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SatAM17
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 Posted: Thu Jun 28, 2007 4:42 pm Post subject: Astronomers Astonished by 'Monstrous' Star Explosion |
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Astronomers Astonished by 'Monstrous' Star Explosion
| Quote: | Scientists have detected a stellar explosion that is the brightest and most energetic ever recorded, and which could be the first evidence of a new type of supernova fueled by an antimatter engine.
The "SN 2006gy" explosion occurred in a galaxy 240 million light-years away, called NGC 1260, and was 100 times more energetic than typical supernovas. It was detected in September 2006 using ground-based telescopes and NASA's Chandra X-ray space observatory. It brightened slowly for 70 days, and at its peak emitted more than 50 billion Suns worth of light-shining 10 times brighter than its host galaxy-before dimming slowly. Most supernovas reach peak brightness in days to a few weeks.
"Of all exploding stars ever observed, this was the king," said Alex Filippenko of the University of California, Berkeley, who led ground-based observations of the supernova at Lick Observatory in California and Keck Observatory in Hawaii. "We were astonished to see how bright it got and how long it lasted."
NASA has released an image and animation of what the explosion might have looked like.
The finding, presented today at a NASA press conference and detailed in an upcoming issue of The Astrophysical Journal, provides evidence for a fundamentally different type of supernova explosion that only occurs with the universe's most massive stars.
The monster supernova suggests the first stars that illuminated the universe died in explosive lightshows. "We may have witnessed a modern-day version of how the first generation of the most massive stars ended their lives," Filippenko said.
Astrophysicists also think the supernova could be a preview of what they will see when a massive star in our own galaxy explodes.
Going out with a bang
Supernovas are stellar swan songs. They occur when ancient, massive stars do as poet Dylan Thomas advised, that is, to "burn and rage at the close of day," and "rage, rage against the dying of the light."
Most supernovas are the result of stars with 8 to 20 times the mass of our Sun collapsing under their own gravity. Astronomers think something different happened with SN 2006gy, whose star was much bigger--about 150 solar masses.
Stars this massive are extremely rare: Scientists estimate there are only a dozen or so such stars in the Milky Way's stellar population of 400 billion.
Supermassive stars are thought to produce so much gamma-ray light at the end of their lives that some of the radiation is converted into matter and antimatter, mostly electrons and positrons. Antimatter particles have the same mass as ordinary matter but opposite atomic properties such as spin and charge. Gamma radiation is the energy that prevents the outer layers of a star from collapsing; once it starts disappearing, the star's outer layer falls inward, triggering a thermonuclear explosion that destroys the star.
The new findings suggest some of the first stars in the early universe, which were also very massive, went out in spectacular explosions like SN 2006gy, instead of bypassing the supernova stage and collapsing directly into black holes.
"In terms of the effect on the early universe, there's a huge difference between these two possibilities," said study leader Nathan Smith, also of UC Berkeley. "One pollutes the galaxy with large quantities of newly made elements, and the other locks them up forever in a black hole."
Eta Carinae
Scientists think SN 2006gy could be a sign of things to come in our own galaxy. Eta Carinae, the most luminous star in our Milky Way, is located some 7,000 light-years away and seems poised to undergo its own explosion at any moment.
"This could happen tomorrow or it could happen 1,000 years from now," said Mario Livio of the Space Telescope Science Institute in Baltimore, who was not involved in the research.
Eta Carinae is an unstable star currently radiating about 5 million times more energy than our Sun and is undergoing eruptions on its surface that are similar to what scientists think happened on the star that produced SN 2006gy just before it blew.
Despite its relatively close proximity to us, Eta Carinae's death is not likely to pose any significant threat to life on Earth, scientists say.
"I think we can sleep quietly tonight for Eta Car not extinguishing life on Earth," Livio said, "but [SN 2006gy] and all the questions it brings about will keep us awake for quite a while." |
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SatAM17
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| Posted: Thu Jun 28, 2007 6:05 pm Post subject: Chandra Takes X-ray Image |
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Chandra Takes X-ray Image
of Repeat Offender
Image of Eta Carina reveals
shocking details of mysterious star
| Quote: | October 8, 1999: I'll huff and I'll puff and I'll blow my house down.
This is no idle threat in the case of the nebula called Eta Carina. Just three years ago the Hubble Space Telescope provided a dazzling image of a star that was blowing off massive quantities of material in a blast that looked like a supernova yet, mysteriously, wasn't one.
Now the Chandra X-ray Observatory has looked at Eta Carina and showed details that are, well, shocking.
Right: The earlier, well-known image of Eta Carina taken by the Hubble Space Telescope is scaled to show how it fits within the much larger X-ray nebula discovered by the Chandra X-ray Observatory. Each of the lobes shown in the Hubble image is about the width of our entire solar system. Links to 1024x1024-pixel, 222KB JPG. Credit: Chandra Science Center and NASA.
"It is not what I expected," said Dr. Fred Seward of the Harvard-Smithsonian Center for Astrophysics. "I expected to see a strong point source with a little diffuse emission cloud around it. Instead, we see just the opposite- a bright cloud of diffuse emission, and much less radiation from the center."
The Chandra image reveals a hot inner core around this mysterious superstar.
Left: Chandra has revealed unexpected structures around the nova at the center of Eta Carina. The colors are artificial to help the viewer sort out details and structure. Links to 512x512-pixel, 82KB JPG. Credit: Chandra Science Center and NASA.
