Cepheid variable stars were named after the first of their kind observed, δ Cepheus. There are actually two classes of Cepheid: Type I Cepheids (δ Cepheus is a classical Cepheid) are population I stars with high metallicities, and pulsation periods generally less than 10 days. Type II Cepheids (W Virginis stars), are low-metallicity, population II stars with pulsation periods between 10 and 100 days. All Cepheids are luminous, yellow, horizontal branch stars that lie in the instability strip of the Hertzsprung-Russell diagram. Instabilities which cause their size and temperature to change give rise to the periodic variations in their luminosity.
: Courtesy of AAVSO
In 1907, Henrietta Leavitt discovered that Cepheid variable stars
in the Small Magellanic Cloud
pulsated at a rate which depended solely on their absolute magnitude
. This period-luminosity relationship
(shown right) allows Cepheids
to be used as standard candles (once the pulsation period
is known) to estimate distances to the objects in which they are located. In fact, Cepheid
formed the first non-direct method of distance
determination, and established the first rung of the distance
ladder. All subsequent rungs in the ladder use Cepheid
distances as a stepping stone.
In the 1920s, Edwin Hubble used variable stars
to measure distances to nearby galaxies
. At the time it was thought that all of these variables were Type I Cepheids
, but in reality the sample also included RR Lyrae
and W Virginis stars
. Although each of these stellar
types possesses a different period-luminosity relationship
, Hubble was able to determine that the Universe
was expanding, though his estimate of the rate of expansion (called the Hubble constant in his honour) was almost a factor of 10 times larger than the value accepted today.