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| Piú votate - The Universe in Super Definition |
Abell-901_and_902-PM.jpgAbell 901 and 902 Supercluster56 visiteAstronomers are using NASA's Hubble Space Telescope to dissect one of the largest structures in the universe as part of a quest to understand the violent lives of galaxies. Hubble is providing indirect evidence of unseen dark matter tugging on galaxies in the crowded, rough-and-tumble environment of a massive supercluster of hundreds of galaxies.
Dark matter is an invisible form of matter that accounts for most of the universe's mass. Hubble's Advanced Camera for Surveys has mapped the invisible dark matter scaffolding of the supercluster Abell 901/902, as well as the detailed structure of individual galaxies embedded in it.
The images are part of the Space Telescope Abell 901/902 Galaxy Evolution Survey (STAGES), which covers one of the largest patches of sky ever observed by the Hubble telescope. The area surveyed is so wide that it took 80 Hubble images to cover the entire STAGES field. The new work is led by Meghan Gray of the University of Nottingham in the United Kingdom and Catherine Heymans of the University of British Columbia in Vancouver, along with an international team of scientists.
The Hubble study pinpointed four main areas in the supercluster where dark matter has pooled into dense clumps, totaling 100 trillion times the Sun's mass. These areas match the location of hundreds of old galaxies that have experienced a violent history in their passage from the outskirts of the supercluster into these dense regions. These galaxies make up four separate galaxy clusters.
"Thanks to Hubble's Advanced Camera for Surveys, we are detecting for the first time the irregular clumps of dark matter in this supercluster," Heymans said. "We can even see an extension of the dark matter toward a very hot group of galaxies that are emitting X-rays as they fall into the densest cluster core."
The dark matter map was constructed by measuring the distorted shapes of over 60,000 faraway galaxies. To reach Earth, the galaxies' light traveled through the dark matter that surrounds the supercluster galaxies and was bent by the massive gravitational field. Heymans used the observed, subtle distortion of the galaxies' shapes to reconstruct the dark matter distribution in the supercluster using a method called weak gravitational lensing. The dark matter map is 2.5 times sharper than a previous ground-based survey of the supercluster.
"The new map of the underlying dark matter in the supercluster is one key piece of this puzzle," Gray explained. "At the same time we're looking in detail at the galaxies themselves." The survey's broader goal is to understand how galaxies are influenced by the environment in which they live.
On Earth, the pace of quiet country life is vastly different from the hustle of the big city. In the same way, galaxies living lonely isolated lives look very different from those found in the most crowded regions of the universe, like a supercluster. "We've known for a long time that galaxies in crowded environments tend to be older, redder, and rounder than those in the field," Gray said. "Galaxies are continually drawn into larger and larger groups and clusters by the inevitable force of gravity as the universe evolves."
In such busy environments galaxies are subject to a life of violence: high-speed collisions with other galaxies; the stripping away of gas, the fuel supply they use to form new stars; and distortion due to the strong gravitational pull of the underlying invisible dark matter. "Any or all of these effects may play a role in the transformation of galaxies, which is what we're trying to determine," Gray said.
The STAGES survey's simultaneous focus on both the big picture and the details can be likened to studying a big city. "It's as if we're trying to learn everything we can about New York City and New Yorkers," Gray explained. "We're examining large-scale features, like mapping the roads, counting skyscrapers, monitoring traffic. At the same time we're also studying the residents to figure out how the lifestyles of people living downtown differ from those out in the suburbs. But in our case the city is a supercluster, the roads are dark matter, and the people are galaxies."
Further results by other team members support this view. "In the STAGES supercluster we clearly see that transformations are happening in the outskirts of the supercluster, where galaxies are still moving relatively slowly and first feel the influence of the cluster environment," said Christian Wolf, an Advanced Research Fellow at the University of Oxford in the U.K.
