II.

Novae

Before the era of modern astronomy, a star that appeared suddenly where none had been seen before was called a nova, or “new star”. This is a misnomer, as the stars involved had existed long before they became visible to the naked eye. Astronomers estimate that perhaps a dozen novae occur in the Milky Way, the Earth's galaxy, each year, but two or three of them are too distant to be seen or are obscured by interstellar matter. Indeed, novae are often more easily observed in other, nearby galaxies than in our own. Novae are named according to the year of their occurrence and the constellation in which they appear. Typically, a nova flares up to several thousand times its original brightness in a matter of days or hours. It next enters a transition stage, during which it may fade and grow bright again and then fade gradually to or near its original level of brightness.

Novae are stars in a late stage of evolution. Like other forms of “cataclysmic variable” stars, they consist of an interacting binary system that contains a small, dense white dwarf and a “normal” companion star. Over many thousands of years, a continuous in-fall of material from the companion builds up on the surface of the white dwarf. Nuclear reactions cause the temperature and pressure in its outer layers to rise until a huge thermonuclear explosion occurs. See Star: Evolution of Stars.

Sometimes the cycle is more rapid and the outbursts are much smaller. In the case of “dwarf novae”, the white dwarf erupts repeatedly at regular intervals of a few to hundreds of days. Other stars experience repeated outbursts over a few decades, so they are called “recurrent novae”. One of the best known of these is RS Ophiuchi, in the constellation of Ophiuchus, which has been seen to “erupt” six times since 1898. During the most recent event, which began in February 2006, the nova was examined in great detail by ground-based telescopes and space observatories. The results showed that RS Ophiuchi is a white dwarf with a mass about 1.4 times that of the Sun. Nearby is a bloated red giant star that spills material onto the white dwarf. When enough of that material has accumulated, a gigantic thermonuclear explosion occurs. During the outburst, material from the white dwarf smashes into the gaseous envelope surrounding the red giant. This creates a shock wave that heats the gas, causing it to emit energetic X-rays. Radio observations suggest that the nova produced twin jets of stellar material that spewed out in opposite directions.

Novae in general show a relationship between their maximum brightness and the time they take to fade by a certain number of magnitudes. By observing nearer novae, whose distance and brightness are known, astronomers can use novae in other galaxies as indicators of the distance to those galaxies.