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Anirregular variable is a type ofvariable star in which variations in brightness show no regularperiodicity. 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, forT Tauri stars, 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, calledslow irregular variables, 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.^[1]

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