Assistant professor Shardha Jogee and graduate student Amanda Heiderman, both of the University of Texas in Austin, concur. "We see more collisions between galaxies in the regions toward which the galaxies are flowing than in the centers of the clusters," Jogee said. "By the time they reach the center, they are moving too fast to collide and merge, but in the outskirts their pace is more leisurely, and they still have time to interact."
The STAGES team also finds that the outer parts of the clusters are where star formation in the galaxies is slowly switching off and where the supermassive black holes at the hearts of the galaxies are most active.
Added Heiderman: "The galaxies at the centers of the clusters may have been there for a long time and have probably finished their transformation. They are now old, round, red, and dead."
The team plans more studies to understand how the supercluster environment is responsible for producing these changes.
Abell 901/902 resides 2.6 billion light-years from Earth and measures more than 16 million light-years across.
MareKromium
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SN-Cassiopeia_A_-PIA10207.jpgCassiopeia "A": Dissecting the Wake of a Supernova Explosion53 visiteThe elements and molecules that flew out of the Cassiopeia A star when it exploded about 300 years ago can be seen clearly for the first time in this plot of data, called a Spectrum, taken by NASA's SST.
The Spectrum, which was created by splitting light into its basic components, reveals the composition of gas and dust that were synthesized in the explosion. It also provides some of the best evidence yet that stellar explosions, called Supernovae, were a significant source of fresh dust in the very young universe.
Prior to these observations, nobody was certain where this early dust — the same dust that ultimately made its way into future stars, planets and people — came from.
One of the most interesting features of the plot is a bump labeled Cassiopeia A Dust Feature. This bump is actually the signature of a collection of dust composed of proto-silicates, Silicon Dioxide and Iron Oxide. The Spectrum reveals that the brightness of the dust feature is correlated to that of Argon gas (yellow vertical line at left), known to have been expelled and synthesized during the star's explosion. The fact that the dust is associated with the expelled gas, or ejecta, tells astronomers that this Supernova manufactured new dust.
Each of the 3 lines of this plot represents a different layer of the Supernova remnant, with the top yellow and red line being the outermost layer.
Similar correlations between gas and dust are also seen in the middle layer (green line). For example, neon gas correlates with dust composed of Carbon and Aluminum Oxide.MareKromium
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HD-113766-PIA09931.jpgHD 11376653 visiteThis artist's conception shows a binary-star, or two-star, system, called HD 113766, where astronomers suspect a rocky Earth-like planet is forming around one of the stars. At approximately 10 to 16 MY old, astronomers suspect this star is at just the right age for forming rocky planets. The System is located approx. 424 LY away from Earth.
The two yellow spots in the image represent the System's two stars. The brown ring of material circling closest to the central star depicts a huge belt of dusty material, more than 100 times as much as in our asteroid belt, or enough to build a Mars-size planet or larger. The rocky material in the belt represents the early stages of planet formation, when dust grains clump together to form rocks, and rocks collide to form even more massive rocky bodies called planetesimals. The belt is located in the middle of the system's terrestrial Habitable Zone, or the region around a star where liquid water could exist on any rocky planets that might form.
Earth is located in the middle of our sun's terrestrial habitable zone.
Using NASA's Spitzer Space Telescope, astronomers learned that the belt material in HD 113766 is more processed than the snowball-like stuff that makes up infant solar systems and comets, which contain pristine ingredients from the early solar system. However, it is not as processed as the stuff found in mature planets and asteroids. This means that the dust belt is made out of just the right mix of materials to be forming an Earth-like planet. It is composed mainly of rocky silicates and metal sulfides (like fool's gold), similar to the material found in lava flows.
The white outer ring shows a concentration of icy dust also detected in the system. This material is at the equivalent position of the asteroid belt in our solar system, but only contains about one-sixth as much material as the inner ring. Astronomers say it is not clear from the Spitzer observations if anything is occurring in the icy belt, but they believe it could be a source of water later on for the planet that grows from the inner warm ring.
