Nova

The word nova, Latin for "new," was assigned by ancient astronomers to any bright star that suddenly appeared in the sky. A nova occurs when

Ultraviolet image of Nova Cygni 1992. On February 19, 1992, this nova was formed by an explosion triggered by the transfer of gases to the white dwarf from its companion star. (Reproduced by permission of

National Aeronautics and Space Administration

.)

one member of a binary star system temporarily becomes brighter. Most often the brighter star is a shrunken white dwarf, the cooling, shrunken core remaining after a medium-sized star (like our sun) ceases to burn. Its partner is a large star, such as a red giant, a medium-sized star in a late stage of its evolution, expanding and cooling.

As the companion star expands, it loses some of its matter—mostly hydrogen—to the strong gravitational pull of the white dwarf. After a time, enough matter collects in a thin, dense, hot layer on the surface of the white dwarf to initiate nuclear fusion reactions. The hydrogen on the white dwarf's surface burns away, and while it does so, the white dwarf glows brightly. This is a nova. After reaching its peak brightness, it slowly fades over a period of days or weeks.

The transfer of matter does not stop after a nova explodes, but begins anew. The length of time between nova outbursts can range from several dozen to thousands of years, depending on how fast the companion star loses matter to the white dwarf.

A nova should not be confused with a supernova, which is the massive explosion of a relatively large star. A nova is much more common than a supernova, and it does not release nearly as much energy. Because novae (plural of nova) occur more often, they can change the way constellations in the night sky appear. For example, in December 1999, a bright, naked-eye nova appeared in the constellation Aquila, the Eagle. At its maximum, the nova was as bright as many of the stars in Aquila. For a few days at least, viewers were treated to the spectacle of a truly "new star" in an otherwise familiar constellation.

[ See also Binary star ; Star ; Supernova ; White dwarf ]