1000 km apart. The smallest resolvable features in the image are about 70 km in size. The image was taken by Prof. Goran Scharmer and processed by Dr. Mats G. Löfdahl, both of the Institute for Solar Physics of the Royal Swedish Academy of Sciences.

"Until recently we thought of the solar photosphere as the relatively flat and featureless 'surface' of the Sun, punctuated only by an occasional sunspot," said Dr. Tom Berger, principal investigator on the study, and solar physicist at the Lockheed Martin Solar and Astrophysics Lab (LMSAL) at the company's Advanced Technology Center in Palo Alto, Calif. "Now, using the newly commissioned Swedish one-meter Solar Telescope (SST) on the island of La Palma, Spain, we have, for the first time, imaged the three-dimensional structure of the convective 'granules' that cover the photosphere."

The solar surface consists mostly of an irregular cellular pattern caused by temperature variations. The cells, called granules, are evidence of convection that transports heat to the surface in the same manner as boiling water on a stovetop or thermal plumes rising over hot fields to form thunderstorms. Each granule on the sun is about the size of Texas. At the 75 km resolution of the SST, sunspots and smaller dark "pores" are seen to be sunken into the surrounding granulation. This so-called "Wilson depression" has been inferred from lower resolution observations of large sunspots but never directly resolved until now.

Most importantly from a terrestrial climate perspective, the images show clearly that the granulation in regions of smaller magnetic fields outside of sunspots is both raised up and has brighter walls than the granulation in non-magnetic regions. Bright structures near the limb of the Sun have been seen for centuries in lower resolution images and are called "faculae" (Latin for "little torches"). Faculae are significant because scientists believe that their brightness is responsible for the increased solar irradiance (on the order of 0.1 to 0.15%) that occurs during periods of maximum solar magnetic activity.

At solar maximum, the Sun is covered by the greatest amount of dark sunspots in its 11-year cycle. It would be expected that the solar irradiance reaching Earth during that time might decrease. But beginning in the 1980s, satellite radiometer instruments, such as the Active Cavity Radiometer Irradiance Monitor instrument (ACRIM I) on the Solar Maximum Mission (SMM) spacecraft, revealed that while sunspots cause a decrease in the solar irradiance on time scales of days to weeks, the long-term solar irradiance actually increases as sunspot (magnetic) activity increases.

The source of this "extra" irradiance has been traced to the bright faculae near the limb of the Sun. Based on earlier low resolution images of faculae, scientists have created models that attribute most of the brightness of faculae to small magnetic "flux tubes" or "micropores". These models suggest that micropores act like tiny holes in the surface of the photosphere. When looking at disk center, we see only the relatively cool "floors" of the flux tubes. When seen at an angle near the limb, the models predict that the "hot walls" of the magnetic holes shine brightly compared to the relatively cooler surrounding granules.

The SST images may help resolve discrepancies between the "hot-wall" flux tube model and observations of facular brightness near the solar limb. Most of the bright structures seen are between 150 and 400 km tall and are typically elongated towards the limb. Simultaneous measurements of the magnetic field establish that the bright faculae are exactly aligned with the magnetic fields. However the faculae in these images appear more like bright walls of granulation that have somehow been "piled up" by the presence of magnetic fields than like micropores seen at an angle.

Theoretical models of solar convection developed by Dr. Neal Hurlburt of LMSAL support this "raised wall" picture. "The model that has been used to explain the brightness of faculae," Dr. Hurlburt reflects, "usually assumed that the rest of the solar atmosphere was an innocent bystander. However it is known that magnetic fields are swept aside as hot gas rises and spreads across the solar surface and confines the field to regions of down-flows. Many groups have modeled the dynamics of such magnetoconvection, but we have never gotten around to detailed comparison with sources of irradiance variations. We frequently find that the gas in our models is denser or hotter at the edges of the magnetic fields -- which might result in brightenings very much like what these images show."

As the ultimate source of all energy input to the Earth, understanding solar irradiance and its variation with magnetic activity on the Sun is an important factor in understanding climate variation on Earth. "Raising the hot material above the photosphere enhances facular emission at low angles to the solar surface" according to Prof. John Lawrence of California State University Northridge. "Low angles cover the greater part of the solar 'sky' as seen from the perspective of a facula, so this discovery impacts our estimate of the contribution of faculae to solar brightness changes. With this new discovery, we can hope to incorporate the effects of magnetoconvection into solar irradiance models to better predict variations in solar output."

Preliminary analyses of the some of the images are in a paper by Dr. Bruce Lites of NCAR, Prof. Goran Scharmer of the Royal Swedish Academy of Sciences, and Drs. Alan Title and Tom Berger of Lockheed Martin Solar and Astrophysics Lab that has been submitted for peer-review to the journal Solar Physics.

Lockheed Martin Space Systems Company is one of the major operating units of Lockheed Martin Corporation. Space Systems designs, develops, tests, manufactures, and operates a variety of advanced technology systems for military, civil and commercial customers. Chief products include a full-range of space launch systems, including heavy-lift capability, ground systems, remote sensing and communications satellites for commercial and government customers, advanced space observatories and interplanetary spacecraft, fleet ballistic missiles and missile defense systems.

Headquartered in Bethesda, Md., Lockheed Martin is a global enterprise principally engaged in the research, design, development, manufacture, and integration of advanced-technology systems, products, and services. The Corporation's core businesses are systems integration, space, aeronautics, and technology services. Employing about 125,000 people worldwide, Lockheed Martin had 2002 sales surpassing $26.6 billion.

Low- and high-resolution JPEG image files of the discovery are available at the following URL: http://www.lmsal.com/Press/SPD2003.html

'Solar tsunamis' used to study the solar corona
SOUTHWEST RESEARCH INSTITUTE NEWS RELEASE
Posted: June 16, 2003

Since the launch of the Solar and Heliospheric Observatory (SOHO) in 1995, scientists have used its Extreme-Ultraviolet Imaging Telescope (EIT) to study flares, filaments and coronal mass ejections. The telescope has also discovered solar tsunamis (also called "EIT waves" by solar scientists), huge propagating waves that are triggered along with coronal mass ejections and can travel the entire diameter of the sun. Researchers at Southwest Research Institute (SwRI) are applying this unusual phenomenon for the first time to new studies of the solar corona.

Voss working aboard the space station in 2001. Credit: NASA

The Transition Region and Coronal Explorer (TRACE) spacecraft, launched in 1998, has provided new data complementing SOHO observations. Its higher resolution and faster cadence give solar physicists the tools to study hitherto unseen details. Dr. Meredith Wills-Davey, a post-doctoral researcher in the SwRI Space Studies Department, uses TRACE data to better understand the nature of solar tsunamis and the structure of the corona through which they travel. Her work is being presented June 16 at the Solar Physics Division Meeting of the American Astronomical Society in Laurel, Md.