MareKromium
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NGC-1333-PIA09966.jpgWater Vapor inside NGC 133354 visiteThis diagram illustrates the earliest journeys of water in a young, forming Star System. Stars are born out of icy cocoons of gas and dust. As the cocoon collapses under its own weight in an inside-out fashion, a stellar embryo forms at the center surrounded by a dense, dusty disk.
The stellar embryo "feeds" from the disk for a few million years, while material in the disk begins to clump together to form planets.
NASA's Spitzer Space Telescope was able to probe a crucial phase of this stellar evolution - a time when the cocoon is vigorously falling onto the pre-planetary disk. The infrared telescope detected water vapor as it smacks down on a disk circling a forming star called NGC 1333-IRAS 4B.
This vapor started out as ice in the outer envelope, but vaporized upon its arrival at the disk.
By analyzing the water in the system, astronomers were also able learn about other characteristics of the disk, such as its size, density and temperature.
How did Spitzer see the water vapor deep in the NGC 1333-IRAS 4B system?
This is most likely because the system is oriented in just the right way, such that its thicker disk is seen face-on from our Earthly perspective. In this "face-on" orientation, Spitzer can peer through a window carved by an outflow of material from the embryonic star. This system in this drawing is shown in the opposite "edge-on" configuration.MareKromium
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NGC-1333-PIA09965.jpgWater Vapor inside NGC 133358 visiteThis plot of infrared data, called a Spectrum, shows the strong signature of water vapor deep within the core of an Embryonic Star System called NGC 1333-IRAS 4B.
The data were captured by NASA's SST using an instrument called Spectrograph.
A spectrograph collects light and sorts it according to color, or wavelength. In this case, infrared light from NGC 1333-IRAS 4B was broken up into the wavelengths listed on the horizontal axis of the plot. The sharp spikes, called spectral lines, occur at wavelengths at which the stellar object is particularly bright. The signature of water vapor is revealed in the pattern of wavelengths at which the spikes appear.
By comparing the observed data to a model (lower curve), astronomers can also determine the physical and chemical details of the region.
F.e.: Astronomers say these data suggest that ice in a cocoon surrounding the forming star is falling inward. The ice then smacks supersonically into a dusty planet-forming disk surrounding the stellar embryo, heats up and vaporizes quickly, releasing the infrared light that Spitzer collected.
MareKromium
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Mira-PIA09958-1.jpgMira: anatomy of a "Celestial Shooting Star"...55 visiteA close-up view of a star racing through space faster than a speeding bullet can be seen in this image from NASA's Galaxy Evolution Explorer. The star, called Mira (pronounced My-rah), is traveling at 130 kilometers per second, or 291,000 miles per hour. As it hurls along, it sheds material that will be recycled into new stars, planets and possibly even life.
In this image, Mira is moving from left to right. It is visible as the pinkish dot in the bulb shape at right. The yellow dot below is a foreground star. Mira is traveling so fast that it's creating a bow shock, or build-up of gas, in front of it, as can be seen here at right.
Like a boat traveling through water, a bow shock forms ahead of the star in the direction of its motion. Gas in the bow shock is heated and then mixes with the cool hydrogen gas in the wind that is blowing off Mira. This heated hydrogen gas then flows around behind the star, forming a wake.
Why is the wake of material glowing? When the hydrogen gas is heated, it transitions into a higher-energy state, which then loses energy by emitting ultraviolet light - a process called fluorescence. The Galaxy Evolution Explorer has special instruments that can detect this ultraviolet light.
A similar fluorescence process is responsible for the Northern Lights -- a glowing, green aurora that can be seen from northern latitudes. However, in that case nitrogen and oxygen gas are fluorescing with visible light.
Streams and a loop of material can also be seen coming off Mira. Astronomers are still investigating what these streams are, but they suspect that they are denser parts of Mira's wind perhaps flowing out of the star's poles.
This image consists of data captured by both the far- and near-ultraviolet detectors on the Galaxy Evolution Explorer between November 18 and December 15, 2006. It has a total exposure time of about 3 hours.
