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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Variable star | 3/10 | https://en.wikipedia.org/wiki/Variable_star | reference | science, encyclopedia | 2026-05-05T13:33:52.261350+00:00 | kb-cron |
Intrinsic variable stars The variability is being caused by changes in the physical properties of the stars themselves. This category can be divided into four subgroups: Pulsating variables, stars whose radius alternately expands and contracts as part of their natural evolutionary aging processes. Eruptive variables, stars who experience eruptions on their surfaces like flares or mass ejections. Cataclysmic or explosive variables, stars that undergo a cataclysmic change in their properties like novae and supernovae. X-ray variables, close binary systems with a hot mass-accreting compact object.
Extrinsic variable stars The variability is caused by external viewing perspectives like rotation or eclipses. There are two subgroups: Eclipsing binaries, double stars or planetary systems where, as seen from Earth's vantage point the stars occasionally eclipse one another as they orbit, or the planet eclipses its star. Rotating variables, stars whose variability is caused by phenomena related to their rotation. Examples are stars with extreme "sunspots" which affect the apparent brightness or stars that have fast rotation speeds causing them to become ellipsoidal in shape.
These subgroups themselves are further divided into specific types of variable stars that are usually named after their prototype. For example, dwarf novae are designated U Gem stars after the first recognized star in the class, U Geminorum. The population of stars in the Milky Way galaxy is divided into two groups based on their age, chemical abundances, and motion through the galaxy. Population I stars are limited to the flat plane of the galactic system, known as thin disk stars. These originate in open clusters and often display high abundances of elements produced by stellar fusion processes – their metallicity. The Population II stars are more often distributed in the thick disk, the galactic halo, globular clusters, and galactic bulge. These are much older stars that show lower abundances of elements more massive than helium. In some cases the classification system of variable stars and their behavior is determined by their population membership.
== Intrinsic variable stars ==
Examples of types within these divisions are given below.
=== Pulsating variable stars ===
Pulsating stars swell and shrink, affecting their brightness and spectrum. Pulsations are generally split into: radial, where the entire star expands and shrinks as a whole; and non-radial, where one part of the star expands while another part shrinks. Depending on the type of pulsation and its location within the star, there is a natural or fundamental frequency which determines the period of the star. Stars may also pulsate in a harmonic or overtone which is a higher frequency, corresponding to a shorter period. Pulsating variable stars sometimes have a single well-defined period, but often they pulsate simultaneously with multiple frequencies and complex analysis is required to determine the separate interfering periods. In some cases, the pulsations do not have a defined frequency, causing a random variation, referred to as stochastic. The study of stellar interiors using their pulsations is known as asteroseismology. The expansion phase of a pulsation is caused by the blocking of the internal energy flow by material with a high opacity, but this must occur at a particular depth of the star to create visible pulsations. If the expansion occurs below a convective zone then no variation will be visible at the surface. If the expansion occurs too close to the surface the restoring force will be too weak to create a pulsation. The restoring force to create the contraction phase of a pulsation can be pressure if the pulsation occurs in a non-degenerate layer deep inside a star, and this is called an acoustic or pressure mode of pulsation, abbreviated to p-mode. In other cases, the restoring force is gravity and this is called a g-mode. Pulsating variable stars typically pulsate in only one of these modes.
==== Cepheids and cepheid-like variables ====
The Hertzsprung–Russell diagram is a scatter plot of stars showing the relationship between the absolute magnitude and the spectral class (luminosity vs. effective temperature). Most ordinary stars like the Sun occupy a band called the main sequence that runs from lower right to upper left on this diagram. Several kinds of these pulsating stars occupy a box called the Cepheid instability strip that crosses the main sequence in the region of A- and F-class stars, then proceeds vertically and to the right on the H–R diagram, finally crossing the track for supergiants. These stars swell and shrink very regularly, caused by the star's own mass resonance, generally by the fundamental frequency. The Eddington valve mechanism for pulsating variables is believed to account for cepheid-like pulsations. The pulsational instability of Cepheid variables correlates with variations in the spectral class, effective temperature, and surface radial velocity of the star. Each of the subgroups on the instability strip has a fixed relationship between period and absolute magnitude, as well as a relation between period and mean density of the star. The period-luminosity relationship makes these high luminosity Cepheids very useful for determining distances to galaxies within the Local Group and beyond. The Cepheids are named only for Delta Cephei, while a completely separate class of variables is named after Beta Cephei.
Classical Cepheid variables
Type I cepheids, also called Classical Cepheids or Delta Cephei variables, are evolved population I (young, massive, and luminous) yellow supergiants which undergo pulsations with very regular periods on the range of 1–100 days. They are relatively rare stars with hydrogen-burning progenitors that had 4 to 20 solar masses and temperatures above a B5 class. Their radial pulsations are driven by the high opacity of ionized helium and hydrogen in their outer layers. Because of their high luminosity, Classical Cepheids can be viewed in nearby galaxies outside the Milky Way. On September 10, 1784, Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of Cepheid variables. However, the namesake for classical Cepheids is the star Delta Cephei, discovered to be variable by John Goodricke a few months later.
Type II Cepheids