"Just as geologists can learn about material in the ground by studying the waves generated by earthquakes," she says, "solar physicists can use these solar tsunamis to learn more about the structure of the solar corona."

TRACE observations of a well-observed event on June 13, 1998, are sufficiently detailed that it is possible to show, through morphology alone, that the propagation must be a "fast-mode magnetoacoustic wave." Analysis of the amplitude and the energy flux of the wave front shows that it actually increases through much of its lifetime. This suggests that, rather than being a single impulse, the wave driver may exist for an extended period. Because this particular event was associated with a coronal mass ejection, it is possible the wave is somehow part of the coronal mass ejection formation, says Wills-Davey.

Comparison between current observations at different coronal temperatures also offers insight into the wave's altitude of propagation. Evidence suggests that the tsunami is skimming along the base of the corona. This idea is also consistent with the lack of measurable dispersion in the wave, a circumstance more easily explained if the front travels at a constant height. Existing models and theories suggest that propagating waves in the corona should be trapped in "wave guides," but this appears to be the first observational evidence.

"These pulse waves serve as 'sonar pulses' that will let us probe the local conditions in up to 30 percent of the sun's atmosphere at once," says Dr. Craig DeForest, a senior research scientist at SwRI. "In addition, they help us study the unknown processes at play in solar flares, the largest explosions in our solar system."

"The study of waves in the corona is a new venture with an exciting future, and the benefits to our understanding of the sun should be substantial," adds Wills-Davey, who recently received NASA funding to continue this work. The research to date has been funded by NASA and the American Association of University Women Educational Foundation.

SwRI is an independent, nonprofit, applied research and development organization based in San Antonio, Texas, with more than 2,800 employees and an annual research volume of more than $339 million.

Clouds Lift on Massive Protostars
Strong stellar winds provide astronomers' first look at very young massive stars.
by Vanessa Thomas

NGC 3603 includes a massive star cluster, pillars of dust, molecular clouds, and very young and massive protostars. ESO The most massive stars live fast lives. Their infant years are over in the cosmic equivalent of a blink of an eye, and then they race through adolescence and hurry through adulthood before exploding and giving way to the next generation of stars. Their rush through life makes it difficult for astronomers to find massive stars when they're still very young — especially when they typically remain swaddled in their nascent clouds of gas and dust until they're nearly fully grown. However, a lucky circumstance has allowed European Southern Observatory (ESO) astronomer Dieter Nürnberger to find a group of high-mass infant stars that have been unveiled by the strong stellar winds of their big brothers. These young stars offer the first look at the earliest formation of massive stars and suggest that they form from accretion rather than by violent collisions. "I know of no other high-mass protostellar candidates which have been revealed at such an early evolutionary stage," Nürnberger says. The stars were found about 22,000 light-years away in the Carina arm of the Milky Way as unidentified point sources lying in the starbirth region of NGC 3603. Collectively called IRS 9, the stellar group lies on the periphery of a molecular cloud called MM 2 and facing a prominent cluster of hot, massive stars. Stellar winds from the fully developed stars in this cluster appear to be dispersing the clouds in which the stars of IRS 9 were born and which would normally shield the stars from view.

The brightest stars of IRS 9 are pointed out in this composite image. ESO Nürnberger used a series of observations at different wavelengths to make this discovery, which he has reported in a recent issue of the journal Astronomy and Astrophysics. Near-infrared images from the 8.2-meter Antu unit of the Very Large Telescope determined that the stars belonged to NGC 3603 and weren't foreground stars. Millimeter observations with the Swedish-ESO Submillimeter Telescope mapped the dense gas of the molecular cloud and found that the cluster's intense radiation and strong winds are creating a cavity in the clouds around IRS 9. Mid-infrared images shows warm dust distribution in the area and indicates that intense star formation continues in IRS 9. "We now have convincing arguments to consider IRS 9 as a kind of Rosetta Stone for our understanding of the earliest phases of the formation of massive stars," Nürnberger states. "The new near- and mid-infrared observations are giving us a first look into this extremely interesting phase of stellar evolution." Nürnberger also studied the three brightest stars of IRS 9 in more detail. Labeled IRS 9A, 9B, and 9C, they each appear to be only about 100,000 years old or younger. The intrinsic brightness of the group's most luminous member, IRS 9A, is about 100,000 times greater than the sun's. The other two are 1,000 times brighter than the sun. While all three stars are quite hot (roughly 20,000 to 22,000 degrees), they appear to be surrounded by relatively cold dust (-20° C to 0° C). The observations also suggest that the stars are  already at least 10 times as massive as the sun and are still growing — accreting up to an Earth mass per day or one solar mass over a millennium. No low- or intermediate-mass stars were found in the area, suggesting that massive stars — or at least those in IRS 9 — grow steadily by accretion, rather than in sudden bursts when low-mass stars collide, as some scientists have theorized.

Sun Beheads Comets
SOHO recently witnessed the decapitation of two comets as they flew past the sun.
by Kelly Kizer Whitt

The Solar and Heliospheric Observatory imaged two comets nearing the sun on May 24, 2003. SOHO / NASA / ESA This faint "puff" is what remains of two comets that lost their heads while nearing the sun. SOHO / NASA / ESA Click on this image to download a 706-kilobyte movie showing the decapitation of these two comets on May 24, 2003. SOHO / NASA / ESA Two comets met their fate on May 24, 2003, during a close encounter with the sun that cost them their heads. The Solar and Heliospheric Observatory (SOHO) witnessed this rare beheading and the survival of the comets' tails, which continued to race away from the sun after the ordeal. The guillotined comets belonged to the Kreutz family of sungrazers, which are often observed by SOHO as they make their final plunge toward the sun. But these comets were different from most Kreutz sungrazers in two important ways: they were "twin" comets traveling together and they were not completely consumed by their close encounter with the sun. The two comets lost their heads when their orbits put them inside the sun's multimillion-degree corona. The severe heat and radiation vaporized the nuclei and coma that made up the heads of the comets. However, like the Headless Horseman galloping through Sleepy Hollow, the tails of the headless comets continued to move away from the sun in their orbits. Comets are nicknamed "dirty snowballs" because they are made mostly of ice and dust. As they near the sun the tails begin to enlarge when heat and radiation pressure releases gas and dust from the nucleus. The solar wind pushes charged particles away from the comet to create the ion tail, while radiation pressure pushes the dust tails out from the comet. (Both tails are easily seen on photographs of bright comets such as Hale-Bopp.) Scientists believe the "puff of smoke" SOHO saw exiting the corona is the comets' dusty remains being expelled by radiation pressure after the nuclei's ice evaporated. Douglas Biesecker of the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder, Colorado, says, "The fact that the tail 'holds together' so well probably means that the dust is mostly the same size." Although SOHO has observed more than 600 sungrazing comets over more than six years, this type of  behavior is unusual. Most sungrazers are simply annihilated altogether by the heat and radiation of a close encounter with the sun. However, in June 1998, SOHO did witness another pair of comets that appeared to succumb to their close pass with the sun except for their tails.

