SDB

A B-type subdwarf (sdB) is a kind of subdwarf star with spectral type B. They differ from the typical subdwarf by being much hotter and brighter. They are situated at the "extreme horizontal branch" of the Hertzsprung–Russell diagram. Masses of these stars are around 0.5 solar masses, and they contain only about 1% hydrogen, with the rest being helium. Their radius is from 0.15 to 0.25 solar radii, and their temperature is from 20,000 to 40,000K.

These stars represent a late stage in the evolution of some stars, caused when a red giant star loses its outer hydrogen layers before the core begins to fuse helium. The reasons why this premature mass loss occurs are unclear, but the interaction of stars in a binary star system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two white dwarfs. The sdB stars are expected to become white dwarfs without going through any more giant stages.

Subdwarf B stars, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters, spiral galaxy bulges and elliptical galaxies. They are prominent on ultraviolet images. The hot subdwarfs are proposed to be the cause of the UV-upturn in the light output of elliptical galaxies.

Subdwarf B stars were discovered by Zwicky and Humason around 1947 when they found subluminous blue stars around the north galactic pole. In the Palomar-Green survey they were discovered to be the commonest kind of faint blue star with a magnitude over 18. During the 1960s spectroscopy discovered that many of the sdB stars are deficient in hydrogen, with abundances below that predicted by the big bang theory. In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung–Russell diagram.

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