Luminous blue variable

Luminous blue variables (LBVs) are rare, massive and evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness.

[1] The original Hubble Sandage paper contains a footnote that S Doradus might be the same type of star, but expressed strong reservations, so the link would have to wait several decades to be confirmed.

[6] LBVs are massive unstable supergiant (or hypergiant) stars that show a variety of spectroscopic and photometric variation, most obviously periodic outbursts and occasional much larger eruptions.

In their "quiescent" state they are typically B-type stars, occasionally slightly hotter, with unusual emission lines.

A few examples have been found where luminosity appears to change during an outburst, but the properties of these unusual stars are difficult to determine accurately.

[7] S Doradus typifies this behaviour, which has been referred to as strong-active cycle, and it is regarded as a key criterion for identifying luminous blue variables.

[8] Some LBVs have been observed to undergo giant eruptions with dramatically increased mass loss and luminosity, so violent that several were initially catalogued as supernovae.

If the star does not lose enough mass before the end of the LBV stage, it may undergo a particularly powerful supernova created by pair-instability.

[19] Luminous blue variable stars can undergo "giant outbursts" with dramatically increased mass loss and luminosity.

η Carinae is the prototypical example,[20] with P Cygni showing one or more similar outbursts 300–400 years ago,[21] but dozens have now been catalogued in external galaxies.

The historical η Carinae and P Cygni outbursts, and several seen more recently in external galaxies, have lasted years or decades whereas some of the supernova imposter events have declined to normal brightness within months.

A candidate luminous blue variable (cLBV) can be identified relatively quickly on the basis of its spectrum or luminosity, and dozens have been catalogued in the Milky Way during recent surveys.

[27] Recent studies of dense clusters and mass spectrographic analysis of luminous stars have identified dozens of probable LBVs in the Milky Way out of a likely total population of just a few hundred, although few have been observed in enough detail to confirm the characteristic types of variability.

In addition the majority of the LBVs in the Magellanic Clouds have been identified, several dozen in M31 and M33, plus a handful in other local group galaxies.