Related Stories
·	Our Solar System: Comets Break Up Far from the Sun
·	Space Missions: SOHO Nabs its 500th Comet

Galaxy Cluster is Mostly Cold and Dark
The Chandra X-ray Observatory maps cold dark matter within a galaxy cluster.
by Vanessa Thomas

Abell 2029 is a massive galaxy cluster in the constellation Serpens. Digitized Sky Survey / Palomar Observatory The most detailed plot of dark matter at the heart of a galaxy cluster suggests that about three-quarters of the cluster's mass and the mass of the universe exists as "cold" dark matter — subatomic particles that were moving slowly when the first galaxies were born. "We still have very little idea as to the exact nature of these particles, but our results show that they must behave like cold dark matter," says Aaron Lewis of the University of California, Irvine (UCI). Lewis heads a team of astronomers that used NASA's Chandra X-ray Observatory to study the x-ray emission from hot gas within the galaxy cluster Abell 2029. Located about a billion light-years from Earth, the cluster includes thousands of galaxies that appear to be centered on one huge elliptical galaxy. The distribution of hot gas in Abell 2029 depends on the gravitational whims of the cluster's strange and unseen dark matter. So the team surveyed the cluster's hot gas to learn about its shady contents. Theories involving cold dark matter predict that the density of dark matter in a cluster will increase steadily toward its center, while other theories predict the density will decrease. Chandra's high-resolution observations showed a gradual rise in the density into Abell 2029's central galaxy.

This Chandra X-ray Observatory image shows hot gas at the center of the Abell 2029 galaxy cluster. NASA / CXC / UCI / A.Lewis et al. "I was really surprised at how well we could measure the dark matter so deep into the core of a rich cluster," said Lewis, who is the lead author of a recent paper describing the results in The Astrophysical Journal. The team benefited from the fact that the goings-on in Abell 2029 are unusually civil, with little activity disrupting the order between the cluster members. "While Abell 2029 might be boring for the average person to look at," said UCI team member David Buote, "it is a pure delight for astrophysicists to study because it allows for a very straightforward and accurate comparison of theory and observation." A similar result was reached in an earlier study of the less orderly Hydra A galaxy cluster. Its dark-matter density also appeared to increase toward the cluster's center. However, those findings were less definitive because astronomers suspected Hydra A's more explosive history had made its environment more messy. But now both investigations seem to suggest that cold dark matter composes the bulk of the universe.

Related Stories
·	Galaxies: Einstein Cross Reveals Dark Matter Puzzle
·	Galaxies: Chandra Finds More Evidence of Dark Matter
·	Galaxies: Dark Matter Missing in Elliptical Galaxies
·	The Milky Way: Astronomers Shine a Light on Milky Way's Dark Matter
·	The Milky Way: First Seen Microlens Opens Dark Matter's Door
·	Cosmology: Through the Universe, Darkly
·	Cosmology: Chandra Spies Invisible Web

Almost as Flat as a Pancake
The bright southern star Achernar is the flattest star ever measured.
by Vanessa Thomas

The member units of the Very Large Telescope can work together as an interferometer. ESO Just like planets, moons, asteroids, and galaxies, stars spin. And just like those other citizens of the cosmos, some stars rotate faster than others. Stellar spins cause these huge balls of gas to bulge out at their middles, and the faster they spin, the wider they are. The sun, for example, is 42 kilometers larger along its equator than it is across its poles. However, many stars spin much faster than our middle-aged sun and are much more oblate. Astronomers using the Very Large Telescope Interferometer (VLTI) in Chile have now identified the flattest star ever measured: Achernar in the southern constellation Eridanus. Its equatorial radius is more than 50 percent greater than its polar radius.

Achernar is the brightest star in Eridanus and is the ninth-brightest star in the sky. DSS Also known as Alpha Eridani, Achernar shines at magnitude 0.5 and is the ninth-brightest star in the sky (ahead of Betelgeuse in Orion and behind Procyon in Canis Minor). Located 145 light-years away, it's a hot B-type star six times as massive as the sun. Achernar was the target when two of the four 8.2-meter telescopes of the VLTI were combined for the first time in October 2001. But from September 11 to November 12 of 2002, astronomers had some real science in mind when they focused on the star. A team headed by French astronomer Armando Domiciano de Souza used two of the VLTI's 40-centimeter testing telescopes to take periodic measurements of Achernar's angular diameter. As Earth itself turned, the astronomers were able to measure the star's angular size in different directions.

This profile shows the measured shape of the star Achernar. ESO The measurements revealed that Achernar's greatest diameter is approximately 0.00253 arcsecond wide and that its shortest diameter is no more than 0.00162 arcsecond across. At Achernar's distance, these angular diameters correspond to radii of about 8.4 million kilometers for its major axis and 5.4 million kilometers for its minor axis, or about 12 and 7.7 solar radii, respectively. This means Achernar is at least 56 percent larger at its equator than along its poles — it may be even flatter. Because we don't know exactly which way the star's rotational axis is pointing, the estimate of Achernar's polar axis is an upper limit. The true polar axis may be even smaller. The incredible flatness of Achernar presents a conundrum for astrophysicists. It is flatter than current models predict. Scientists will have to adapt their models to explain the new observations.

Related Stories
·	Stars: Altair's Chubby Middle
·	Observatories: Two VLT Giants Collaborate

Nasa adia lançamento de segundo robô para Marte

 da Folha Online

A Nasa (agência espacial norte-americana) disse hoje que o lançamento do segundo robô de exploração de Marte, Opportunity ("oportunidade", em inglês), será lançado no dia 26 e não na véspera, como anunciado anteriormente.

O adiamento por 24 horas já era esperado, após o lançamento do primeiro robô, Spirit ("espírito"), no dia 10, ter atrasado dois dias.

Os "robôs geológicos", como são chamados pela Nasa, chegam ao planeta vermelho em janeiro para explorar dois extremos de Marte em busca de água. Eles serão alimentados com energia solar e podem se deslocar 40 metros por dia marciano. Essa distância corresponde a todo o trajeto percorrido pelo robô Sojourner, o primeiro a se locomover sobre a superfície de Marte em 1997.

Com agências internacionais

Sonda japonesa passa pela Terra, a caminho de Marte

 da Folha de S.Paulo

A sonda japonesa Nozomi fez um vôo rasante pela Terra anteontem e agora viaja na direção de Marte, onde deve chegar no início de 2004.

