APS News

February 2003 (Volume 12, Number 2)

This Month in Physics History

February 1987: Discovery of Supernova 1987A

The nebula of SuperNova 1987A photographed in 2002
The nebula of SuperNova 1987A photographed in 2002
On February 23, 1987, Canadian astronomer Ian Shelton (then with the University of Toronto) was engaged in what he thought was merely routine work at the Las Campanas Observatory in Chile, taking a telescopic photo of a small galaxy 167,000 light-years from Earth called the Large Magellanic Cloud. But when he developed the photographic plate, he noticed an extremely bright star that he had not seen in previous observations of the same area: a star of about the fifth magnitude. He realized that it was not a new star, but an aging massive star that had blown apart in a supernova explosion.

Data taken by a small telescope aboard the International Ultraviolet Explorer (IUE) satellite helped astronomers identify the exploding star's location as Sanduleak -69° 202, the former site of a blue supergiant about 20 times the mass of the sun. They named the exploding star Supernova 1987A. Astronomers believe the star swelled up to become a red supergiant, puffed away some of its mass, and then contracted and reheated to become a blue supergiant. In less than a second, the star's core collapsed, and a wave of neutrinos heated the inner core to 10 billion degrees. The process triggered a shock wave that ripped the star apart, propelling a burst of neutrinos into space.

Supernova 1987A is the closest supernova to have exploded in modern times, and the brightest since Johannes Kepler observed a supernova in 1604 in the Milky Way Galaxy; it is also the first supernova visible to the naked eye since 1885. In addition, research over the last 15 years has yielded a wealth of new observational data that has provided astronomers with unprecedented insight into the processes that govern stellar bodies.

By May 1987, the IUE team had discovered an abundance of chemical elements in the supernova debris, indicating that the progenitor star had already passed through the red giant phase, and verifying the original theory. By July, a Japanese satellite and West German telescope detected x-rays emanating from the debris. And from August to November, several other research missions detected high-energy gamma rays, typically released in the decay of radioactive elements formed in nuclear reactions at the core of a dying star. The data confirmed a widely held theory that supernovas produce the heavy chemical elements that make up most of the matter on Earth.

Two years later, optical observations in La Silla, Chile, showed a bright ring around the Supernova, an observation that was confirmed a year later by the Faint Object Camera aboard the newly deployed Hubble Space Telescope. Since its discovery, the remnant from Supernova 1987A has been expanding. Its rings are thought to be parts of shells of gas ejected by the star long before the explosion. In May 1997, Hubble's imaging spectrograph produced a detailed ultraviolet image of the inner ring, identifying specific gases such as oxygen, nitrogen, hydrogen and sulfur, which astronomers hope will help them assemble a picture of how the ring was created.

Radio waves were detected for two weeks after the supernova was first observed, but in 1990, scientists at the Australia Telescope National Facility detected rapidly brightening radio emissions, which they traced to an area that lies between the ring and the glowing debris of the supernova at its center, where the most rapidly moving debris is crashing into gas. Two years later, the ROSAT satellite detected rapidly brightening x-rays from the supernova, coming from the same collision area as the radio waves. In May 1994, Hubble revealed two additional loops of glowing gas. In January 1997, it showed two blobs of debris in the Supernova's center racing away from each other at nearly 6 million mph. Later that year, astronomers made the first measurements of the fast-moving gas ejected by the supernova explosion-gas that had been invisible until Hubble's imaging spectrograph enabled scientists to observe it in ultraviolet light.

The collisions were predicted by theory, and expected to occur sometime between 1995 and 2010, as the ring absorbed the full force of the crash. While the radiation from the original explosion traveled out at the speed of light, material from the star itself was ejected at a much lower speed, and was only now catching up and colliding with material blown out some 20,000 years earlier by the star. The collision caused the gases in the ring to glow as they heated to millions of degrees, compressed by a blast wave estimated to be moving at 40 million mph.

As recently as February 2000, Supernova 1987A was still yielding surprising discoveries on stellar processes. New images taken by Hubble revealed four bright new knots of heated gas in an area that had been fading slowly for a decade. This was significant because the hot spots were not confined to a single location, but distributed around the circumstellar ring, indicating that a large fraction of the ejected material was finally colliding with the entire ring, marking the beginning of the formation of a Supernova remnant. Astronomers hope that the light from the collisions will illuminate matter surrounding the Supernova that until then had been invisible, providing useful information on the true structure of the gas around the Supernova, and enabling them to determine how it might have gotten there.

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Editor: Alan Chodos
Associate Editor: Jennifer Ouellette

February 2003 (Volume 12, Number 2)

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This Month in Physics History
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