variable star star that varies, either periodically or irregularly, in the intensity of the light it emits. Other physical changes are usually correlated with the fluctuations in brightness, such as pulsations in size, ejection of matter, and changes in spectral type, color, or temperature. The class to which a variable star belongs is determined by a plot of its light curve, which is a graph of the star's apparent brightness versus time. The light curve reveals its maximum and minimum brightness and gives evidence for periodicity, if any exists. The approximately 30,000 known variable stars are grouped into three broad classes: the pulsating variables and the eruptive variables (both of which are intrinsic variables, because the variation is caused by a physical change within the star) and the eclipsing variables (which are extrinsic variables, because the variation is caused by two or more bodies eclipsing one another).

Intrinsic Variable Stars

Pulsating Variables

Pulsating variables account for more than half of the known variable stars. They are characterized by slight instabilities that cause the star alternately to expand and contract. This pulsation is accompanied by changes in absolute luminosity and temperature. The pulsating variables can be further divided into the following subclasses: short-term, long-term, semiregular, and irregular. Short-term variables have well-defined periods ranging from less than one day to more than 50 days.

Relatively rare among this subclass are the Cepheid variables ; these yellow supergiant stars are historically important because, having periods roughly proportional to their absolute brightness, they provide a means of measuring galactic and extragalactic distances. A key research program of the Hubble Space Telescope is the measurement of Cepheid variables in distant galaxies in order to refine our concept of the size and age of the universe. Cepheid variables are classed as either population I Cepheids, which are found in the spiral arms of galaxies, or population II Cepheids, also known as W Virginis stars, which are found in star clusters (see also stellar populations ). About 700 Cepheids of both types have been found in our galaxy.

A more common short-term variable is of the RR Lyrae group; about 6,000 of this type are known in our galaxy and are concentrated in globular clusters. They have periods of less than one day, and all have roughly the same intrinsic brightness. The latter feature, along with their wide distribution throughout the galaxy, makes them another useful distance indicator.

The long-term variables are the most numerous of all pulsating stars. They are red giant and supergiant stars with periods ranging from a few months to more than a year. The best known of these stars is Omicron Ceti, also known as Mira. Over a period of about 11 months, it brightens by about 7 magnitudes and then gradually fades. Semiregular variables are stars whose periodic variations are occasionally interrupted by sudden bursts of light. The best-known example is the red supergiant Betelgeuse, in Orion. Irregular variables show no periodicity in their variations in brightness. The amplitude of their fluctuations in brightness is in general smaller than the fluctuations of the long-term regular variables.

Eruptive Variables

The eruptive variables are highly unstable stars that suddenly and unpredictably increase in brightness. T Tauri stars, also known as nebular variables because they are young objects still embedded in nebulosity, are the least violent of these explosive stars. Novas and supernovas are much more dramatic. Novas are small, very hot stars that suddenly increase thousands of times in luminosity. Their decline in luminosity is much slower, taking months or even years. Most novas probably repeat their outbursts, the dwarf novas every few months, the recurring novas every few years or decades, and the standard novas over thousands of years. Supernovas , upon exploding, increase millions of times in brightness and are totally disrupted. More than 30 supernovas events are observed annually in distant galaxies. Three supernovas have been seen in our own galaxy, in 1054, 1572, and 1604; in 1987 a supernova erupted in a neighboring galaxy, the Large Magellanic Cloud.

Extrinsic Variable Stars

Eclipsing variables are not true (intrinsic) variables but rather are binary star systems, i.e., pairs of stars revolving around a common center of mass. The apparent brightness of an eclipsing variable fluctuates because the orbit of the pair is seen edgewise, so that first one star and then the other regularly blocks the light of its companion. Best known of this type is Algol (Beta Persei).

Bibliography

See D. Levy, Observing Variable Stars (1989).

Variable star

Variable stars are stars that vary in brightness over time. In most cases, these changes occur very slowly over a period of months or even years. In some cases, however, the changes take place in a matter of hours.

The category variable stars encompasses several different types of stars that vary in brightness for entirely different reasons. Examples include red giants, eclipsing binaries, Cepheid variables, and RR Lyrae.

The most common variables, with the longest bright-dim cycles, are red giants. Red giants are stars of average size (like the Sun) in the final stages of life. During the last several million years of its multibillion-year lifetime, a red giant will puff up and shrink many times. It becomes al ternately brighter and dimmer, generally spending about one year in each phase until it completely runs out of fuel to burn.

The apparent variable behavior of a second group of stars, eclips ing binaries, is caused by a very different process. A binary star is a double star system in which two stars orbit each other around a central point of gravity. An eclipsing binary occurs when the plane of a binary's orbit is nearly edgewise to our line of sight (that is, from a viewpoint on Earth). Each star is then eclipsed by the other as they complete their orbits.