A primeira investida do Japão na exploração marciana custou mais de US$ 800 milhões. Lançada em 1998, a Nozomi passou boa parte do tempo perdida no espaço, o que obrigou técnicos e engenheiros a pensarem em outra maneira de levar a sonda até seu destino. O jeito foi usar a gravidade da Terra para acelerá-la na direção do planeta vermelho

Foguete russo levará dois turistas juntos ao espaço

da Folha Online

A empresa Space Adventures, que já levou dois milionários ao espaço, anunciou na quarta-feira (18) que pretende aumentar o número de civis nas viagens até a Estação Espacial Internacional (ISS).

A intenção é enviar duas pessoas, e não apenas uma, em uma viagem exclusivamente turística até 2005. Eles iriam a bordo de uma nave russa Soyuz, pilotada por um cosmonauta. Apesar de o número de assentos dobrar, o valor continua o mesmo: cerca de US$ 20 milhões por pessoa.

Segundo a companhia, já há procura pelo passeio --os selecionados serão anunciados em dois ou três meses, após rigorosos treinamentos e testes médicos. Além disso, os candidatos devem cumprir uma lista de exigências feitas pela Nasa (agência espacial norte-americana), parceira da Rússia na ISS.

Até hoje, dois turistas estiveram na estação: o norte-americano Dennis Tito, em abril de 2001, e o sul-africano Mark Shuttleworth, um ano depois. Ambos foram encaixados em missões científicas.

Com agências internacionais

Explosão acaba com enigma da astrofísica

SALVADOR NOGUEIRA
da Folha de S.Paulo

Chega ao fim um dos maiores mistérios da astrofísica. Três grupos independentes de astrônomos acabam de confirmar como são produzidas as explosões mais intensas do Universo.

Elas ocorrem em média uma vez por dia e, por poucos instantes, brilham mais que a luz produzida conjuntamente por todas as estrelas do cosmos. São conhecidas como disparos de raios gama ("gamma ray bursts"), mas o nome não é tão impressionante quanto a energia que carregam.

Desde os anos 1960, satélites em órbita têm observado esses disparos, vindos das mais longínquas regiões do espaço, mas ninguém sabia o que os produzia. Teorias para explicá-los sobravam, mas faltavam dados observacionais que apontassem a hipótese certa e excluíssem as erradas. Apesar das inúmeras observações desses disparos, normalmente eles vinham de distâncias gigantescas, impedindo que astrônomos chegassem a conclusões firmes.

A sorte grande veio no último dia 29 de março, quando um disparo de raios gama foi detectado por um satélite em órbita. Analisando as distorções que as partículas sofrem durante o percurso até a Terra, descobriu-se que ele se originou a 2,6 bilhões de anos-luz do planeta. A distância ainda é enorme (cerca de um décimo do diâmetro de todo o Universo observável), mas foi próxima o suficiente para associá-lo a uma estrela que acabou de virar supernova.

Um disparo de raios gama desses, vindo de tal distância, acontece em média uma vez por década. Nunca houve chance de analisar tão bem a origem da explosão quanto agora. "Estivemos esperando esse [disparo] por um longo, longo tempo", conta Jens Hjorth, da Universidade de Copenhague, autor principal de um dos três estudos publicados ontem na revista britânica "Nature" (www.nature.com). "O disparo do dia 29 tem toda a informação faltante. Foi criado pelo colapso do núcleo de uma estrela maciça."

Apesar de não serem organismos vivos, estrelas nascem, crescem e morrem. Normalmente, geram seu brilho transformando hidrogênio em hélio, via fusão nuclear. A pressão resultante desse processo compensa a gravidade e mantém a estrela estável.

Ao final do ciclo de vida, as que têm massa muito maior que a do Sol (25 vezes ou mais) extinguem seu combustível e iniciam um processo de implosão, que no final levará à formação de um buraco negro (objeto cuja gravidade é tão intensa na superfície que nem a luz pode escapar). Enquanto isso está acontecendo, a estrela dá um último rugido --o violento disparo de raios gama.

A energia proveniente dessas explosões súbitas é tão intensa que, se partisse de uma supernova da Via Láctea, poderia afetar a vida terrestre. Há quem pense que alguns desses eventos causaram extinções e agiram como motor da evolução, no passado.

Estima-se que um disparo de raios gama ocorra na galáxia em que a Terra está localizada a cada milhão de anos. Uma estrela gigante em seu estágio final de vida, Eta Carinae, está a apenas 8.000 anos-luz e pode encerrar suas atividades daqui a 10 mil anos, na pior das hipóteses. Sabe-se lá o que um disparo de raios gama vindo daquela direção poderá causar à vida no planeta.

A curvatura do espaço-tempo

MARCELO GLEISER
Especial para a Folha de S.Paulo

O animador de televisão americano David Letterman gosta de fazer a lista das "Dez Mais", que podem ser perguntas, fatos absurdos, notícias estranhas etc. Eu também gosto, modestamente, de fazer uma lista, a das "dez perguntas mais populares em cosmologia". Recentemente (na coluna de 8 de junho), escrevi sobre a questão do começo, onde a pergunta (uma das mais populares da lista) era "O que ocorreu antes do começo?" Ou seja, se o Universo surgiu mesmo do Big Bang, o que havia antes disso?

Essa questão aborda a natureza do tempo, sua origem e seu significado. Mas ficou faltando algo. A resposta depende da teoria da relatividade geral, desenvolvida por Einstein em 1915 (a versão especial da teoria data de 1905). Nela, para falar de tempo deve-se necessariamente falar de espaço: o que existe é um contínuo espaço-temporal de quatro dimensões, três espaciais (norte-sul, leste-oeste, acima-abaixo) e uma temporal.

Esse contínuo se chama espaço-tempo. Segundo a teoria da relatividade, qualquer evento ocorrendo na natureza, seja uma bola caindo ao chão, seja a explosão de uma estrela, deve ser caracterizado pela sua posição nesse espaço-tempo quadridimensional. Portanto, um ponto no espaço-tempo é descrito por quatro números, três para a sua posição e um relacionado ao instante em que o evento ocorre. "Calma aí!" -exclama o leitor. "Você está me confundindo: no dia-a-dia também descrevemos o movimento dos objetos usando quatro números, três para a sua posição no espaço e um para o tempo. Qual a diferença entre esses quatro números e o espaço-tempo da teoria da relatividade?"

A pergunta é boa, mesmo que não esteja na lista. A diferença é enorme. Na física aplicável ao dia-a-dia, carros, trens, elevadores etc., espaço e tempo são vistos como entidades separadas, absolutas, uma distinta do outra. Na relatividade, o tempo é tratado como uma dimensão espacial, uma distância no espaço-tempo. Para isso, ele é multiplicado pela velocidade da luz. (Lembre-se de que velocidade tem unidade de distância dividida por tempo, como em km/h. Portanto, multiplicar tempo por velocidade resulta em distância.)

Vejamos um exemplo. Uma bola cai de uma altura de um metro. Segundo a física não-relativística, falamos de sua posição inicial, de sua posição final e de quanto tempo ela demorou para cair. Em relatividade, falamos de dois pontos no espaço-tempo, separados por uma distância.

