What is it about a variable star that astronomers, starting with Hubble, wish to measure?

What is it about a variable star that astronomers, starting with Hubble, wish to measure?








-the brightness and the period of its change





but why?|||Cepheid variables all pulsate, but at different rates.





It was discovered that the period of these variable stars (that is, the exact time between the pulsations) was proportional to the brightness of them. In other words, the longer the period, the brighter the star. Because of this relationship between the period and the brightness, we can tell how bright the star is from a certain distance (which is 32.6 light years).





Now, if we know the exact brightness of a star from that distance, and we also know how bright it appears to us, then its distance from us can be worked out.





This is why the period and apparent brightness are important. The period tells us its "absolute magnitude" from a distance of 32.6 ly, we can see its "apparent magnitude" from where we are, and there's a formula for using those two numbers to work out the distance.





This is why Cepheid variables are useful for working out hte distance to star clusters they belong to, and even close-by galaxies. Hubble found a variable in the "Andromeda Nebula", and when he measured its distance using the above relationship he found it was too far away to be in our galaxy. Therefore, it must have been a different galaxy altogether. Because the relatinship wasn't as well understood then, he was wrong about the distance, which was actually half again what Hubble had calculated. But that's how he did it.|||To determine the nature of the stars photosphere. Note that spectroscopic measurements were/are also taken to determine the 'chemistry' of the photosphere.





Both Luminosity and Spectral analysis is used to determine if the star is a multiple system. We can only measure the mass of a staff if it is a binary or multiple system.





Edit: There are more variable stars than just cepheids! Hubble did attempt to use Cepheids to measure the distance to nearby galaxies BUT misidentified most of them and got the distances wrong. There are other standard candles classified as Variables such as RR Lyrae and Supernova.|||Period, changes in spectum, changes in luminosity.





All of these measurements can lead to diagnosis of the star's "problem".