Any star that changes or varies in some way, usually brightness, is called a variable star. Stars can vary because of something intrinsic to the star itself or because of something extrinsic to the star, such as another nearby star. Here are a few of the major types of variable stars.
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Variable Stars: Facts and Information
According to Wikipedia, a variable star is defined as a star whose physical properties, such as brightness, radial velocity, and spectral type, vary with time. The time length between maximum and minimum brightness for a variable star can be anything from minutes to years.
Major Types of Variable Stars
There are five main groups of variable stars: cataclysmic, eclipsing, eruptive, pulsating, and rotating.
- Cataclysmic variables are binary star systems in which a white dwarf is receiving mass transfer from a companion.
- Eclipsing binary stars are two (or more) stars in a system in which one of the members passes in front of and then behind the other star, thereby temporarily reducing its total light output.
- Eruptive variables are stars that brighten thru flares or thru the ejection of gaseous shells.
- Pulsating variables are stars that are undergoing a physical change, expanding and contracting, which causes the light output to increase and decrease.
- Rotating variables change brightness due to irregular surface features, spots, and markings that dim and brighten the light output depending on what surface is currently facing the viewer.
The above categories can also describe extrinsic variables, or those which change in brightness due to a process external to the star (such as an eclipse), or intrinsic variables, when the change comes from the star itself (such as pulsation). It is possible for a variable star to be both extrinsic and intrinsic (pulsating and eclipsing, for example).
Eclipsing binaries
Eclipsing binary systems are systems of two stars whose orbits are inclined so that each star occasionally passes in front of the other. They orbit each other closely enough that they cannot be resolved as two separate stars. When one of the stars is behind the other, it is eclipsed, and the system is fainter because we only see light from one star. The individual stars themselves do not actually vary in brightness.
Pulsating variable stars
Pulsating variable stars change in brightness because the star is changing in size. These size changes are called pulsations. When the star is larger, it has more surface area radiating light, so it is brighter. When the star is smaller, it is fainter. The most common types of pulsating variables are: Cepheid variables, RR Lyrae variables, Mira (or red) variables, RV Tauri stars, Beta Canis Majoris stars, and Delta Scuti (or dwarf Cephied) variables.
T Tauri stars
T-Tauri stars are young stars that have not yet reached the main sequence stage of the HR diagram. The brightness variations in these stars are caused by large spotted regions. These spotted regions are similar to sunspots but much more extreme. When the spotted region faces us, the star appears fainter. T-Tauri stars also often have flares, which are rapid, brief increases in brightness.
RS CVn stars
RS CV stars are binary systems with individual components similar to the Sun. When the two solar-type stars are in close proximity, the spot activity increases to an extreme level compared to the spot activity on the Sun. When the spotted hemisphere is facing us, the star appears fainter. RS CV stars also have occasional flares. In addition, RS CVN systems might be eclipsing systems, but they don’t have to be.
Flare stars
Flare stars have flares similar to those found on the sun. On the Sun, flares are rapid increases in brightness on a small part of the solar surface. For red, less luminous stars, a flare that would not significantly increase the brightness of the Sun represents a very significant increase in the total energy output of the star.
Cataclysmic Variable Stars
Cataclysmic variable stars are basically stellar explosions. These explosions can range from relatively small eruptions that repeat themselves to major explosions that destroy the entire star. The relatively small repeatable eruptions are dwarf novae. Novae are explosions involving only the surface layers of a white dwarf star. Supernovae are the most violent stellar explosions that can often outshine an entire galaxy. A type I supernova occurs when a white dwarf star exceeds the maximum mass limit of 1.4 solar masses. Type II supernovae are the death explosions of the most massive stars. The outer layers are blasted into space, and the core collapses into a neutron star or a black hole.
Famous Variable Stars
There are many examples of variable stars of all types. Here are a few of the more well-known variable stars.
- Algol-Beta Persei-The Demon Star. Algol is an eclipsing (triple) binary system. Algol’s magnitude of 2.1 drops to 3.4 for 10 hours at a time as it is eclipsed by a companion. These eclipses occur a little less than every three days.
- Mira: Mira is a pulsating variable star that has a whole category of variables named for it (the Mira variables). Mira is a red giant whose surface oscillates such that it can be a bright magnitude 2.0 star or one that is not visible to the unaided eye.
- Delta Cephei: Delta Cephei is famous for giving its name to an entire class of variable stars, the Cepheid variables. Delta Cephei is a regular period variable, changing from magnitude 3.5 to 4.3 and back over a period of five days, eight hours, 47 minutes, and 32 seconds, exact enough that you can set your clock to it. Knowing a cepheid’s luminosity, which is related to its period of pulsation, allows an astronomer to calculate its distance, which has become enormously helpful in determining distances in the universe.
- S Doradus: S Doradus is one of the brightest known stars in the universe. It lies in the Large Magellanic Cloud and its radiation pressure is blowing off much of its mass in a strong stellar wind. S. Doradus will eventually end its life in a supernova. S. Doradus belongs to a type known as a lumimous blue variable. S. Doradus is considered an eruptive variable.
There are many other variable stars, many with unique features that affect their variability. The American Association of Variable Star Observers (AAVSO) is for people who enjoy tracking the change in brightness over time of the many variable stars in the sky.