Essa junção do espaço com o tempo causa efeitos peculiares. Na sua maioria, eles passam despercebidos, limitados que somos em nossa percepção da realidade. Mas, quando movimentos ocorrem com velocidades próximas da velocidade da luz, ou quando a força gravitacional é muito intensa, a natureza unificada do espaço-tempo se torna palpável, mesmo para nossos olhos míopes. A pergunta da lista está ligada com esses efeitos gravitacionais fortes.

"Como o espaço pode encurvar?" Espaço é a entidade que usamos para medir distâncias entre pontos. Um espaço deformável, portanto, é aquele em que as distâncias podem mudar, como em uma superfície elástica puxada nessa ou naquela direção. Segundo Einstein, a presença de massas deforma o espaço-tempo, alterando a geometria do espaço e o fluir do tempo. Uma analogia muito comum é o de uma bola de chumbo sobre um colchão: na vizinhança mais imediata da bola o colchão se deforma.

Claro, a analogia é apenas sugestiva, já que o que deforma o colchão é o peso da bola na gravidade terrestre. Mas a idéia é que campos gravitacionais fortes alteram a geometria do espaço-tempo. Esse é o caso perto de estrelas muito maciças, ou dos misteriosos buracos negros, onde a curvatura é tal que o espaço-tempo se fecha sobre si mesmo, como um casulo.

Pode parecer estranho que algo tão intangível como o espaço (ou o tempo) responda à presença de massas. Mas é importante lembrar que teorias físicas são descrições da natureza criadas com um propósito muito claro, o de ajudar na compreensão de fenômenos mensuráveis e quantificáveis. Se elas ocasionalmente resultam em explicações surpreendentes, é porque nossa miopia é grande. Cada teoria pode ser vista como uma lente um pouco mais forte, que permite desvendar um ou outro novo detalhe da natureza, de suas infinitas formas e criatividade.

Marcelo Gleiser é professor de física teórica do Dartmouth College, em Hanover (EUA), e autor do livro "O Fim da Terra e do Céu"

The secret lives of galaxies unveiled in deep survey
NASA NEWS RELEASE
Posted: June 19, 2003

Two of NASA's Great Observatories, bolstered by the largest ground-based telescopes around the world, are beginning to harvest new clues to the origin and evolution of galaxies. It's a bit like finding a family scrapbook containing snapshots that capture the lives of family members from infancy through adolescence to adulthood.

NASA's Hubble Space Telescope reached back to nearly the beginning of time to sample thousands of infant galaxies. This image, taken with Hubble's Advanced Camera for Surveys, shows several thousand galaxies, many of which appear to be interacting or in the process of forming. Some of these galaxies existed when the cosmos was less than about 2 billion years old. The foreground galaxies, however, are much closer to Earth. Two of them -- the white, elongated galaxies, left of center -- appear to be colliding. Credit: NASA, ESA, the GOODS Team and M. Giavalisco (STScI)

"This is the first time the cosmic tale of how galaxies build themselves has been traced reliably to such early times in the universe's life," said Mauro Giavalisco, head of the Hubble Space Telescope (HST) portion of the survey, and research astronomer at the Space Telescope Science Institute (STScI) in Baltimore.

The HST has joined forces with the Chandra X-ray Observatory to survey a relatively broad swath of sky encompassing tens of thousands of galaxies stretching far back into time. The Space Infrared Telescope Facility (SIRTF), scheduled for launch in August, will soon join this unprecedented survey. Called the Great Observatories Origins Deep Survey (GOODS), astronomers are studying galaxy formation and evolution over a wide range of distances and ages. The project is tracing the assembly history of galaxies, the evolution of their stellar populations, the gusher of energy from star formation and active nuclei powered by immense black holes.

HST astronomers report the sizes of galaxies clearly increase continuously from the time the universe was about 1 billion years old to an age of 6 billion years; approximately half the current age of the universe, 13.7 billion years. GOODS astronomers also find star birth rate rose mildly, by about a factor of three, between the time the universe was about one billion years old and 1.5 billion years old. It remained high until about 7 billion years ago, when it quickly dropped to one-tenth the earlier "baby boomer" rate. This is further evidence major galaxy building trailed off when the universe was about half its current age.

This increase in galaxy size is consistent with "bottom-up" models, where galaxies grow hierarchically, through mergers and accretion of smaller satellite galaxies. This is also consistent with the idea the sizes of galaxies match hand- in-glove to a certain fraction of the sizes of their dark- matter halos. Dark matter is an invisible form of mass that comprises most of the matter in the universe. The theory is dark matter essentially pooled into gravitational "puddles" in the early universe, then collected normal gas that quickly contracted to build star clusters and small galaxies. These dwarf galaxies merged piece-by-piece over billions of years to build the immense spiral and elliptical galaxies we see today.

The Chandra Deep Field-North image was made by observing an area of the sky over half the size of the full moon for 23 days. It is the most sensitive or "deepest" X-ray exposure ever made. By combining the Chandra and Hubble data for this field, astronomers can take a census of the fraction of young galaxies that contain active supermassive black holes back to a time when the universe was only about one billion years old, less than 10 percent of its present age. The data show that these very distant supermassive black holes are rare, more so than some expected. Credit: NASA/CXC/Penn State/D.M. Alexander, F.E. Bauer, W.N. Brandt et al.

The Chandra observations amounted to a "high-energy core sample" of the early universe, allowing us to "study the history of black holes over almost the entire age of the universe," said Niel Brandt of Penn State University, a co- investigator on the Chandra GOODS team. One of the fascinating findings in this deepest X-ray image ever taken is the discovery of mysterious black holes, which have no optical counterparts.

"We found seven mysterious sources that are completely invisible in the optical with Hubble," said Anton Koekemoer of the STScI, a co-investigator on both the HST and Chandra GOODS teams. "Either they are the most distant black holes ever detected, or they are less distant black holes that are the most dust enshrouded known, a surprising result as well."

When comparing the HST and Chandra fields, astronomers also found active black holes in distant, relatively small galaxies were rarer than expected. This may be due to the effects of early generations of massive stars that exploded as supernovae, evacuating galactic gas and thus reducing the supply of gas needed to feed a super massive black hole.

Observatories continue to reach farther back in time to study the evolution of stars and galaxies. This illustration shows that the Chandra X-ray Observatory and the Hubble Space Telescope's Advanced Camera for Surveys looked back billions of years to see the first galaxies. Their combined effort was part of the Great Observatories Origins Deep Survey (GOODS). Hubble's successor, the James Webb Space Telescope (JWST), will gaze even farther back in time to the birth of the first stars. Credit: NASA and Ann Feild (STScI)

These and other results from the GOODS project will be published in a special issue of the Astrophysical Journal Letters, entirely devoted to the team's results. The Chandra results are found in papers led by Koekemoer and Stefano Cristiani of the Trieste Astronomical Observatory. Hubble's findings came from papers led by Giavalisco, Mark Dickinson, and Harry Ferguson of the STScI.

