Cepheid Variables

Overview

Cepheid Variables are a class of stars that exhibit a regular and predictable pattern of brightness change, making them extremely important for astronomers.
Named after the star δ Cephei, whose variability was discovered in 1784 by English astronomer John Goodricke.

Cepheid Variables are giant or supergiant stars, much larger and brighter than the sun.
Their brightness changes are due to the pulsation of the star; it expands and contracts regularly, causing changes in temperature and size.
These variables have a very well-defined relationship between their pulsation period (the time taken to go through one cycle of high to low brightness) and their absolute luminosity (their true brightness).
This relationship is known as the Period-Luminosity Relation.

The longer the pulsation period of a Cepheid Variable, the more luminous it is.
Due to this relationship, once the period of a Cepheid Variable is measured, its luminosity can be estimated, which makes them excellent distance indicators.
By comparing the estimated absolute luminosity with the apparent brightness observed from Earth, astronomers can calculate the distance to the star.
This method of distance measurement is known as the Cepheid Distance Scale.

Cepheid Variables were keys in the determination of the size of the Milky Way and other galaxies, and the scale of the universe.
American astronomer Henrietta Leavitt, in 1908, established the relationship between the luminosity and period of Cepheid Variables – a significant milestone in astronomy.
Edwin Hubble used this relationship to determine that the Andromeda Nebula was in fact a separate galaxy, leading to the understanding that the universe extended far beyond the Milky Way.

There are two types of Cepheid Variables: classical Cepheids and type II Cepheids.
Classical Cepheids are young, massive stars, and are generally found in the spiral arms of galaxies.
Type II Cepheids are old, low-mass stars, and are generally found in globular clusters.
The period-luminosity relationship is slightly different for these two types, so astronomers must be careful to distinguish between them when determining distances.