A Cepheid variable is a variable star that pulsates radially, varying in both diameter and temperature causing changes in brightness with a well-defined stable period and amplitude. Cepheid variables are extremely luminous so very distant ones can be observed and measured and are used to measure distances in astronomy.
There are 2 subclasses of Cepheid variables which have different masses, ages and evolutionary histories:
Classical Cepheids (also known as Population I Cepheids, type I Cepheids or DeltaCepheid variables):
pulsate with very regular periods over days to months.
4 to 20 times more massive than the Sun and up to 100 000 times more luminous.
yellow bright giants or super giants (F6 to K2)
radii change by millions of kilometres during a pulsation cycle.
used to determine distances to galaxies in the Local Group and beyond and are used to establish the Hubble Constant.
Type II Cepheids (known as Population II Cepheids)
pulsate with periods between 1 and 50 days.
usually metal-poor, old, low mass objects (about half the mass of the Sun).
used to establish the distance to the Galactic Centre, globular clusters and galaxies.
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