Serendipidity catches a supernova.

Supernovas are giant explosions of stars. Supernovas are also giant frustrations for astronomers. There is no way to predict when or where a supernova will happen, so astronomers usually don't notice them until the explosion is virtually over and their light has passed its maximum brightness and is starting to fade away.

Now a group of astronomers working at institutions in South America report that they have discovered a type II supernova on its way to maximum brightness and have been able to follow it through maximum brightness and into its decline.

Supernovas are divided into two types according to the way their light emission changes. Theory says type I supernovas occur in binary star systems, whereas type II supernovas result from the explosions of lone giant stars: red supergiants or the class known as Wolf-Rayet stars. The South American observers--Mark M. Phillips of the Cerro Tololo Inter-American Observatory at La Serena, Chile, Maria Teresa Ruiz of the University of Chile in Santiago and Virpi S. Niemela of the Institute of Astronomy and Space Physics in Buenos Aires, Argentina--say their find has contributed "unique" data in support of the theory about type II supernovas. An announcement by the (U.S.) National Optical Astronomy Observatories, which operates Cerro Tololo, calls it "[t]he first detailed observations of a star in its death throes."

Supernovas occur at a rate of about one per galaxy per hundred years. (None has been seen in our galaxy since the 17th century.) This latest one is designated 1983k and is located in the galaxy NGC 4699. Although the observations were made in early (southern) winter of 1983, it took until now to get them analyzed and published in THE ASTROPHYSICAL JOURNAL (Vol. 289, p. 52).

On June 6, 1983, astronomer Marina Wischnjewsky made a plate of NGC 4699 as part of a supernova survey conducted by the University of Chile's Cerro Calan Observatory in Santiago. On the plate she noticed an incipient supernova, a star that was 300 times as bright as it should be. She got in touch with her colleague, Ruiz, then working as a visiting astronomer at Cerro Tololo.

"It was serendipitous that we had this team of experts at Cerro Tololo," says Phillips. "Maria Teresa specialies in supernova remnants, and Virpi is a specialist in Wolf-Rayet stars."

The astronomers at Cerro Tololo got their first spectra of the object on June 14, 16 and 17, while the supernova was getting brighter, and then followed with post-maximum spectra on July 16 and 18. They believe their data show evidence for a shock wave beginning deep within the star, rising to the surface and then blowing the star's surface material outward. The star, which most likely had a diameter of about 17 billion kilometers on June 6, had expanded to some 80 billion km by late July.

Before maximum light (which occurred on June 23) the star showed spectral patterns characteristic of a Wolf-Rayet star, particularly prominent signs of enrichment with nitrogen. These characteristics abruptly disappeared at maximum light. These Wolf-Rayet emissions are a hitherto unobserved phase of the development of type II supernovas, according to the Cerro Tololo observers. Such a finding raises the suspicion that the progenitor star was of the Wolf-Rayet type. However, the observers say, those spectral features could be an artifact of the explosion rather than a property of the progenitor.

They conclude that "SN 1983k most likely resulted from a massive star with an extended envelope, which had undergone significant mass loss prior to exploding and whose surface layers contained nitrogen-enriched material."