Astronomers from Columbia University and the Cerro Tololo Inter-American Observatory have measured events that will help scientists discover what led to the brilliant explosion of Supernova 1987A, the brightest supernova observed from Earth in nearly four centuries.
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.
For further information, contact Dr. Arlin Crotts, Columbia Astrophysics Laboratory, 550 W. 120th St., New York, NY 10027 (phone: 212-854-7899, fax: 212-854-8121, e-mail: arlin@astro.columbia.edu).
This document is available at http://www.columbia.edu/cu/pr/. Working press may receive science and technology press releases via e-mail by sending a message to rjn2@columbia.edu.
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