A slow irregular variable (ascribed the

GCVS The General Catalogue of Variable Stars (GCVS) is a list of variable stars. Its first edition, containing 10,820 stars, was published in 1948 by the Academy of Sciences of the USSR and edited by B. V. Kukarkin and P. P. Parenago. Second and thi ...

types L, LB and LC) is a

variable Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

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 about them, they are reclassified into other categories such as

semiregular variable In astronomy, a semiregular variable star, a type of variable star, is a giant or supergiant of intermediate and late (cooler) spectral type showing considerable periodicity in its light changes, accompanied or sometimes interrupted by various irre ...

Nomenclature

Irregular variable stars were first given acronyms based on the letter "I": ''Ia'', ''Ib''. and ''Ic''. These were later refined so that the I codes were used "nebular" or "rapidly irregular" variable stars such as T Tauri and Orion variables. The remaining irregular stars, cool slowly varying giants and supergiants of type Ib or Ic were reassigned to Lb and Lc. When the

General Catalogue of Variable Stars The General Catalogue of Variable Stars (GCVS) is a list of variable stars. Its first edition, containing 10,820 stars, was published in 1948 by the Academy of Sciences of the USSR and edited by B. V. Kukarkin and P. P. Parenago. Second and thi ...

standardised its acronyms to be all uppercase, the codes LB and LC were used.

Type Lb

''Slow irregular variables of late spectral types ( K, M, C, S); as a rule, they are giants'' The GCVS also claims to give this type to slow irregular red variables where the luminosity or spectral type is not known, although it also uses the type L for slow irregular red stars where the spectral type or luminosity is unclear. The K5 star CO Cygni is given as a representative example.

Type Lc

''Irregular variable supergiants of late spectral types having amplitudes of about 1 mag in V'' The M2 supergiant TZ Cassiopeiae is given as a representative example.

List

Other irregular variables

There are a number of other types of variable stars lacking clearly detectable periods, and which are sometimes referred to as

irregular variable 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 categor ...

s: *

γ Cassiopeiae variable A shell star is a star having a spectrum that shows extremely broad absorption lines, plus some very narrow absorption lines. They typically also show some emission lines, usually from the Balmer series but occasionally of other lines. The bro ...

s, eruptive

shell star A shell star is a star having a spectrum that shows extremely broad absorption lines, plus some very narrow absorption lines. They typically also show some emission lines, usually from the Balmer series but occasionally of other lines. The broa ...

s *

Orion variable An Orion variable is a variable star which exhibits irregular and eruptive variations in its luminosity and is typically associated with diffuse nebulae. It is thought that these are young stars which will later become regular, non-variable stars o ...

s, pre-main-sequence stars, including

T Tauri star 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 ...

s and YY Orionis stars *Rapid irregular variables, possibly similar to Orion variables with shorter period *Poorly-defined irregular variable stars, of unknown type In addition, many types of eruptive or

cataclysmic variable In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since ones with an outburst brightness visible to ...

are highly unpredictable.

References

External links

Classifying variable stars
- Astronomical Society of South Australia
Why observe variable stars
- British Astronomical Association {{Variable star topics *