ESA atrasa testes do módulo de exploração de Marte

 da Folha Online

O primeiro contato com o robô Beagle 2, que ruma em direção à Marte a bordo da Mars Express, foi adiado em uma semana. Segundo a versão on-line da revista "New Scientist", a decisão foi tomada após o centro de controle da missão receber uma mensagem inesperada da sonda.

Os engenheiros da ESA (agência espacial européia) enviaram comandos à Mars Express, mas receberam uma resposta que não esperavam --os dados precisos não foram divulgados. Por isso, os testes que seriam realizados com os sistemas do Beagle-2 serão suspensos enquanto a Mars Express não for checada.

Segundo a ESA, ações do tipo são comuns e não causam preocupação. A sonda e o módulo de exploração devem se separar no dia 19 de dezembro, após entrarem na órbita de Marte.

Enquanto o Beagle-2 recolhe dados sobre o solo e a atmosfera de Marte, a Mars Express vai tirar fotos e analisar o planeta de cima durante um ano marciano (687 dias). O custo da missão é estimado em cerca de US$ 200 milhões.

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Frota de naves terrestres vai "invadir" o planeta Marte em 2004

Sonda européia de exploração de Marte é lançada com sucesso

Universe slightly simpler than expected
UNIVERSITY OF FLORIDA NEWS RELEASE
Posted: June 21, 2003

A pair of black holes, center left, devour all light at the center of a giant elliptical galaxy in this artist's depiction. Credit: Gabriel Perez Diaz, MultiMedia Service, Instituto de Astrofísica de Canarias

The universe just became a little less mysterious.

Using images from the Hubble Space Telescope, astronomers at the University of Florida have concluded that two of the most common types of galaxies in the universe are in reality different versions of the same thing. In spite of their similar-sounding names, astronomers had for decades considered "dwarf elliptical" and "giant elliptical" galaxies to be unique. The findings, which appear in this month's edition of The Astronomical Journal, fundamentally alter astronomers' understanding of these important components of the universe, making it easier to understand how galaxies form in the first place.

"This helps to simplify the universe because we replace two distinct galaxy types with one," said Alister Graham, a UF astronomer and lead author of the paper. "But the implications go beyond mere astronomical taxonomy. Astronomers had thought the formation mechanisms for these objects must be different, but instead there is a unifying construction process."

Galaxies, the building blocks of the visible universe, are enormous systems of stars bound together by gravity and scattered throughout space. There are several different types, or shapes. For example, the Milky Way galaxy, in which the Earth resides, is a "spiral" galaxy, so named because its disk-like shape has an embedded spiral arm pattern. Other galaxies are known as "irregular" galaxies because they do not have distinct shapes. But together, dwarf and giant elliptical galaxies are the most common.

For the past two decades, astronomers have considered giant elliptical galaxies, which contain hundreds of billions of stars, and dwarf elliptical galaxies, which typically contain less than one billion stars, as completely separate systems. In many ways it was a natural distinction: Not only do giant elliptical galaxies contain more stars, but the stars also are more closely packed toward the centers of such galaxies. In other words, the overall distribution of stars appeared to be fundamentally different.

Graham, a postdoctoral research associate, and Rafael Guzmán, a UF associate professor of astronomy, decided to take a second look at the accepted wisdom. The pair analyzed images of dwarf elliptical galaxies taken by the Hubble Space Telescope and combined their results with previously collected data on over 200 galaxies. The resulting sample revealed that the structural properties of the galaxies varied continuously between the allegedly different dwarf and giant galaxy classes - in other words, these two types were just relatively extreme versions of the same object.

Sidney van den Bergh, former director and researcher emeritus at the Dominion Astrophysical Observatory at the National Research Council of Canada in Victoria, said Graham and Guzmán's result puts to rest a "very puzzling" question.

"In astronomy, like in physical anthropology, there is a deep connection between the classification of species and their evolutionary connections," van den Bergh said. "The bottom line is that the new work of Graham and Guzmán has made life a little bit simpler for those of us who want to understand how galaxies are formed and have evolved."

Graham and three colleagues expand on his and Guzmán's conclusions with a separate article that appears in the same issue of The Astronomical Journal.

In recent years, Graham said, a number of studies had revealed that the innermost centers of giant elliptical galaxies - the inner 1 percent - had been scoured out or emptied of stars. Astronomers suspect that massive black holes are responsible, gravitationally hurling away any stars that ventured too near and devouring the stars that came in really close. This scouring phenomenon had tended to dim the centers of giant elliptical galaxies, which ran counter to the trend that bigger galaxies tend to have brighter centers. The dimming phenomenon was also one reason astronomers had concluded dwarf and giant galaxies must be different types.

Building on recent revelations showing a strong connection between the mass of the central black holes and the properties of their host galaxies, Graham and his colleagues introduced a new mathematical model that simultaneously describes the distribution of stars in the inner and outer parts of the galaxy.

"It was only after allowing for the modification of the cores by the black holes that we were able to fully unify the dwarf and giant galaxy population," Graham said.

Peter Erwin and Andres Asensio Ramos of the Instituto de Astrofísica de Canarias in Spain, and Ignacio Trujillo of the Max-Planck Institut fur Astronomie in Germany, worked with Graham to support his conclusions. Both research projects were funded in part by NASA and the American Astronomical Society.

Under the Sea
Three astronauts and one NASA scientist currently call the ocean floor home.
by Matt Quandt

The crew of NEEMO 5 pose for a picture before beginning their underwater mission. UNCW For six months in 2002, Peggy Whitson lived aboard the International Space Station, floating 241 miles above Earth's surface. As of June 16, though, she resides 62 feet below sea level. Whitson is currently in command of four "aquanauts" during a two-week stay on the Atlantic Ocean floor. Fellow astronauts Clay Anderson and Garret Reisman join her, as well as NASA space station support scientist Emma Hwang. The foursome are serving as part of a six-person crew living in the Aquarius Underwater Research Facility from June 16 to 29. At first glance, the density of the ocean depths seems to be the last place to simulate activities carried out in the vacuum of space. However, NASA scientists expect to learn much about how the body reacts to prolonged exposure to extreme conditions. For this reason, the mission is entitled NASA Extreme Environment Mission Operations (NEEMO) 5.

Aquarius is an underwater laboratory located in the Florida Keys National Marine Sanctuary. NOAA / UNCW "NEEMO 5 goes beyond the bounds of a space analog experience," said Bill Todd, NEEMO project manager at NASA's Johnson Space Center. "We have ratcheted up the isolation factor, complexity, and science objectives to a level that closely parallels a space mission experience." Some of the crew's activities include a study of how their environment affects sleep and the body's immune system, the growth of bacteria in the habitat, the use of wireless medical monitoring equipment, as well as nutrition-related studies. "The science we are performing may very well help answer several critical path questions on our road map for journeying to Mars and beyond," Todd said.