The new X-ray observation shows three distinct structures: an outer, horseshoe-shaped ring about 2 light years in diameter, a hot inner core about 3 light-months in diameter, and a hot central source less than 1 light-month in diameter which may contain the superstar that drives the whole show. The outer ring provides evidence of another large explosion that occurred over 1,000 years ago.
All three structures are thought to represent shock waves produced by matter rushing away from the superstar at supersonic speeds. The temperature of the shock-heated gas ranges from 60 million deg Kelvin in the central regions to 3 million K on the outer structure.
Eta Carinae is one of the most interesting and perplexing objects in the entire Milky Way. It was discovered, as a variable star, in 1677 by Sir Edmund Halley, then made its mark on modern astronomy during the "Great Eruption" of 1837-56. The star brightened to magnitude -1, second only to Sirius in brightness. It is also known as NGC 3372 and as the Homunculus Nebulae which sits inside the larger Keyhole Nebula.
Right: Eta Carina as seen in infrared light. Links to 432x432-pixel, 79KB JPG. Credit: Elisha Polomski, U. Florida/CTIO.
It dimmed until only telescopes could detect it - until 1940 when it started brightening again and became visible to the naked eye.
Located about 10,000 light-years from Earth (some estimates are as low as 7,500 light-years), the star is more than 150 times as massive as our Sun, and emits energy at a rate 4 million times greater than the Sun (most of Eta Carinae's radiation is at infrared wavelengths from dust in the bipolar nebula). In 1996, Hubble was trained on the star and revealed a massive cloud resembling two balloons extending from either side of a pie plate. These are two spectacular bubbles of gas - each as wide as our entire solar system - expanding in opposite directions away from a central bright disk at speeds in excess of 600,000 km/h. The odd shape is believed to be partly due to the star's intense magnetic field channeling plasma.
Eta Carina is located at Right Ascension 10h45m04s, Declination: -59d41m03s9h55m51s. The constellation is named for the keel of the giant ship, Argo Navis, sailed by Jason and his Argonauts in search of the Golden Fleece. Links to 648x493-pixel, 524KB JPG. Credit: Chandra Science Center (right) and John Gleason & Steve Mandel (below).
"The Chandra image contains some puzzles for existing ideas of how a star can produce such hot and intense X-rays," agreed Prof. Kris Davidson of the University of Minnesota. Davidson is principal investigator for the Eta Carina observations by Hubble. "In the most popular theory, X-rays are made by colliding gas streams from two stars so close together that they'd look like a point source to us. But what happens to gas streams that escape to farther distances? The extended hot stuff in the middle of the new image gives demanding new conditions for any theory to meet."
And a Death Star as well?
As if its huffing and puffing behavior weren't weird enough, Eta Carina also appears to be a Death Star powerful enough to make Darth Vader turn in his light saber. Sveneric Johansson, a specialist in atomic spectroscopy at the University of Lund in Sweden, has proposed that Eta Carinae also is acting as a massive ultraviolet laser.
Johansson, using Hubble observations made with the Goddard High-Resolution Spectrograph, reported in 1996 that his interpretation is not yet proven, but that it appears to be the most plausible explanation of the data.
A laser (Light Amplification by Stimulated Emission of Radiation) creates an intense coherent beam of light when atoms or molecules in a gas, liquid or solid medium, force an incoming mix of wavelengths (or colors) of light to work in phase, or, at the same wavelength. Laser light is analogous to a loud, single-pitch note, while normal white light is analogous to audio static on a radio.
Left: An artist's concept of multiple laser beams pouring from the heart of Eta Carinae. Links to 750x600-pixel, 59KB JPG. Credit: James Gitlin, Space Telescope Science Institute.
Johansson's explanation is not that far-fetched. Natural masers (the microwave version of a laser) have been seen in space since the mid-1960s, and an infrared laser was discovered around the hot young star MWC 349, earlier this year.
"Each ultraviolet light particle (photon) generated in the Eta Carinae laser has almost 700 times the energy of a photon in MWC 349, and so the total energy output is far greater," said Davidson. "Natural infrared lasers are very rare in space; this ultraviolet laser is even more difficult for nature to arrange, and nothing like it has been seen before."
In a Hubble Space Telescope investigation led by Davidson, including nine other collaborators in the U.S. and Sweden, ultraviolet light from the same gas is being closely studied. Johansson was particularly interested in emission by iron ions that seemed unnaturally bright in the new data. He has found the only plausible explanation of the relative brightness of the iron emission lines is a natural laser emitting at energetic ultraviolet light.
"The spectrum of singly-ionized iron (an iron atom with one electron removed) has almost a thousand known energy states and some of these are apparently well-suited to making a laser effect," said Davidson.
If it's not a supernova, then what is Eta Carinae? Astronomers still do not know what lies at the heart of Eta Carinae, but most believe that it is powered by an extremely massive star that may be a 150 times as massive as the Sun. Such stars produce intense amounts of radiation that cause violent instabilities before they explode as a supernova. The outer rings seen by Chandra are evidence of an ancient eruption that preceded the 19th century "Great Eruption." So, we can expect Eta Carinae will huff and puff a few times before it finally blows its own candle out in a true supernova.
Further Chandra observations of Eta Carina are planned for the near future and should give astronomers deeper insight into this cryptic colossus.
NASA's Marshall Space Flight Center in Huntsville, AL, manages the Chandra program. TRW, Inc., Redondo Beach, CA, is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, MA. |
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