Variable Star

> Variable Stars

All stars probably vary slightly in their brightness on a more or less periodic basis, including the Sun. Such variations may be scarcely measurable. Some stars, however, change greatly in brightness and are called variable stars. There are many types. Some repeat cycles with almost clock-like precision; others are highly irregular. Some may require only hours or days to return to a given brightness; others may require years. The brightness of such stars may change almost imperceptibly or violently.

The most spectacular variables are the novae and supernovae. Novae may brighten to as much as 200,000 times the Sun's brightness by blowing off perhaps a hundredth or a thousandth of 1 per cent of the Sun's mass at speeds up to 960 km/s (600 mi/s). Some novae repeat this process periodically until they lose too much mass to continue.

Although supernovae are similarly named, they are a far more catastrophic phenomenon and not periodic at all. They represent the true explosion of a star, sometimes brightening for a few days to 10 billion times the Sun's true brightness before fading away permanently. They leave behind expanding wreckage seen as bright gaseous clouds, or nebulae; the Crab Nebula is an example, first observed from the Earth as a supernova in 1054. Sometimes a pulsar is also left as a remnant at the centre of the wreckage. Novae occur fairly frequently in the Milky Way, perhaps one or two being observed each year, but supernovae are much rarer. The most recent supernova in the Milky Way appeared in 1604, although one in a nearby galaxy drew great attention in 1987.

Many variable stars change their brightness because they pulsate, that is, expand and contract somewhat like a balloon. One important type, named Cepheid variables after δ Cephei, repeat their brightness cycles rather accurately. Their periods range from about a day to hundreds of days, and they are all hundreds of times more luminous than the Sun. The longer the period of a Cepheid variable, the greater the average brightness of the star. This period-luminosity relation, discovered by Henrietta Leavitt of the Harvard College Observatory, has proved invaluable in measuring stellar distances, particularly to nearby galaxies. Only the period and average apparent brightness of a Cepheid need be observed to provide a measure of its distance. Novae and especially supernovae are also important distance measures because their incredible brilliance at maximum light makes them observable at huge distances.

Crab Nebula

An exploding supernova leaves behind a rapidly expanding cloud of gaseous material called a nebula. The Crab Nebula was produced when a star in our galaxy exploded; the light from the explosion was observed by Chinese astronomers in 1054. At the centre of the Nebula lies a pulsar, a dense, rapidly spinning star.

Variable stars are of unusual interest because their variation is usually caused by some peculiarity of their internal structure that develops with age. Variable stars can thus reveal information about stellar evolution. Supernovae, for example, have burned up their nuclear fuel and must blow off matter because they become unstable as they collapse gravitationally.

The eclipsing variable, mentioned in the previous section, varies because of external rather than internal causes. The star Algol in the constellation Perseus is typical. Algol is a double star composed of one bright and one comparatively faint component, orbiting each other in a plane almost exactly in the line of sight from the Earth. As the darker component eclipses the brighter, the apparent brightness of the pair falls off sharply, and a similar but less marked diminution occurs when the brighter component eclipses the darker. Astronomers have observed many thousands of eclipsing variable stars, which are valuable in measuring stellar masses.

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