Posted on 05/29/2003 8:32:47 PM PDT by NormsRevenge
NASHVILLE -- Astronomers have peered through a secretive cloak of dust to find five exploded stars in a region of space believed to be among the most energetic in the universe.
They're calling it a supernova factory, the first of its kind ever witnessed.
The supernovae, forged at the intersection of two merging galaxies, help confirm a long-held theoretical expectation that galaxy collisions generate episodes of intense star birth and rapid death. Astronomers call these merging objects starbirth galaxies, and they are thought to be a primary driving force of cosmic evolution.
The work represents an initial step toward a better understanding of the early universe, when galaxy mergers were common, the birth of stars occurred frenetically in cramped spaces and explosive deaths were the norm, researchers said.
The results were presented here today at the 202nd meeting of the American Astronomical Society.
Just exploded
One of the newfound supernovae only recently exploded and was only spotted weeks ago. It did not show up in observations made last year.
"This supernova is likely to be part of a group of super star clusters that produce an average of one such stellar explosion every two years," said James Ulvestad of the National Radio Astronomy Observatory.
Normal galaxies like our Milky Way typically produce supernovae no more often than once a century.
Ulvestad and his colleagues used a giant array of radio telescopes, which peer through thick dust that surrounds the scene and renders traditional telescopes ineffective. He likened the effort to trying to detect sunlight through a brick. The technique is expected to lead to several similar findings in other suspected starburst galaxies.
The five supernovae are all within 350 light-years of one another in the merging galaxy, called Arp 229. Two of them are just 7 light-years apart.
"Arp 229 is an excellent time machine for looking at the early universe," said Susan Neff of NASA (news - web sites)'s Goddard Space Flight Center. She said there are probably more supernova in the region that have yet to be detected.
About supernovae
Supernova events are the incendiary deaths of huge stars several times as massive as the Sun. They use up their nuclear fuel quickly, in just a few million years, and gravity pulls remaining material rapidly inward.
Upon collapse, a dying star's core rebounds to generate a shock wave that blasts its outer layers into space. For a moment, it can shine as bright as a 100 billion suns.
The first stars in the universe were very massive and made almost entirely of hydrogen with some helium. They were the first chemical factories, forging new elements -- heavier with each generation -- that were cast into space when the stars exploded. Subsequent generations of stars formed from this detritus.
More modern stars, like are Sun, are the beneficiaries of stellar evolution, containing an abundance of heavier elements.
The very ingredients for life were created in supernova, astronomers like to say.
Confirming expectations
Supernova discoveries are common nowadays. But most of those spotted are pretty boring, Ulvestad said, because they occur in relative isolation. In Arp 299, things are so crowded that the explosion is confined. So the supernova are smaller than most and pack more power into their confined punch.
More refined observations will be needed to figure out what that means.
Meanwhile, the new findings are the first to involve a supernova factory that resembles those suspected in the earliest epochs of the cosmos, said Cornell University's Daniel Weedman, who was not involved in the research.
"It confirms the concept of starburst," Weedman said, adding that the majority of all the universe's energy comes from starburst galaxies. Other researchers are eager to learn how starburst activity relates to black hole mergers and overall galaxy formation.
Weedman cautioned that modern galaxies like Arp 299, and the stars that develop in them, may experience different physical, chemical and environmental conditions compared to similar structures in the early universe. So it is not clear, he said, how useful the apparent window to the past will prove to be. More observations and theoretical advances will be needed. But he said the window represents a very important first step.
The discoveries were made with two National Science Foundation (news - web sites) observatories, the Robert C. Byrd Green Bank Telescope and the Very Long Baseline Array, which combines the efforts of 10 antennas spread from Hawaii to the Virgin Islands to gain greater sensitivity.
The colliding galaxies of Arp 299 are about 140 million light-years from our planet, so the activity recently observed from Earth actually took place 140 million years ago, its light having just arrived here.
Previously, optical observations had detected four other supernovae in Arp 299 over the past 13 years, but each sat around the outskirts of the obscuring dust clouds, so astronomers had not known what really went on at the heart of the action.
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ALBUQUERQUE, N.M. - The center of our Milky Way Galaxy is inching toward an era of intense fireworks when stars will be born 100 times more frequently than today and many will die quick, explosive deaths, according to new research.
A huge and dense ring of interstellar gas is collecting near the galactic center and approaching a density that will, in about 200 million years, generate a burst of star formation that could transform the very appearance of our galaxy as seen from afar.
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SCIENCE TUESDAY | |
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"They'll quickly use up their fuel and explode as supernovae," said Antony Stark of the Harvard-Smithsonian Center for Astrophysics. "Right now, we see one supernova in our galaxy about every 100 years. When the starburst happens, we'll see one supernova every year."
