I.

Introduction

Novae and Supernovae (Latin, stella nova, “new star”), two kinds of explosive event that take place in some stars. A nova is a star that suddenly increases greatly in brightness and then slowly fades, but may continue to exist for some time. A supernova exhibits the same pattern of behaviour, but the explosion destroys or profoundly alters the star. Supernovae are much rarer than novae, which are observed fairly frequently in photographs of the sky.

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.

III.

Supernovae

A supernova explosion is far more spectacular and destructive than a nova and much rarer. Such events may occur no more than once every few years in our galaxy, but despite their increase in brilliance by a factor of billions, only a few are ever observable to the naked eye. Only four have been so observed in the past 1,000 years (1006, 1054, 1572, and 1604), although there is also evidence for a supernova taking place in the constellation of Cassiopeia around 1680. The best known of these was recorded by Chinese astronomers in ad 1054 and its remains are now known as the Crab Nebula. Supernovae, like novae, are more often seen in other galaxies. Thus, the most recent naked-eye supernova, which appeared in the southern hemisphere on February 24, 1987, occurred in the Large Magellanic Cloud, a companion galaxy to the Milky Way. This supernova, which exhibits some unusual traits, is now the object of intense astronomical scrutiny.

The mechanisms that produce supernovae are less well understood than those that result in novae. What is clear is that stars that are much more massive than the Sun sometimes explode in the late stages of their rapid evolution. This is caused when the star runs out of nuclear fuel in its core. This leads to a catastrophic gravitational collapse, since the pressure created by nuclear processes within the star is no longer able to withstand the weight of the star's outlying layers. This is called a Type II supernova.

A Type I supernova is brought about in a similar way to a nova. In the case of a Type IA supernova, a white dwarf that belongs to a binary system receives an influx of fresh fuel when it captures material from its swollen companion. When its mass becomes too great (that is, it passes the Chandrasekhar limit), the white dwarf collapses, causing a huge explosion. Type IB and IC supernovae show similar light curves to Type IA but tend to be dimmer. Type IB supernovae are thought to be produced by explosions in the cores of massive stars that have been stripped of their hydrogen. Type IC supernovae have been stripped of their helium.

Type I supernovae are used in cosmology to measure distance, and hence the age of the universe. Because their absolute magnitude and rate of brightening and dimming follow a consistent pattern, they can be used as “standard candles” to measure distance. Since distant objects in the universe are moving away from us (and the more distant they are the faster they are receding), light waves from them are progressively lengthened as they travel through the universe, thus appearing to move towards the red end of the electromagnetic spectrum. This red shifting can therefore also be used to estimate distance.

A 1998 study of type I supernovae at distances of up to 10 billion light years showed that many of the red shifts were greater than expected, indicating that the expansion of the universe is accelerating. Subsequent studies have confirmed these observations and shown that the expansion rate of the universe was decelerating until about 5 to 6 billion years ago, and then began speeding up. This acceleration is thought to be due to a repulsive force known as dark energy.

Little may remain after the explosion of a supernova except the expanding shell of gases. A famous example is the Crab Nebula. At its centre is a pulsar, or rapidly rotating neutron star. Supernovae are significant contributors to the interstellar material that forms new stars. It has been thought for some time that supernovae explosions are also responsible for the creation of oxygen and all of the heavier elements and their spread throughout the interstellar medium. This was confirmed by observations of a supernova in the Small Magellanic Cloud using the Chandra X-Ray Observatory.

In 2003 it was suggested that supernovae could be the source of long-duration gamma-ray bursts observed in extremely distant galaxies. Recent studies using the SWIFT spacecraft, launched in 2004, indicate that they may be particularly associated with Type IC supernovae. This is presumably because the presence of a hydrogen envelope around the collapsing core can block the emergence of a gamma-ray burst jet.