MareKromium
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PIA09955_fig1.jpgFearsome Foursome (Figure 1)54 visiteOne of the biggest galaxy collisions ever observed is taking place at the center of this image. The four yellow blobs in the middle are large galaxies that have begun to tangle and ultimately merge into a single gargantuan galaxy. The yellowish cloud around the colliding galaxies contains billions of stars tossed out during the messy encounter. Other galaxies and stars appear in yellow and orange hues.
NASA's Spitzer Space Telescope spotted the four-way collision, or merger, in a giant cluster of galaxies, called CL0958+4702, located nearly 5 BLY away.
The dots in the picture are a combination of galaxies in the cluster; background galaxies located behind the cluster; and foreground stars in our own Milky Way galaxy.
Infrared data from Spitzer are colored red in this picture, while visible-light data from a telescope known as WIYN are green. Areas where green and red overlap appear orange or yellow.
Since most galaxies in the cluster contain old stars that are visible to Spitzer and WIYN, those galaxies appear orange.
MareKromium
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NGC-2237_and_NGC-2244.jpgHot Stars in the Rosette Nebula53 visiteWinds and radiation from massive hot stars in the Rosette Nebula have cleared the natal gas and dust from the center of the nearby star-forming region. They also pose a danger to planet forming disks around young, cooler stars in the neighborhood. This Spitzer Space Telescope (SST) infrared image of dust clouds near the Rosette's central region, shows the cleared-out cavity.
The view spans about 45 LY at the the nebula's estimated distance of 5.200 LY. MareKromium
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HD-15115.jpgLopsided debris disk around the young star HD 1511554 visiteThe disk, seen edge-on, is the dense blue line extending from the star to the upper right and lower left of the image. As seen from Earth, the edge-on disk resembles a needle sticking out from the star. The disk appears thicker and longer at upper right than at lower left, evidence of the disk's lopsided structure.
Astronomers think the disk's odd imbalanced look is caused by dust particles following a highly elliptical orbit around the star, which is slightly brighter than the Sun. The lopsidedness may have been caused by planets sweeping up debris in the disk or by the gravity of a nearby star.
Astronomers used an occulting mask on Hubble's Advanced Camera for Surveys to block out the bright starlight so they could see the dim disk. The occulting masks can be seen in the image as the dark circle in the center and the dark bar on the left. The star is behind the central mask.
HD 15115 is among nearly 30 stars that belong to the Beta Pictoris Moving Group. Moving groups are expanded clusters of stars believed to have a common birthplace and age, in this case about 12 million years, that are traveling together loosely through space. HD 15115 is 150 light-years from Earth.
Dusty disks are known to exist around at least 100 stars, but because of the difficulty in observing material close to the brightness of a star, less than a dozen have been studied closely.
Astronomers described the disk as one of the most peculiar debris disks that Hubble has ever imaged. They in fact made follow-up observations with the W.M. Keck Observatory in Hawaii to confirm the disk's presence.
Hubble's Advanced Camera for Surveys snapped the image on July 17, 2006.
MareKromium
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NGC-0362-PIA09653.jpgNGC 362 - Globular Star Cluster53 visiteThe Galaxy Evolution Explorer's ultraviolet eyes have captured a globular star cluster, called NGC 362, in our own Milky Way galaxy. In this new image, the cluster appears next to stars from a more distant neighboring galaxy, known as the Small Magellanic Cloud.
Globular clusters are densely packed bunches of old stars scattered in galaxies throughout the universe. NGC 362, located 30,000 light-years away, can be spotted as the dense collection of mostly yellow-tinted stars surrounding a large white-yellow spot toward the top-right of this image. The white spot is actually the core of the cluster, which is made up of stars so closely packed together that the Galaxy Evolution Explorer cannot see them individually.
