Cepheids are a class of variable stars, named after the prototype star Delta Cephei in the constellation Cepheus. They were first discovered in 1784 by John Goodricke, who noticed that the brightness of this particular star varied over time.
Since then, many more cepheid variables have been found, and they have become important tools for astronomers. Cepheids are used to measure distances to other galaxies and to determine the expansion rate of the Universe.
Cepheid variables are giant stars with masses several times that of our Sun. They are very hot and bright, and their outer layers expand and contract as they pulsate. This causes their brightness to vary over time, with a typical period of a few days to weeks. The brighter a cepheid is, the longer its period will be.
These stars are thought to form in binary systems, with two stars orbiting each other closely. As one star ages and expands into a red giant, it can begin to transfer mass onto its companion star. This can cause the companion star to become more massive than it would otherwise be, and also causes it to spin faster due to conservation of angular momentum. This fast rotation can make the star’s outer layers expand further than they would otherwise, leading to an increase in luminosity and making the star appear bluer than it would otherwise be. The extra mass also makes the star’s core hotter and more luminous, which makes it pulsate faster and causes its light curve (the plot of brightness versus time) to be narrower than for an equivalent non-binary system without this extra mass transfer.