An irregular variable is a type of
variable star
A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...
in which variations in brightness show no regular
periodicity
Periodicity or periodic may refer to:
Mathematics
* Bott periodicity theorem, addresses Bott periodicity: a modulo-8 recurrence relation in the homotopy groups of classical groups
* Periodic function, a function whose output contains values tha ...
. There are two main sub-types of irregular variable: eruptive and pulsating.
Eruptive irregular variables are divided into three categories:
* Group I variables are split into subgroups IA (spectral types O to A) and IB (spectral types F through M).
*
Orion variables, GCVS type IN (irregular and nebulous), indigenous to star-forming regions, may vary by several magnitudes with rapid changes of up to 1 magnitude in 1 to 10 days, are similarly divided by spectral type into subgroups INA and INB, but with the addition of another subgroup, INT, for
T Tauri stars
T Tauri stars (TTS) are a class of variable stars that are less than about ten million years old. This class is named after the prototype, T Tauri, a young star in the Taurus star-forming region. They are found near molecular clouds and iden ...
, or INT(YY) for YY Orionis stars.
* The third category of eruptive irregulars are the IS stars, which show rapid variations of 0.5 to 1 magnitude in a few hours or days; again, these come in subgroups ISA and ISB.
Pulsating irregular giants or supergiants, called
slow irregular variable
A slow irregular variable (ascribed the GCVS types L, LB and LC) is a variable star that exhibit no or very poorly defined periodicity in their slowly changing light emissions. These stars have often been little-studied, and once more is learnt a ...
s, are all of late spectral types (K, M, C, or S), and classed as type L-LB for giants and LC for supergiants. How many of these are actually semi-regular variables that simply need more study, remains unclear.
References
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