The light blue dots surrounding the cluster core are called extreme horizontal branch stars. These stars used to be very similar to our sun and are nearing the end of their lives. They are very hot, with temperatures reaching up to about four times that of the surface of our sun (25,000 Kelvin or 45,500 degrees Fahrenheit).
A star like our sun spends most of its life fusing hydrogen atoms in its core into helium. When the star runs out of hydrogen in its core, its outer envelope will expand. The star then becomes a red giant, which burns hydrogen in a shell surrounding its inner core. Throughout its life as a red giant, the star loses a lot of mass, then begins to burn helium at its core. Some stars will have lost so much mass at the end of this process, up to 85 percent of their envelopes, that most of the envelope is gone. What is left is a very hot ultraviolet-bright core, or extreme horizontal branch star.
Blue dots scattered throughout the image are hot, young stars in the Small Magellanic Cloud, a satellite galaxy of the Milky Way located approximately 200,000 light-years away. The stars in this galaxy are much brighter intrinsically than extreme horizontal branch stars, but they appear just as bright because they are farther away. The blue stars in the Small Magellanic Cloud are only about a few tens of millions of years old, much younger than the approximately 10-million-year-old stars in NGC 362.
Because NGC 362 sits on the northern edge of the Small Magellanic Cloud galaxy, the blue stars are denser toward the south, or bottom, of the image.
Some of the yellow spots in this image are stars in the Milky Way galaxy that are along this line of sight. Astronomers believe that some of the other spots, particularly those closer to NGC 362, might actually be a relatively ultraviolet-dim family of stars called "blue stragglers." These stars are formed from collisions or close encounters between two closely orbiting stars in a globular cluster.
This image is a false-color composite, where light detected by the Galaxy Evolution Explorer's far-ultraviolet detector is colored blue, and light from the telescope's near-ultraviolet detector is red.
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BHR71-PIA09338.jpgProtostellar Jet in BHR 71 Dark Cloud54 visiteTwo rambunctious young stars are destroying their natal dust cloud with powerful jets of radiation, in an infrared image from NASA's Spitzer Space Telescope.
The stars are located approximately 600 light-years away in a cosmic cloud called BHR 71. In visible light (left panel), BHR 71 is just a large black structure. The burst of yellow light toward the bottom of the cloud is the only indication that stars might be forming inside. In infrared light (center panel), the baby stars are shown as the bright yellow smudges toward the center. Both of these yellow spots have wisps of green shooting out of them. The green wisps reveal the beginning of a jet. Like a rainbow, the jet begins as green, then transitions to orange, and red toward the end. The combined visible-light and infrared composite (right panel) shows that a young star's powerful jet is responsible for the rupture at the bottom of the dense cloud in the visible-light image. Astronomers know this because burst of light in the visible-light image overlaps exactly with a jet spouting-out of the left star, in the infrared image.
The jets' changing colors reveal a cooling effect, and may suggest that the young stars are spouting out radiation in regular bursts. The green tints at the beginning of the jet reveal really hot hydrogen gas, the orange shows warm gas, and the reddish wisps at the end represent the coolest gas. The fact that gas toward the beginning of the jet is hotter than gas near the middle suggests that the stars must give off regular bursts of energy -- and the material closest to the star is being heated by shockwaves from a recent stellar outburst. Meanwhile, the tints of orange reveal gas that is currently being heated by shockwaves from a previous stellar outburst. By the time these shockwaves reach the end of the jet, they have slowed down so significantly that the gas is only heated a little, and looks red. The combination of views also brings out some striking details that evaded visible-light detection. For example, the yellow dots scattered throughout the image are actually young stars forming inside BHR 71. Spitzer also uncovered another young star with jets, located to the right of the powerful jet seen in the visible-light image. Spitzer can see details that visible-light telescopes don't, because its infrared instruments are sensitive to "heat."
The infrared image is made up of data from Spitzer's infrared array camera. Blue shows infrared light at 3.6 microns, green is light at 4.5 microns, and red is light at 8.0 microns.
