Variable Stars, or "Stars That Change"

When looking at most stars, their brightness stays the same during our lifetime. However, some stars do change and these are called variable, or pulsating, stars. The time these stars take to change can be as short as a few hours or as long as a few months, but the brighter stars always take longer. If we measure this time, called the period, we can work out how bright the star actually is. We can then compare this to how bright the star looks from Earth, which lets us work out the distance to the star. This means we can use these stars to find the distances to the galaxies where they live. The stars usually used for these types of calculations are called Cepheids. These are very large bright stars and can be seen from a very long way away, so we can find the distance to very far-off galaxies.