A variable star is a star whose brightness as seen from Earth fluctuates.
This variation may be caused by a change in emitted light or by something partly blocking the light, Variable stars may be either intrinsic or extrinsic.
- Intrinsic variable stars: stars where the variability is being caused by changes in the physical properties of the stars themselves. This category can be divided into three subgroups.
- Pulsating variables, stars whose radius alternately expands and contracts as part of their natural evolutionary ageing processes.
- Eruptive variables, stars who experience eruptions on their surfaces like flares or mass ejections.
- Cataclysmic or explosive variables, stars that undergo a cataclysmic change in their properties like novae and supernovae.
- Extrinsic variable stars: stars where the variability is caused by external properties like rotation or eclipses. There are two main subgroups.
- Eclipsing binaries, double stars where, as seen from Earth’s vantage point the stars occasionally eclipse one another as they orbit.
- Rotating variables, stars whose variability is caused by phenomena related to their rotation. Examples are stars with extreme “sunspots” which affect the apparent brightness or stars that have fast rotation speeds causing them to become ellipsoidal in shape.
Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.
An ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol.
Of the modern astronomers, the first variable star was identified in 1638 when Johannes Holwarda noticed that Omicron Ceti (later named Mira) pulsated in a cycle taking 11 months; the star had previously been described as a nova by David Fabricius in 1596.
This discovery, combined with supernovae observed in 1572 and 1604, proved that the starry sky was not eternally invariable as Aristotle and other ancient philosophers had taught. In this way, the discovery of variable stars contributed to the astronomical revolution of the sixteenth and early seventeenth centuries.
Since 1850 the number of known variable stars has increased rapidly, especially after 1890 when it became possible to identify variable stars by means of photography. The latest edition of the General Catalogue of Variable Stars (2008) lists more than 46,000 variable stars in the Milky Way, as well as 10,000 in other galaxies, and over 10,000 ‘suspected’ variables.
The most common kinds of variability involve changes in brightness, but other types of variability also occur, in particular changes in the spectrum. By combining light curve data with observed spectral changes, astronomers are often able to explain why a particular star is variable. Variable stars are generally analysed using photometry, spectrophotometry and spectroscopy. Measurements of their changes in brightness can be plotted to produce light curves.
For regular variables, the period of variation and its amplitude can be very well established; for many variable stars, though, these quantities may vary slowly over time, or even from one period to the next. Peak brightnesses in the light curve are known as maxima, while troughs are known as minima.