Subdwarf
Unterdwarf (abbreviated sd from English subdwarf ) are stars of luminosity class VI. They are significantly weaker than "normal" main sequence stars with the same surface temperature and are therefore 1.5 to 2 magnitudes below the dwarf stars of the main sequence in the Hertzsprung-Russell diagram (hence the name). They have a smaller radius and a higher surface temperature than ordinary stars of comparable mass.
The term Unterzwerg was coined in 1939 by Gerard Peter Kuiper , who used it to describe a series of stars that were previously regarded as "intermediate white dwarfs ".
A distinction is made between the two classes, cool and hot sub-dwarfs , whose members are in completely different stages of development.
Cool sub-dwarfs
Cool sub-dwarfs are generally very old Population II stars , which mostly belong to the galactic halo of the Milky Way and have high speeds relative to the Sun. They are also relatively low in metal and are in the stable hydrogen burning phase . They have a surface temperature of around 2000 to 6000 K - accordingly they are divided into the spectral classes (sd) M to (sd) G.
Hot or blue sub-dwarfs
Hot or blue sub-dwarfs are helium-burning stars that only have a very thin hydrogen shell. Normally , a star in the red giant helium stage fuses under a massive hydrogen shell. According to the current state of research, the hot sub-dwarfs are the nuclei of such stars that have almost completely lost their hydrogen-rich shell. The masses of the hot sub-dwarfs are 0.46 solar masses with a small scatter , and they have radii of a few tenths of the sun. This is in stark contrast to the giant stars , which are in a similar phase of stellar evolution. Due to the burning of helium, they have a very high surface temperature: main sequence stars with similarly high surface temperatures (spectral classes O and B) have several solar masses.
Hot or blue sub-dwarfs are hotter than 10,000 K on their surface - accordingly they have spectral classes in the range from (sd) B to (sd) O.