A Cepheid, more commonly referred to as a Cepheid variable, is a type of pulsating variable star whose periods of variability is directly related to their absolute luminosity. This precise relationship between the star's pulsation period and luminosity makes the stars invaluable indicators of astronomical distances. Today Cepheid variables are one of the methods astronomers use to determine the distances to celestial objects in the universe, known collectively as the cosmic distance ladder. They are seen as standard candles, that is astronomical objects that have a known luminosity. The best known and closest Cepheid variable to us is the current North Star Polaris.
Cepheid stars pulsate in regular intervals typically in the range from 1 to 100 days. The stars themselves are yellowish giants or supergiants, five to twenty times more massive than and up to 30,000 times as luminous as the Sun. This intense brightness allows for Cepheids to be measured in other galaxies for extra-galactic distances.
Discovery and History
The star Eta Aquilae was the first star to be discovered to have observable variability in 1784, and is the first known representative of a Cepheid. A few months later, Delta Cepheid was also discovered to be a regular variable star and became the namesake of Cepheid variables. It wasn't until 1908 though, that the regular period-luminosity relation of Cepheids were discovered when Henrietta Swan Leavitt observed the effect in her observations of thousands of variable stars in the Magellanic Clouds.
In the early 20th century it was debated wither or not other galaxies existed independent of the Milky Way, which accumulated in the 1920 Curtis-Shapley debate, of which the period-luminosity relationship of Cepheid variables was a point of contention. This debate was finally settled in the discovery of Cepheid variables in the Andromeda galaxy by Edwin Hubble in 1924, showing once and for all the Milky Way and the Universe were not synonymous, and that other galaxies do exist.
Characteristics and Causes of Cepheid variation
The cause of this regular variation in luminosity is the result of the Cepheid regularly oscillating between two states, compact and expanded. In the compact state increasing temperature and pressure builds up in the star, eventually causing it to expand and become more luminous. As the star expands to maximum, there pressure from within the star weakens considerably. At this point there is no longer enough pressure against gravity to maintain the expanded size, and the star contracts once again. This cycle repeats itself in regular, timed intervals.
The shape of the light curve of a Cepheid is commonly referred to as the shark fin when plotted as luminosity versus periods of time. This characteristic shape shows during the Cepheid's regular period of variation, there is a rapid rise to maximum brightness, followed by a brief period at peak brightness before a smooth and slow decline to minimum. Although this curve is considered the normal behavior of such a star, in observations and plotting, there is considerable scatter around such a curve.
Types of Cepheids
Cepheid variables fall into one of two categories, Type I, also known as classical Cepheids or Delta Cepheid stars, and Type II, also known as W Virginis stars. The Type I Cepheids are extreme population I stars located in the spiral arms of galaxies. Metal rich and some 4 times as luminous as Type II Cepheids, these stars typically have a variation period lasting 5 to 10 days in lenght. Type II Cepheids on the other hand are metal-poor population II stars. Their periods of variability typically ranges from 10 to 30 days in length, and are primarily found in elliptical galaxies, globular clusters, and galactic halos.