Avariable star is astar that is sometimesbrighter ordimmer. Most stars have only very small differences in brightness, like theSun; but some stars can vary hugely.
There are generally two types of variable stars:
A star beingeclipsed, where the changes in brightness are fromperspective, are called extrinsic variables. For example, in asolar eclipse the moon gets between the sun and Earth. Somebinary star systems sometimes show eclipses, as do planetary systems with high-mass planets such as gas giants.
Stars whose brightness actually changes are called intrinsic variables. These stars appear to get bigger and smaller over time. Some pulsate at a constant rate, some do not. All of them change the rate at which energy is put out, which changes their appearance to us.
Mira. Typical of a class of stars with pulsation periods longer than 100 days. They arered giant stars in the very late stages of stellar evolution. They will expel their outer envelopes as planetary nebulae and becomewhite dwarfs in a few million years.
Protostars are young objects that have not yet completed the process of contraction from a gas nebula to a true star. Most protostars show irregular brightness variations.
Supernovae are the most dramatic events in theuniverse. A supernova can briefly emit as much energy as an entiregalaxy, brightening by more than 20 magnitudes (over one hundred million times brighter). Supernovae result from the death of an extremely massive star, many times heavier than the Sun.
The supernova explosion is caused by a white dwarf or a star core reaching a certain mass/density limit, theChandrasekhar limit. Then the star collapses in a fraction of a second. This collapse "bounces" and causes the star to explode and emit an enormous amount of energy.
The outer layers of these stars are blown away at speeds of many thousands of kilometers an hour. The expelled matter may formnebulae calledsupernova remnants. A well-known example of such a nebula is theCrab Nebula, left over from a supernova that was observed inChina andNorth America in 1054. The core of the star or the white dwarf may either become aneutron star (generally apulsar) or disintegrate completely in the explosion.
A supernova may also result from mass transfer onto awhite dwarf from a star companion in a double star system. The infalling matter pushes the white dwarf over the Chandrasekhar limit. The absolute luminosity of this type of supernova can be calculated from its light curve, so these explosions can be used to fix the distance to other galaxies. One of the most studied supernovae isSN 1987A in theLarge Magellanic Cloud.
.jpg)