MareKromium
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NGC-2808-0.jpgNGC 2808 - Globular Star Cluster53 visiteAstronomers have long thought that globular star clusters had a single "baby boom" of stars early in their lives and then settled into a quiet existence.
New observations by NASA's Hubble Space Telescope, however, are showing that this idea may be too simple. The Hubble analysis of the massive globular cluster NGC 2808 provides evidence that star birth went "boom, boom, boom," with three generations of stars forming very early in the cluster's life.
"We had never imagined that anything like this could happen," said Giampaolo Piotto of the University of Padova in Italy and leader of the team that made the discovery. "This is a complete shock."
Globular clusters are the homesteaders of our Milky Way Galaxy, born during our galaxy's formation. They are compact swarms of typically hundreds of thousands of stars held together by gravity.
"The standard picture of a globular cluster is that all of its stars formed at the same time, in the same place, and from the same material, and they have co-evolved for billions of years," said team member Luigi Bedin of the European Space Agency, the European Organization for Astronomical Research in the Southern Hemisphere (ESO), in Garching, Germany, and the Space Telescope Science Institute in Baltimore, Md. "This is the cornerstone on which much of the study of stellar populations has been built. So we were very surprised to find several distinct populations of stars in NGC 2808. All of the stars were born within 200 million years very early in the life of the 12.5-billion-year-old massive cluster."
Finding multiple stellar populations in a globular cluster so close to home has deep cosmological implications, the researchers said.
"We need to do our best to solve the enigma of these multiple generations of stars found in these Hubble observations so that we can understand how stars formed in distant galaxies in our early universe," Piotto explained.
The astronomers used Hubble's Advanced Camera for Surveys to measure the brightness and color of the cluster stars. Hubble's exquisite resolution allowed the astronomers to sort out the different stellar populations. The Hubble measurements showed three distinct populations, with each successive generation appearing slightly bluer. This color difference suggests that successive generations contain a slightly different mix of some chemical elements.
"One assumption, although we have no direct proof," said team member Ivan King of the University of Washington in Seattle, "is that the successively bluer color of the stellar populations indicates that the amount of helium increases with each generation of stars. Perhaps massive star clusters like NGC 2808 hold onto enough gas to ignite a rapid succession of stars."
The star birth would be driven by shock waves from supernovae and stellar winds from giant stars, which compress the gas and make new stars, King explained. The gas would be increasingly enriched in helium from previous generations of stars more massive than the Sun.
Astronomers commonly assume that globular clusters produce only one stellar generation, because the energy radiating from the first batch of stars would clear out most of the residual gas needed to make more stars. But a hefty cluster like NGC 2808, which is two to three times more massive than a typical globular cluster, may have enough gravity to hang onto that gas, which has been enriched by helium from the first stars. Of the about 150 known globular clusters in our Milky Way Galaxy, NGC 2808 is one of the most massive, containing more than 1 million stars.
Another possible explanation for the multiple stellar populations is that NGC 2808 may only be masquerading as a globular cluster. The stellar grouping may have been a dwarf galaxy that was stripped of most of its material due to gravitational capture by our galaxy.
Omega Centauri, the only other stellar system Piotto's group found to have multiple generations of stars, is suspected to be the remnant core of a dwarf galaxy, Bedin said.
Although the astronomers' search is only in its infancy, they say multiple stellar populations may be a typical occurrence in other massive clusters.
"No one would make the radical step of suggesting that previous work on other clusters is no longer valid," King said. "But this discovery shows that the study of stellar populations in globular clusters now opens up in a new direction."
The team plans to use ESO's Very Large Telescope in Chile to make spectroscopic observations of the chemical abundances in NGC 2808, which may offer further evidence that the stars were born at different times and yield clues to how they formed. They also will use Hubble to hunt for multiple generations of stars in about 10 more hefty globular clusters.
The team's results have been accepted for publication in the Astrophysical Journal Letters.
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