An irregular variable
is a type of variable star
in which variations in brightness show no regular periodicity
. 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).
- So-called IN (irregular nebulous) variables, 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, 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, all of late spectral types (K, M, C, or S), are 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.