Feb 16, 2000
This collision is beginning to cause the gases in the ring to glow as they are heated to millions of degrees and compressed by the sledgehammer blow of a 40 million mile-per- hour blast wave. In new pictures taken on February 2, Hubble's sharp view revealed four bright new knots of heated gas at places that had been fading slowly for a decade. Under an observing program called the supernova intensive survey, a team of astronomers has been monitoring SN1987A with Hubble since it was launched in 1990.
One of the first clues to the celestial fireworks came in 1997 when Hubble saw a single knot in the ring shine like a bright diamond as it was first impacted by the shockwave. "That was the opening jab. Now the dancing around is over and the slugfest will begin," says Robert Kirshner of Harvard-Smithsonian Center for Astrophysics in Cambridge, MA.
"The real fireworks show is finally starting and over the next ten years things will get spectacular. It helps that Hubble is giving us an unparalleled view," adds Peter Garnavich of the University of Notre Dame.
Previous Hubble spectroscopic observations, and radio and x-ray telescopic observations of the expanding supernova shockwave all led astronomers to anticipate that the titanic collision was only a matter of time. As far back as 1992 astronomers predicted that the ring would become ablaze with light as it absorbs the full force of the crash.
Upon seeing the new Hubble pictures, Kirshner remarked, "It's about time. We saw that first hotspot two years ago, but I was getting nervous that we might have been mistaken about its location. It's great to see the shock wave start to light up the ring."
The supernova, called SN 1987A, has long puzzled astronomers. They believe the ring is made up of old gas that was ejected by the star 20,000 years ago, long before it exploded. The ring's presence was given away when it was heated by the intense burst of light from the 1987 explosion. The ring has been slowly fading ever since then as the gas cools.
The initial supernova flash only lit up a small part of the gas that surrounds the supernova. Much of it is still invisible. But the light from the crash should illuminate this invisible matter for the first time, and help unravel the mystery of a pair of outer rings seen around the supernova as well.
"Now as the central ring begins to light up again, we can see how this old material is arranged around the star. We can map its distribution," Kirshner says. "This event gives us another chance to see the true structure of the gas around the supernova and to puzzle out how it got there."
Kirshner and colleagues plans to use Hubble to do follow- up observations later this year to track the ongoing drama of one of the biggest celestial collisions ever witnessed by astronomers.
The Supernova Intensive Survey team includes Dick McCray, University of Colorado, Boulder; Nino Panagia, Space Telescope Science Institute, Baltimore, MD; Nick Suntzeff, Cerro Tololo Inter-American Observatory, Chile; and George Sonneborn and Jason Pun, NASA Goddard Space Flight Center, Greenbelt, MD.
Images to accompany this release are available at:
www.stsci.edu
Columbia University
New York, New York
February 16, 2000
The activity was sighted on Dec. 25, 1999, by a team of Columbia astrophysicists, Stephen Lawrence, Arlin Crotts, Ben Sugerman, and Robert Uglesich, led by Patrice Bouchet of the National Optical Astronomy Observatories' Cerro Tololo Inter-American Observatory (CTIO). A "hot spot" that appeared in the circumstellar ring around the supernova in 1997 was believed to be the first impact of supernova ejecta, but no other activity sites had been observed until the CTIO and Columbia team sighted this one in December. The new hot spot is about the same brightness as the first was when it was originally found. Other, fainter impact sites are present in their observations. The scientists also determined that the original hot spot had brightened significantly since their last observation over a year ago.
"The first collision of ejecta may have been a jet of material striking the circumstellar ring, shocking the gas into emission, much like bullets hitting a target," said Crotts, a Columbia professor of astronomy. "Now the entire ring is beginning to be engulfed with shocked material from the supernova, lighting up the ejecta and circumstellar material as a supernova remnant. We have observed many examples of supernova remnants -- for example, the Crab Nebula -- but all were formed long ago. We have never before seen one in the making in any meaningful degree of detail."
The significance of the newly discovered hot spots is that they are not confined to a single location, but are distributed around the circumstellar ring. The distribution around the ring indicates that a large fraction of the ejected material is finally colliding with the whole ring, instead of a fast moving "bullet" of ejecta making a single hot spot. If so, this is the beginning the long awaited formation of a supernova remnant. Other teams making follow-up observations with the Hubble Space Telescope in late January and early February have confirmed the new hot spot and found a number of other faint, new impact sites. These other hot spots are also found in the CTIO data, at a more subtle level.
