Cepheid variable stars have a regular cycle of expansion and contraction, affecting their brightness. The cycle length increases with brightness, allowing astronomers to calculate their actual brightness and measure distances to other galaxies. They contain helium, causing regular trends in brightness. There are two types of Cepheids, with classical Cepheids being more useful for determining galactic distances. Henrietta Leavitt discovered the fixed relationship between brightness and cycle length. Cepheid variables confirmed the universe extends beyond our galaxy and provided evidence for the universe’s expansion.
In astronomy, Cepheid variable stars are variable stars whose brightness changes characteristically and regularly over a certain period. Normally, the outward pressure from nuclear fusion in a star’s center is balanced by the inward pressure from the star’s gravity, and the star remains at a constant size and luminosity. Variable stars go through a cycle of expansion and contraction which affects their brightness. In Cepheid variable stars, the cycle length increases with the star’s brightness in a predictable way, so that when the period is measured, astronomers can tell the Cepheid’s actual brightness, and from its apparent brightness on Earth, calculate how bright it is. These variable stars are an important tool for measuring distances to other galaxies.
These stars are thought to expand and contract in a regular cycle due to the properties of helium, which they contain in large quantities. When helium is fully ionized, it is less transparent to electromagnetic radiation, causing it to heat up and expand. As it expands, it cools and becomes less ionized, absorbing less heat and contracting. The result is a regular trend of expansion and contraction, with parallel variations of brightness, having a period ranging from one to about 50 days.
There are two main types of Cepheid variables. Type I, or classical Cepheids, are relatively young and very bright stars containing a relatively large proportion of heavier elements, indicating that they formed in regions where these elements were created by supernova explosions of older stars. Type II Cepheids are older stars, less luminous and poor in heavy elements. There are also Abnormal Cepheids, which have more complex cycles, and Dwarf Cepheids. Classical Cepheids, due to their greater luminosity and simple, regular cycles, are more useful to astronomers in determining galactic distances.
The regular variations in brightness and the fixed relationship between brightness and cycle length were discovered by astronomer Henrietta Leavitt in 1908 while studying these stars in the Small Magellanic Cloud, a small nearby galaxy. The term Cepheid variables comes from one of the stars studied by Leavitt, called delta Cephei. Since it was possible to determine the effective luminosity of a Cepheid variable from its period, it was also possible to determine its distance from the fact that the amount of light reaching the Earth is inversely proportional to the distance from the source. Such objects of known luminosity are known as “standard candles”.
Comparing the results of these calculations for Cepheid variables within our galaxy with the distances calculated from parallax confirmed that the method worked. Type I Cepheids are up to 100,000 times more luminous than the Sun. This means they can be detected, by Earth-based telescopes, in other galaxies up to about 13 million light-years away. The Hubble Space Telescope was able to detect these stars at a distance of 56 million light years. Cepheid variables confirmed in the early 20th century that the universe extended far beyond our galaxy, which was just one of many.
These stars have also provided the first strong evidence that the universe is expanding. In 1929, Edwin Hubble compared distance measurements of a number of galaxies, obtained using Cepheid variables, and redshift measurements, which indicated how fast they were receding from us. The results showed that the galaxies’ receding velocities were proportional to their distance and led to the formulation of Hubble’s Law.
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