Re: what kind of stars are pulsating varible stars

Stars are basically just huge, spherical, balls of gas which are held together by their own gravity. Any such ball of gas can vibrate in a number of different ways, rather like a bell vibrating and producing a sound. The simplest kind of vibrations are just what astronomers call "radial pulsations", where the star just expands and contracts and expands again in a regular cycle.

So, if all stars can vibrate like that, why don't they? The answer is that they all do, at some level, but in the large majority of stars, such as the Sun, the size of the pulsations is very small and you have to study them very carefully to notice it.

However, in some stars the radial pulsations are large, and the whole star becomes alternately brighter and fainter during the course of a cycle. It seems that in these stars there's something which is amplifying the pulsations, rather like a bell which is being repeatedly struck to keep it vibrating (and making a noise). The mechanism causing this amplification is rather complicated, but basically what's happening is that as the star contracts the heat from its interior builds up, pushing the star out again, when it can cool, leading to another phase of contraction. You could think of this as being a bit like a geyser, where again heat builds up to a point where the geyser suddenly erupts, although in the case of a star, it happens in a more steady kind of way.

There are actually several classes of stars which become pulsating variables, most famous of which are the Cepheid variables, which were used by Edwin Hubble in the 1920s to estimate the size scale of the universe, and are still used by astronomers today to determine distances to nearby galaxies. Another famous kind of pulsating variable, similar to Cepheids, are the RR Lyrae stars.

Interestingly, when we observe such stars in optical light, the point at which they are brightest doesn't correspond to when the star is at its biggest, as you might think, but instead it corresponds more closely to when the star is at its hottest, which of course is when it is contracted.