The International Space Station flies high over Miami, Florida. NASA Other objectives of the mission include developing new ways of interacting with researchers from a remote laboratory location; developing a communications system for use when a spacewalking crew is working at a significant distance from the habitat; and exercising teambuilding, interpersonal, and leadership skills. On June 25, the crew is expected to make a ship-to-ship link-up with its space-faring brother, the International Space Station. Aquarius, owned by the National Oceanic and Atmospheric Administration (NOAA) and operated by the University of North Carolina at Wilmington, is the world's only underwater research laboratory. Located three and a half miles off the coast of Tavernier in the Florida Keys National Marine Sanctuary, the 81-ton structure is 45 feet long and 9 feet in diameter. The size of its living area is similar to the size of the living quarters on the space station. Aquarius is equipped with an array of household amenities including shower and toilet, instant hot water, a microwave, a trash compactor, refrigerator, air conditioning, and a computer that is hooked up to the Internet.

To learn more about Aquarius and NEEMO 5, visit the Aquarius website.

Engineers battle to overcome antenna problem on SOHO
BY STEPHEN CLARK
SPACEFLIGHT NOW
Posted: June 22, 2003

One of the world's premiere Sun-watching observatories has suffered a glitch that threatens to hamper its future studies of our nearest star.

An illustration of the SOHO mission to study the Sun. Credit: NASA/ESA

The joint NASA/ESA Solar and Heliospheric Observatory was launched into space in 1995, and has since collected information and images of the Sun and its sweltering surroundings for almost eight years. But a recent problem with the high-gain antenna's pointing mechanism has ground controllers scratching their heads for a solution.

A spacecraft maneuver on June 18 confirmed an earlier reading the SOHO fault monitoring system had detected -- that the high-gain antenna was not moving as it should. Extensive testing since then has narrowed the problem to the horizontal axis of the dish-shaped antenna's movement.

Because similar attempts to move the antenna failed in two different strings, engineers have determined that the source of the problem is likely the Moog-built mechanical drive motor or gear assembly that physically steers the antenna.

The communications beam provided by the high gain antenna is about 14 degrees wide, but can be expanded to 25 degrees using still-functioning motors to allow for longer periods of contact with SOHO.

An artist's concept of the SOHO orbit. Credit: NASA/ESA

The operations team were able to guide the sluggish antenna toward what officials call the "sweet spot" in space that allows for the least interruption in communications through the high gain antenna, which requires pointing in the vicinity of Earth to make contact.

The current position of the dish antenna would only limit communications during 19 days out of every three months. The first communications cutout is expected to begin late this week.

SOHO orbits around a LaGrange point located about a million miles toward the Sun from the Earth where the gravitational pull from both bodies is equal, forming a stable point in space.

Although this latest development has no direct impact on the health and safety of the spacecraft itself, science operations will be cut by at least some amount because the now-fixed high gain antenna communications swath will not be able to send or receive data or commands.

During these periods of communications blackout through the high gain system, ground controllers will still be able to control the spacecraft and assess its status and health through omni-directional low gain antennas. But SOHO officials believe they will be unable to receive science data during these blackouts.

SOHO's antennae configuration is shown in this illustration. Credit: NASA/ESA

Engineers are unable to ascertain whether the problem can be resolved in any fashion, partly because the exact nature of it is still unknown. Ideas have been floated around to further minimize the scientific loss, including altering the craft's orbit around its L1 LaGrange point if there is enough fuel available.

Now in its eighth year, SOHO has far outlived its original mission baseline of two years. Propellant was originally loaded aboard SOHO to provide for six years of operations, but the launch and orbital injection were accurate enough to leave enough fuel on-board for several decades, ESA says.

Two subsequent mission extensions until 2003, then most recently until March 2007, will keep the spacecraft operational so it can observe the Sun during a complete 11-year solar cycle.

SOHO was manufactured by Matra Marconi Space -- now Astrium -- under contract from the European Space Agency. NASA has been responsible for the launch of the craft aboard an Atlas rocket, and also for mission operations.

It was five years ago this week that the ground team inexplicably lost contact with SOHO for about six weeks before finally regaining communications with the observatory. The craft again suffered another problem in late 1998 when its final gyroscope failed and engineers had to devise a software patch to allow for continued science operations in a gyro-less mode. Other relatively minor periodic science interruptions have also occurred throughout SOHO's mission.

Britânicos mandam pilotos para limite da atmosfera em um balão

da Folha Online

O Reino Unido anunciou hoje a primeira missão espacial exclusivamente britânica. A QinetiQ 1 consiste em um grande balão de hélio que levará dois astronautas até o limite da atmosfera terrestre.

Por quatro horas, os pilotos --Andy Elson e Colin Prescot-- estarão a 40 quilômetros do solo, em uma região que os deixam extremamente vulneráveis à radiação e a temperaturas baixas, entre - 70ºC e - 25ºC.

Segundo os britânicos, a missão servirá de teste para as roupas espaciais --já que a atmosfera nessa altitude é semelhante à marciana-- e para estudar a reação do corpo humano.

O balão tem 387 metros de altura --o tamanho do Empire State Building, em Nova York-- e é composto por cinco toneladas de polietileno. Em uma emergência, os pilotos podem se soltar do balão e voltar para a Terra. A missão deve ser lançada entre julho e setembro deste ano, de acordo com as condições climáticas.

Com Reuters

All Eyes on Eta Carinae
By Joshua Roth

An artist's concept of Eta Carinae as a binary star containing two hot, blue-white supergiants. In this proposed model, winds from the stars collide, forming an X-ray-emitting shock front (thick white arc at center) that wraps around the less powerful of the two stars.  Courtesy André Fonseca Silva.

June 20, 2003 | If astronomers running one of the most complex observing campaigns ever attempted are correct, the wildly variable, far-southern star Eta Carinae is about to fulfill a tantalizing prediction: that it should nearly disappear to X-ray "eyes" after erratically brightening for several months.

Thus far it appears the predictions are correct, and intensive observations with everything from the Hubble Space Telescope to amateur equipment seem poised to write a new chapter in this famous star's history. But it may be some time before the experts agree on what their copious data really mean, says Ted Gull (NASA/Goddard Space Flight Center), who is focusing on ultraviolet spectroscopy with Hubble and the Far Ultraviolet Spectroscopic Explorer.

This image of Eta Carinae, obtained last October by the Hubble Space Telescope's Advanced Camera for Surveys, shows not only the dusty Homunculus nebula, but dust-piercing ultraviolet light from the star itself. Though this image shows fine detail in a sky patch only about 20 arcseconds wide, it lacks the resolution needed to determine whether Eta Carinae is one star or two. Courtesy the HST Treasury Team, STScI, and NASA.

