Rare, luminous variable star classes in the Hertzsprung-Russell diagram

Image credit & copyright: APOD, Rogelino Bernal Andreo
The Hertzsprung-Russell diagram is the fundamental decryption of stellar astrophysics. In the above example, Rigel is a B8Iab, namely a hot white (B8) supergiant (luminosity class I); midway in luminosity between the lesser and greater supergiants (ab). Rigel easily illuminates the “Witch Head” reflection nebula, just 14 parsecs away from it. The very modern HR diagram below is from the website of James B. Kaler, “Stars“. The full M, L and T dwarf spectral sequence is shown extending the main sequence (class V) to lower luminosities and masses. The locations of ultra-luminous massive stars such as eta Carinae and P Cygni, which are luminous blue variables (LBV), are also described together with ρ Cassiopeiae, a yellow hypergiant (YHG). The former are among the most extreme stars known and undergo huge eruptive outbursts. The latter is likely in a short-lived phase of evolution thought to occur in 40 to 60 solar mass stars, as the star oscillates between being a red and a blue supergiant and spending only a little time in the so-named “yellow evolutionary void” in between. In the centre of the diagram are the more commonly known RR Lyrae and Cepheid variables, populating the instability strip.


The image below (courtesy ESO) contains another star which has been called a hypergiant (class Ia-0), ζ1 Scorpii. Only 6.5 Myr old, it is likely also on its way to becoming an LBV. It has a close line of sight companion, ζ2, which is actually nearer to us, and less intrinsically luminous.


Stellar remnants such as neutron stars and black holes lack photospheres and the temperatures measured for them by other techniques put them way off the left of the diagram. However another class of stars exists, the PG 1159 stars, for which photospheric temperatures can be measured conventionally using absorption lines, but which still fall way off the left side of the diagram: they can easily exceed 100,000 degrees Kelvin*. They are related to the central stars of planetary nebulae, and are evolving towards the white dwarfs (luminosity class VII or D). Lastly, and upon moving to the right of the diagram, the title contender for the lowest temperature red supergiant was certainly V838 Monocerotis. During recent years its temperature fell below the canonical limit for red giants (M10) down nearly to 1000 K, well into the regime normally reserved for brown dwarfs. It is now evolving back across the diagram to the left, reaching about 3700 K currently.

*Wolf-Rayet stars are often nearly as hot, but they only show emission lines so their temperatures have to be derived by more indirect methods. They are a post-red supergiant phase in the evolution of the O stars.


Note: the most extreme red supergiants (or hypergiants) are so diffuse their sizes are difficult to define. They are all losing mass at high rates, as suggested by this image of VY CMa in polarised light. Just as clearly showing mass loss, the image at right shows a shell of molecular material detached from the Mira-type variable T Leporis, observed interferometrically using the full complement of VLTI.


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