Editing Variable star (section)

===Eruptive variable stars===

Eruptive variable stars show irregular or semi-regular brightness variations caused by material being lost from the star, or in some cases being accreted to it. Despite the name, these are not explosive events.

====Protostars====
{{Main|Pre–main-sequence star}}

Protostars are young objects that have not yet completed the process of contraction from a gas nebula to a veritable star. Most protostars exhibit irregular brightness variations.

=====Herbig Ae/Be stars=====
[[File:V1025 Tauri Taurus Molecular Nebula from the Mount Lemmon SkyCenter Schulman Telescope courtesy Adam Block.jpg|thumb|right|[[Herbig Ae/Be star]] [[V1025 Tauri]]]]
{{Main|Herbig Ae/Be stars}}
Variability of more massive (2–8 [[Sun|solar]] mass) [[Herbig Ae/Be stars]] is thought to be due to gas-dust clumps, orbiting in the circumstellar disks.

=====Orion variables=====
{{Main|Orion variable}}

Orion variables are young, hot [[pre–main-sequence star]]s usually embedded in nebulosity. They have irregular periods with amplitudes of several magnitudes. A well-known subtype of Orion variables are the [[T Tauri star|T Tauri]] variables. Variability of [[T Tauri star]]s is due to spots on the stellar surface and gas-dust clumps, orbiting in the circumstellar disks.

=====FU Orionis variables=====
{{Main|FU Orionis star}}

These stars reside in reflection nebulae and show gradual increases in their luminosity in the order of 6 magnitudes followed by a lengthy phase of constant brightness. They then dim by 2 magnitudes (six times dimmer) or so over a period of many years. ''[[V1057 Cygni]]'' for example dimmed by 2.5 magnitude (ten times dimmer) during an eleven-year period. FU Orionis variables are of spectral type A through G and are possibly an evolutionary phase in the life of ''[[T Tauri star|T Tauri]]'' stars.

====Giants and supergiants====

Large stars lose their matter relatively easily. For this reason variability due to eruptions and mass loss is fairly common among giants and supergiants.

=====Luminous blue variables=====
{{Main|Luminous blue variable}}

Also known as the [[S Doradus]] variables, the most luminous stars known belong to this class. Examples include the [[hypergiant]]s [[Eta Carinae|η Carinae]] and [[P Cygni]]. They have permanent high mass loss, but at intervals of years internal pulsations cause the star to exceed its Eddington limit and the mass loss increases hugely. Visual brightness increases although the overall luminosity is largely unchanged. Giant eruptions observed in a few LBVs do increase the luminosity, so much so that they have been tagged [[supernova impostor]]s, and may be a different type of event.

=====Yellow hypergiants=====
{{Main|Yellow hypergiant}}

These massive evolved stars are unstable due to their high luminosity and position above the instability strip, and they exhibit slow but sometimes large photometric and spectroscopic changes due to high mass loss and occasional larger eruptions, combined with secular variation on an observable timescale. The best known example is [[Rho Cassiopeiae]].

=====R Coronae Borealis variables=====
{{Main|R Coronae Borealis variable}}

While classed as eruptive variables, these stars do not undergo periodic increases in brightness. Instead they spend most of their time at maximum brightness, but at irregular intervals they suddenly fade by 1–9 magnitudes (2.5 to 4000 times dimmer) before recovering to their initial brightness over months to years. Most are classified as yellow supergiants by luminosity, although they are actually post-AGB stars, but there are both red and blue giant R CrB stars. [[R Coronae Borealis]] (R CrB) is the prototype star. [[DY Persei variable]]s are a subclass of R CrB variables that have a periodic variability in addition to their eruptions.

====Wolf–Rayet variables====
{{Main|Wolf–Rayet star}}

Classic population I Wolf–Rayet stars are massive hot stars that sometimes show variability, probably due to several different causes including binary interactions and rotating gas clumps around the star. They exhibit broad emission line spectra with [[helium]], [[nitrogen]], [[carbon]] and [[oxygen]] lines. Variations in some stars appear to be stochastic while others show multiple periods.

====Gamma Cassiopeiae variables====
{{Main|Gamma Cassiopeiae variable}}

[[Gamma Cassiopeiae]] (γ Cas) variables are non-supergiant fast-rotating B class emission line-type stars that fluctuate irregularly by up to 1.5 magnitudes (4 fold change in luminosity) due to the ejection of matter at their [[equator]]ial regions caused by the rapid rotational velocity.

====Flare stars====
{{Main|Flare star}}

In main-sequence stars major eruptive variability is exceptional. It is common only among the [[flare star]]s, also known as the [[UV Ceti]] variables, very faint main-sequence stars which undergo regular flares. They increase in brightness by up to two magnitudes (six times brighter) in just a few seconds, and then fade back to normal brightness in half an hour or less. Several nearby red dwarfs are flare stars, including [[Proxima Centauri]] and [[Wolf 359]].

====RS Canum Venaticorum variables====
{{Main|RS Canum Venaticorum variable}}

These are close binary systems with highly active chromospheres, including huge sunspots and flares, believed to be enhanced by the close companion. Variability scales ranges from days, close to the orbital period and sometimes also with eclipses, to years as sunspot activity varies.