A special class of variables, discovered by American astronomer Henrietta Swan Leavitt (1868–1921), consists of blinking yellow supergiants called Cepheid (pronounced SEF-ee-id) variables. They are so named because they were first found in the constellation Cepheus. The pulsing of Cepheids seems to be caused by the expansion and contraction of their surface layers. They become brighter (expansion) and dimmer (contraction) on a regular cycle (lasting 3 to 50 days). For this reason, astronomers use Cepheids as a way of measuring distances in space. If two Cepheids have the same cycle of variation, then the brighter one is closer to Earth.

Similar to Cepheids but older are a group of stars known as RR Lyrae stars. They are so named because one of the first stars of this type was discovered in the constellation Lyra. RR Lyrae are usually found in densely packed groups called globular clusters. Because of their age, RR Lyrae stars are relatively dim. They also have very short light variation cycles, lasting usually less than one day.

Two American astronomers have been instrumental in tracking variable stars. Leavitt, in a search of the southern skies in the early 1900s, discovered about 2,400 variable stars. In 1939, Helen Sawyer Hogg (1905–1993) created the first complete listing of the known 1,116 variable stars in the Milky Way galaxy. In 1955, she updated this catalogue, adding 329 new variables, one-third of which she discovered herself.

Words to Know

Cepheids: Pulsating yellow supergiant stars that can be used to measure distance in space.

Eclipsing binaries: Double star system in which the orbital plane is nearly edgewise to a viewpoint on Earth, meaning that each star is eclipsed (partially or totally hidden) by the other as they revolve around a common point of gravity.

Globular clusters: Tight grouping of stars found near the edges of the Milky Way.

Red giants: Stage in which an average-sized star (like the Sun) spends the final 10 percent of its lifetime; its surface temperature drops and its diameter expands to 10 to 1,000 times that of the Sun.

RR Lyrae: A class of giant pulsating stars that have light variation periods of about a day.

Supergiant: Largest and brightest type of star, which has more than fifteen times the mass of the Sun and shines over one million times more brightly.

[See also Binary star; Red giant; Star ]

variable star Any star that varies in brightness. Two broad categories are recognized: extrinsic variables, which vary for a mechanical reason (e.g. rotation); and intrinsic variables, which undergo a real change in luminosity of either an individual star or some element in a binary system. Certain stars may combine both forms of variation. The standard reference for the classification of types of variable stars and their nomenclature is the General Catalogue of Variable Stars (GCVS). The current total of designated variables is over 40 000.

The classification of variable stars was originally based upon the form of their light-curve, its amplitude (1), and periodicity (or lack of it). Increasingly, however, the physical mechanisms that underlie the different forms of variation, or the physical structure of the stars (and binaries) are used to define six groups of variables, each with specific features. The groups are subdivided into individual types of variation, often named after specific stars, and commonly referred to by capital-letter abbreviations, as listed in Table 8, Appendix. Some stars exhibit more than one form of variability, in which case a combined abbreviation is used, such as E+UG, BY+UV, or EA+UV+BY. Eclipsing binaries are classified in three ways: by their light curve, their physical characteristics, and the evolutionary state of their components. For these, combined abbreviations of the form E/DM, EA/DS/RS, EB/AR, EW/KW are used.

Eruptive variables exhibit unpredictable changes in the form of flares or fades, most of which originate in chromospheric or coronal activity. The group includes flare stars, Gamma Cassiopeiae stars, Orion variables, R Coronae Borealis stars, and T Tauri stars.

Pulsating variables expand and contract or experience wave-like motion of the surface because of fluctuations in the flow of energy from their interiors. Notable examples are Cepheid variables, Mira stars, RR Lyrae stars, RV Tauri stars, semiregular variables, and ZZ Ceti stars.

Rotating variables are a small group whose variations arise from non-uniform surface brightness or ellipsoidal shape.

Cataclysmic variables generally exhibit powerful outbursts with a sudden release of energy. The group includes novae, dwarf novae, and supernovae, and should not be confused with eruptive variables.

Eclipsing variables are binary stars which exhibit partial or total eclipses of one or both components. They are subdivided, on the basis of the shape of the light-curve, into Algol stars, Beta Lyrae stars, and W Ursae Majoris stars. This group also includes some systems that do not eclipse, but where the distorted shape of the components produce fluctuations in the light-curve.

Optically variable X-ray sources bear many similarities to cataclysmic variables (specifically to cataclysmic binaries). The optical variations are often induced by the X-ray variability. Two examples are the AM Herculis stars (or polars) and the HZ Herculis stars (or X-ray pulsars).

variable star Star whose brightness varies with time. Intrinsic variables are stars that vary because of some inherent feature. In extrinsic variables, external factors, such as eclipses or dust, affect the amount of light reaching us from the star. See also magnitude