Variable Stars: Astronomers Classify Stars that Change Brightness


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Algol, the second brightest star in the constellation Perseus, is an eclipsing binary and one of the oldest known variable stars. Image credit: Johannes Hevelius in Uranographia 1690

Astronomers call any star that changes or varies in some way, usually brightness, 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.

Eclipsing Binary Systems

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 can not 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 may not actually vary in brightness, although they can.

Pulsating variables

Pulsating variables change in brightness because the star is changing in size. Astronomers call these size changes 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. Large regions with cooler dark spots cause brightness variations in these stars. 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 CVn 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 CVn 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. On red cool 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 variables

Cataclysmic variables 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. Type I supernovae occur 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.

These are the major types of variable stars, but there are many sub-classifications of each of these types.

This article was originally published on Suite101.

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