"Variable" refers to stars that change their apparent luminosity, as seen from Earth. To some extent, all stars exhibit variability; even the Sun changes its luminosity by about 0.1% over the 11 years sunspot cycle. To be classified as a variable star, however, requires a more profound change. There are two main categories of variables; Intrinsic and Extrinsic. Additional classification is complex and based on the physical processes that cause the variability. Some sub-classes of variable star are named after a prototype; for example the RR Lyrae type.
These are stars where the variability is due to physical changes in the star. There are three main sub-types:
These are stars whose radius alternately expands and contracts as part of their natural evolutionary aging processes. Periods of variability can be from a few hours to several years. One of the most important are the Cepheid and Cepheid types that pulsate very regularly. The luminosity of a Cepheid is related directly to its period; the more luminous the star, the longer the pulsation period. The W Virginis and RR Lyrae variables are population II stars similar to cepheids but with a different luminosity/period relationship. The Long Period and Semiregular variables pulsate over cycles varying from weeks to years. The RV Tauri variables have a dual variability with a strong change alternating with a much weaker change. There are many other sub-types that a searh on the web will uncover.
These stars have surface eruptions, often huge flares or other ejections of mass. Most protostars, as they contract before becoming main-sequence stars, undergo periods of eruptive instability. Faint main sequence stars that flare up regularly are known as Flare Stars. They can increase up to six times in brightness within a few seconds, and return to normal brightness in less than 30 minutes. I discuss Wolf-Rayet stars under their own classification.
These are stars that undergo a cataclysmic change in their properties like novae and supernovae.
These are stars where the variability is apparent rather than an actual change, so is due to causes external to the star. Again; there are three main sub-types:
Binary, or other multiple star systems, where the stars eclipse each another as they orbit. Obviously, this type of variability is highly dependent on the orientation of the star system to the observer, and is most pronounced when the plane of the orbit is towards us. At the other extreme, when the plane is perpendicular to our viewpoint, there would be no eclipsing, so no apparent variability.
In this type, the variability is due to rotation. Stars that undergo very strong sunspot activity change their apparent brightness as they rotate; that is, whether the sunspots are on the side facing us or away from us. Another example is a star that rotates so quickly that it becomes flattened at its poles.
This is due to a planet, or planets, orbiting the star where the planet passes across the star in its orbit as seen from the Earth. These changes in brightness are extremely small.
KY Cygni is a Pulsating Variable red supergiant star of types M3m located approximately 5,200 light-years away. At between 1,420 & 2,850 times the Sun's diameter, it is one of the largest and most luminous stars known. It is about 25 solar masses.
Delta Cephei is a yellow giant star in a binary system approximately 890 light-years away. It is the prototype for the Cepheid Variable class of stars. As a Cepheid Variable, its variability comes from pulsations in the star. It varies from magnitude 3.6 to 4.3 over a very specific period of 5.36634 days, rising quickly to its maximum brightness and then dimming back more slowly to its minimum. At the same time it varies in type from a F3 at about 6,800K to a G3 at a lower temperature of 5,500°K. It is a large star, some 41.6 times the diameter of the Sun with perhaps a litle less than 5 solar masses.
Beta Cephei is a hot blue white star, type B2III. It is the prototype star of the Beta Cepheid sub-class of variable stars. Its magnitude varies between +3.16 and +3.27, much less of a difference than with a Delta Cepheid, and not enough to be visible to the eye. Its period from maximum to minimum and back is very precise at 4.57 hours. Overlaid on this are several other smaller variations of differing periods of up to six days. 595 light years from Earth, it is about 12 solar masses.
Astronomy & Cosmology
Stars - Stellar Classes
Examples of Variable Stars