The CTIO observations used an innovative imaging system on a Blanco 4-m telescope that achieved better spatial resolution than is commonly possible from ground-based observatories. The CTIO system tips and tilts the secondary mirror of the telescope to take the "twinkle" out of starlight, producing steadier, sharper images. They also used a novel image processing technique developed by the Columbia team.
Supernova 1987A occurred when the star known as Sanduleak -69 202 ended its life in a gigantic explosion, which was observed on earth on February 23, 1987, and became known as supernova 1987A. While the radiation from that explosion traveled out at the speed of light, material from the star itself was ejected at a much lower speed, some tens of millions of miles per hour. This material is now beginning to catch up and collide with material blown out some twenty thousand years earlier by the star in a relatively gentle, slow, cool stellar wind. This collision of supernova ejecta with the wind material, now forming the circumstellar shell, was predicted to occur sometime between 1995 and 2010.
IMAGE CAPTION:
Supernova 1987A's nebula (colored ring) is now filling with exploding spots (mostly in red) as the supernova destroys it.
Columbia University
New York, New York
January 8, 1998
Arlin Crotts, associate professor of astronomy at Columbia, and Steve Heathcote, research astronomer at Cerro Tololo in La Serena, Chile, announced their results today (Thursday, Jan. 8, 1998) at the American Astronomical Society meeting in Washington, D.C. The two astronomers were able to measure the velocity of gases ejected during an early phase of supernova formation before the latter phases obscured them. The research will help astronomers confirm one among the many theories advanced for how the star exploded.
In one terrific second, a supernova emits as much energy as the stars within a billion galaxies, affecting a huge volume of surrounding space. Fortunately, Professor Crotts said, there are no such stars in our immediate environs, although there are some that might someday be worth worrying about.
Scientists can learn much about when and how stars explode by studying the phases they go through before their instant of destruction. Stars eject a huge amount of material to maintain stability while great changes are proceeding within them. This material resides in a nebula around the dying star, but will quickly be swept away when the explosion occurs. This is why the process can be difficult to study: the explosion destroys the clues to what led to the star's demise.
In the decade since February 23, 1987, when the explosion in the Large Magellanic Cloud (our galaxy's satellite, seen in the southern constellation Dorado) was seen from Earth, it has been possible to map the surrounding nebula before the debris from the explosion arrives to completely destroy its structure.
"This is a unique opportunity, because the history of these massive stars is recorded in the surrounding material and is usually wiped away by the supernova explosion before we have a chance to study it," Professor Crotts said.
Astronomers know how far the material sits from the dying star and roughly how it is configured throughout that space. In order to know how long these structures were emitted prior to the explosion, scientists must know how fast the structures are expanding outward from the star, hence allowing them to convert the distance from the star into an estimate of the surrounding structure's age, which corresponds to how long the material has been traveling outward in order to reach its current position at the observed velocity. Knowing these ages, scientists can relate the expulsion of these parts of the nebula to the changes in the star leading up to the explosion.
Now, the Columbia-Cerro Tololo team has measured the velocities of the major components of the nebula around Supernova 1987A, allowing their formation to be tied to the evolutionary phases on the star prior to explosion. Using the four-meter reflecting telescope at Cerro Tololo, they found that the entire nebula is at least 200,000 years old and probably several times older. This age presumably corresponds to the end of the stable phase of the burning of hydrogen in the star's core, and the first epoch of mass loss from the star.
The researchers also found that a number of structures in the nebula's interior at different distances are expanding at different velocities, but all are consistent with the finding of being created the same time much more recently, about 200,000 years ago.
The distances to these nebular structures were measured in earlier work by Drs. Crotts and Heathcote, together with William Kunkel of Las Campanas Observatory. This latter age measurement conflicts with earlier hypotheses about the age of portions of the nebula based on indications of different ratios of carbon and nitrogen in the expanding gas.
While this study does not mean that astronomers can now predict when a star will explode, it will provide important clues to how a star capable of becoming a supernova changes during the complex stages in its last million years. The case of Supernova 1987A might be particularly complicated, because astronomers suspect that it may not have been a single star, but the product of two interacting ones. Such a system has even more possible ways of distributing its matter through space as it dies. However, Professor Crotts said, the new findings are likely to eliminate some of these theoretical possibilities.
Previous information on ASTRONET:
Solved: the mystery of the star that blew up as supernova 1987A
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