The explosions will be too far away for most of them to be visible to the unaided eye from Earth, Stark told SPACE.com. "It would, however, be a whopping big signal to current scientific instrumentation."
Supernovae are among the most energetic events in the universe and can momentarily outshine an entire galaxy.
Critical density
Stark and his colleague, Chris Martin, used a telescope in Antarctica to map so-called submillimeter wavelength radiation -- between radio waves and infrared on the electromagnetic spectrum -- coming from carbon monoxide in a portion of the ring of gas. While others have made similar measurements, the new data involves very high frequencies that allowed for a better estimate of gas density in a region of space 400 light-years square and roughly 25,000 light-years away.
Martin presented the work here last week at the 200th meeting of the American Astronomical Society.
"We can combine our map with the recent X-ray map of the galactic center taken by NASA's Chandra X-ray Observatory, as well as with observations at other wavelengths of light," Martin said. "Together, the data allow us to build a complete picture of the environment near the center of our galaxy."
The gas ring contain the diffuse mass of millions of Suns. It is near a critical density, the new study shows, beyond which it will form one or two clouds that will collapse into the center of the galaxy, triggering the star formation. It won't be the first time; every 500 million years or so, enough material makes its way toward the galactic center -- drawn there by the tremendous gravity of a large concentration of stars and a central black hole -- to create a similar scenario.
Other researchers have observed similar starburst activity in other galaxies. Notably, a galaxy called M82 is a tremendous star factory, observations show.
"A large starburst could transform the Milky Way from its present state to a galaxy that looks more like M82," Stark said.
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Dramatic Increase in Supernova Explosions Looms (cont.) |
In addition to the expected stellar baby boom, the rush of gas toward the galactic center will feed the relatively dormant supermassive black hole that anchors the Milky Way.
This gravity well, which contains a mass equal to several million Suns, does not actively spew X-rays to the extent that some black holes in other large galaxies do as they feed. When material spirals into a black hole, it accelerates to nearly the speed of light, becomes superheated, and some of it is converted into X-rays.
Stark said current models do not allow for a precise prediction of how much weight the black hole might gain.
"I would guess that only a fraction will get into the black hole," he said. Similar periods of starburst have occurred about 20 times so far in the life of the Milky Way, Stark said. "My best guess is that the black hole grows 5 percent with each episode."
Some matter that was converted to radiation on the way in will be spat out in two intense jets that travel in opposite directions along the axis of the galaxy's rotation.
"We're lucky that the energy from these jets is directed out away from the plane of the Milky Way," Stark said. "If it weren't, the Earth might be periodically sterilized of all life."
Seeding life
While the work by Stark and Martin predicts supernovae to come, separate research has allowed new insight into what happens to them after they explode.
Supernovae are the endpoints for stars that are more than 10 times as massive as our Sun. After using up their primary thermonuclear fuel in just 15 million years or less, the stars implode in about one second. Then they rebound, stall momentarilly, and explode.
Evidence has shown that the ensuing violent outburst sweeps up dust and debris like an interstellar snowplow. A sphere of gas and dust can reach more than 100 light-years into space.
After about 100,000 years, however, the expansion slows and the supernova remnants merge with their surroundings. This final stage is not well understood, in part because it is hard to observe it in the galaxy within which we sit.
So Rosa Murphy Williams, a postdoctoral researcher at the University of Massachusetts and faculty member Q. Daniel Wang led a study using data from three space-based telescopes to examine supernova remnants in galaxies outside but close to the Milky Way.
"What seems to happen is that as the supernovae sweep up more material, that affects their aging process," Williams said.
Williams described the work as forensic science on dead stars in which the researchers look for "clues as to what happens when the body returns to the soil." She said the new study is just beginning to reveal how the elements from a supernovae explosion get mixed into space.
Importantly, supernovae are the primary sources for many of the heavy elements in the universe. They are the only source for some elements, such as iron and calcium, upon which human life depends. In that sense, the looming starburst era in the Milky Way will seed the formation of future stars that might develop planets on which human cousins could one day emerge.
Humans might even get a close-up view of the fireworks, Stark suggests.
"We're talking about 200 million years in the future," he noted, "so it's possible some of our descendants would be at the galactic center when this happens."
Great name for a band.
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This is somewhat misleading; the fact that the 10 antennae are spread out from the Virgin Islands to Hawaii contributes nothing to the sensitivity of the array; the same 10 antennae parked next to each other would have the same sensitivity. What spreading them out does (which is why it is called the Very Long Baseline Array) is it increases the effective resolution of the array, far greater than can be obtained by a single dish of equivalent apeture.
This means the VLBA can resolve more detail than a single dish of equivalent apeture, but cannnot detect signals that are any weaker than what the equivalent single dish would detect.
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