Some 7,500 light-years from Earth, Eta Carinae is one of the most massive and luminous stars known. In the 1840s astronomers watched it undergo what University of Minnesota astrophysicist Kris Davidson, the lead scientist on the Hubble campaign, calls "the biggest explosion any star is known to have survived." Flaring from relative obscurity to become nearly the brightest star in the sky for many months, Eta Carinae shed several solar masses worth of material, which is now seen as the twin-globed "Homunculus" nebula (shown at right).

In the mid-1990s Brazilian astronomer Augusto Damineli (University of São Paolo) discovered that Eta Carinae's spectrum changes with clocklike precision every 5½ years. He predicted that the changes would next recur when 1997 drew to a close. They did; and the star's X-ray brightness simultaneously ramped up — only to crash abruptly.

The increasingly popular explanation is that Eta Carinae is two stars, not one, in an eccentric 5½-year orbit that somehow causes the source of the X-rays either to be eclipsed or to turn off when the two stars are closest together. This scenario makes a clear prediction: the late-1997 behavior should stage a repeat performance now.

The spectral changes documented in 1997 indeed are returning on schedule, says Damineli, and Eta Carinae's high-energy light curve (measured by the Rossi X-Ray Timing Explorer satellite) has obligingly brightened and declined in recent months. However, "there have been some surprises," admits Michael F. Corcoran (Universities Space Research Association), who is leading the X-ray observing campaign. In particular, though Eta Carinae's X-rays appeared to have peaked in late May, the star abruptly flared in mid-June. "It's not really clear what's causing that," Corcoran admits, though he believes that the star is still on track to bottom out this weekend.

Eta Carinae's X-rays presumably originate from million-degree gases in a shock front of some kind. The result of particles blasting into a gaseous medium at supersonic speeds, a shock front might form when two very massive stars approach one another and their fierce winds collide. However, Davidson stresses, a shock also could be generated "even if there is no companion star" — a slow, dense wind from a solitary star's equator could bump up against a faster, more rarefied wind from its poles.

For his part, Damineli is convinced that the clocklike repeatability of Eta Car's spectral shifts can be explained only by orbital motion. "There is no theoretical or observational basis to support a single luminous star," he told Sky & Telescope earlier this week. But the evidence for binarity, however compelling, remains indirect. Stronger signs will be sought in the spectra that Hubble, FUSE, RXTE, the Chandra X-Ray Observatory, the XMM-Newton X-ray satellite, the gamma-ray-sensing Integral spacecraft, and ground-based instruments will obtain in the coming days, weeks, and months. The answers will bear heavily on astronomers' understanding of how extremely massive stars form and evolve.

Want to know more? Read "Eta Carinae's Year of Glory" in the July 2003 issue of Sky & Telescope, now on newsstands.

Fonte: http://skyandtelescope.com/news/article_984_1.asp

 

Céu claro para todos!

José Geraldo Mattos - Moderador

 

"Não se pode ensinar tudo a alguém, pode-se apenas ajudá-lo a encontrar por si mesmo"

(Galileu Galilei )

 

Leia Astronomynews na WEB: http://br.groups.yahoo.com/group/Astronomynews/messages

Thu Jun 19, 8:00 PM ET

By PAUL RECER, AP Science Writer

WASHINGTON - The Earth, moon, sun and all visible stars in the sky make up less than one percent of the universe. Almost all the rest is dark matter and dark energy, unknown forces that puzzle astronomers.

Observations in recent years have changed the basic understanding of how the universe evolved and have emphasized for astronomers how little is known about the major forces and substances that shaped our world.

Astronomers now know that luminous matter — stars, planets and hot gas — account for only about 0.4 percent of the universe. Nonluminous components, such as black holes and intergalactic gas, make up 3.6 percent. The rest is either dark matter, about 23 percent, or dark energy, about 73 percent.

Dark matter, sometimes called "cold dark matter," has been known for some time. Only recently have researchers come to understand the pivotal role it played in the formation of stars, planets and even people.

"We owe our very existence to dark matter," said Paul Steinhardt, a physicist at Princeton University and a co-author of a review on dark matter appearing this week in the journal Science.

Steinhardt said it is believed that following the Big Bang, the theoretical beginning of the universe, dark matter caused particles to clump together. That set up the gravitation processes that led to the formation of stars and galaxies. Those stars, in turn, created the basic chemicals, such as carbon and iron, that were fundamental to the evolution of life.

"Dark matter dominated the formation of structure in the early universe," Steinhardt said. "For the first few billion years dark matter contained most of the mass of the universe. You can think of ordinary matter as a froth on an ocean of dark matter. The dark matter clumps and the ordinary matter falls into it. That led to the formation of the stars and galaxies."

Without dark matter, "there would be virtually no structures in the universe," he said.

The nature of dark matter is unknown. It cannot be seen or detected directly. Astronomers know it is there because of its effect on celestial objects than can be seen and measured.

But the most dominating force of all in the universe is called dark energy, a recently proven power that astronomers say is causing the galaxies in the universe to separate at a faster and faster speed. It is the force that is causing the universe to expand at an accelerating rate.

Robert P. Kirshner, an astronomer at the Harvard-Smithsonian Center for Astrophysics, said the presence of dark energy was proved only five years ago when astronomers studying very distant exploding stars discovered they were moving away at a constant acceleration. It was a stunning discovery that has since been proved by other observations.

Kirshner said it is clear now that dark matter and dark energy engaged in a gravitational tug of war that, eventually, dark energy won.

Following the Big Bang some 14 billion years ago, matter in the universe streaked outward. It formed galaxies, thinned out and then began to slow down.

"Dark matter was trying to slow things down and dark energy was trying to speed it up," said Kirshner, the author of a review article on dark energy in Science.

"We think dark matter was winning for the first seven billion years, but then universe went from slowing down to speeding up. ... Dark energy took over."

Kirshner said astronomers do not really understand dark energy. Albert Einstein first proposed a form of the idea, but discarded it later. Now, researchers know it exists, but its exact form and nature are mysterious, although it is thought to be related to gravity.

"What this is pointing to is a deep mystery at the heart of physics," said Kirshner. "We don't understand gravity in the same way we understand other forces."

He said there are virtually no experiments on Earth that would explore the nature of dark energy. It can only be studied across vast stellar distances by observing the motion of objects extremely far away, a skill that has been possible only in recent decades with the development of very powerful telescopes.

"Dark energy will cause the universe to expanded faster and faster and eventually, over time, we will see less and less of it," Kirshner said. Over millions of years, familiar stars and nearby galaxies will disappear from view and the sky, now choked with stars, will slowly darken.

"The piece of the universe that we can see will get lonelier and lonelier," he said.