Serendipity: supernova in Centaurus A.

Serendipity: Supernova in Centaurus A

Supernovas--giant stellar explosions --are not particularly rare in the universe. They are a staple item for Astronomical Telegrams, astronomers' system for quick notification of new developments. Highly active galaxies are also fairly numerous. However, the combination --a supernova in an active galaxy--is much rarer. And when the galaxy is the nearest active galaxy to us, Centaurus A, which also happens to be one of the strongest and longest studied celestial sources of radio waves, the combination provides a rather unique excitement for astronomers.

The current supernova, officially designated supernova 1986G, was first reported on May 4 by Robert Evans, an amateur astronomer in Hazelbrook, New South Wales, Australia, and confirmed by observers at the Anglo-Australian Telescope at Siding Spring Mountain in New South Wales. Centaurus A, also known as NGC 5128, is visible only from the Southern Hemisphere. Observatories there have been turning toward it: The National Optical Astronomy Observatories say all major telescopes at their Cerro Tololo Inter-American Observatory near La Serena, Chile, are observing it, an unusual concentration of resources on a single object for a major observatory.

According to Mark Phillips of Cerro Tololo, the supernova's maximum brightness occurred about a week after it was detected. To better understand how supernovas happen, astronomers want to find them before maximum light, while the explosion is on the way up, but most are not noticed until after maximum. However, in a report on International Astronomical Union Circular 4216 (May 15), G. Meurer of Mt. Stromlo Observatory in the Australian Capital Territory says spectra he took indicate that brightness peaked April 21, well before the first sighting. Two more reports on the same circular indicate that the International Ultraviolet Explorer satellite has taken ultraviolet spectra.

Spectra will tell which type of supernova this is--Meurer says his show it to be Type I--and track its development. They may also tell something about the nature and dynamics of the "lane' of dust that lies across the center of Centaurus A. As luck would have it, the supernova lies behind the dust. Such dust lanes are features of several galaxies, and astronomers are interested in their nature and their relation to the dynamics and evolution of the galaxies that have them. Spectra and a profile of the supernova's brightness over time will also help refine the figure for the distance to Centaurus A.

Photo: Centaurus A before (left) and after supernova.