5.7 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Variable star | 10/10 | https://en.wikipedia.org/wiki/Variable_star | reference | science, encyclopedia | 2026-05-05T13:33:52.261350+00:00 | kb-cron |
These stars are high temperature analogs of α2 CVn variables, consisting of magnetic chemically peculiar Bp stars with effective temperatures above 10,000 K. They exhibit brightness fluctuations of some 0.1 magnitude caused by chemical spots, with a cyclic period matching the rotation rate.
Optically variable pulsars
Few pulsars have been detected in visible light. These neutron stars change in brightness as they rotate. Because of the rapid rotation, brightness variations are extremely fast, from milliseconds to a few seconds. The first and the best known example is the Crab Pulsar. The exact cause of this pulsed emission is unclear, but it may be related to synchrotron radiation of electrons in the outer magnetosphere.
=== Eclipsing binaries ===
Extrinsic variables have variations in their brightness, as seen by terrestrial observers, due to some external source. One of the most common reasons for this is the presence of a binary companion star, so that the two together form a binary star. When seen from certain angles, one star may eclipse the other, causing a reduction in brightness. One of the most famous eclipsing binaries, and the first to be discovered, is Algol, or Beta Persei (β Per). Detached, double-lined eclipsing binaries are a useful tool for testing the validity of stellar evolution models. By examining the spectral types of the components and their combined light curve, the masses and radii of both stars can be precisely determined. Under the assumption that both stars formed at the same time, the model can then be used to extrapolate their history and see if it matches the current radii of the components.
==== Algol variables ====
Algol variables undergo eclipses with one or two minima separated by periods of nearly constant light. The prototype of this class is Algol in the constellation Perseus. Most systems with Algol-type light curves are detached binaries. However, some definitions of Algol variables apply to semi-detached binary systems, where one of the components has filled its Roche lobe and is transferring mass to the companion star. These are dubbed classical Algol systems, with Algol itself being one such semi-detached system.
==== Double Periodic variables ====
Double periodic variables (DPVs) are Algol-like, consisting of a semidetached binary system where the primary has at least seven times the mass of the Sun and the secondary, now with 1–3 times the mass of the Sun, is overflowing its Roche lobe. The primary component is orbited by a massive, optically-thick accretion disk. The system shows a long period of photometric variation that is about 35 times the orbital period. The smaller donor star is tidally locked to the orbital period, and this rapid rotation may be driving a magnetic dynamo. This in turn may be causing the longer period variation by modulating the mass transfer rate. An example of a DPV is V393 Scorpii.
==== Beta Lyrae variables ====
Beta Lyrae (β Lyr) variables are close eclipsing binaries, named after the prototype star Beta Lyrae, or Sheliak. The gravitational interaction of the components causes at least one member to form an ellipsoidal shape, resulting in a light curve that varies continually even outside the eclipse. In the Beta Lyrae system, one of the components is overflowing its Roche lobe, creating a mass exchange. This flow of material is adding features to the light curve. Many Beta Lyrae-type variables are also double periodic variables, including Beta Lyrae itself.
==== W Serpentis variables ==== W Serpentis is the prototype of a class of semi-detached binaries including a giant or supergiant transferring material to a massive more compact star. The latter star has an optically-thick accretion disk, and the mass transfer rate is higher than in Algol variables. They are characterised, and distinguished from the similar β Lyr systems, by strong UV emission from accretions hotspots on a disc of material. The light curve can be noisy and the orbital period is often variable.
==== W Ursae Majoris variables ====
The stars in the W UMa group of eclipsing variables show periods of less than a day. The components are so closely situated to each other that their surfaces are almost in contact with each other. They are termed over-contact eclipsing binaries, and they share a common envelope. As a result, the spectral type does not change during an orbit. Their orbit is circular and the components are in synchronous rotation with the orbital period.
=== Planetary transits ===
Stars with planets show brightness variations if their planets pass between Earth and the star. For the Sun, this occurs with the planets inferior to the Earth, namely Mercury and Venus. With exoplanets, these variations are much smaller than those seen with stellar companions, but are detectable using photometric observations with a sufficiently high signal-to-noise ratio. Examples include HD 209458, WASP-43, and all of the planets and planet candidates detected by the Kepler and TESS space-based missions.
== See also == Guest star Irregular variable List of transiting exoplanets List of variable stars
== References ==
== Further reading == Eddington, A.S.; Plakidis, S. (1929). "Irregularities of Period of Long Period Variable Stars". Monthly Notices of the Royal Astronomical Society. 90 (1). London, UK: 65–71. doi:10.1093/mnras/90.1.65. Retrieved February 17, 2023.
== External links == The American Association of Variable Star Observers GCVS Variability Types Society for Popular Astronomy – Variable Star Section