Description: A subdwarf is a class of star that is fainter than a star of the same spectral type on the main sequence, but which is brighter than a white dwarf of the same temperature.

Subdwarfs typically fall into two distinct groups: cool subdwarfs and hot subdwarfs.

Most subdwarfs fall into the cool subdwarf group and lie one to two magnitudes below the main sequence on a Hertzsprung–Russell diagram due to their very low content of elements heavier than helium (called metals in astronomy). This changes how light and heat travel through their interiors and atmospheres, making them smaller, hotter, and fainter.

In the region of the Milky Way near the Solar System, subdwarfs are members of either the galactic halo or the older part of the galactic disk and were formed at an early stage of the Milky Way. This is because each generation of stars produces more metals through nuclear fusion, returning a portion of these to the interstellar medium, the gas reservoir from which stars form. As a result younger stars have more metals than older stars. Hence subdwarfs, which have fewer metals, are typically older stars.

There are also a small number of hot subdwarfs of spectral types O and B (known as sdO and sdB stars). Astronomers are not sure how these objects form but one possible reason is giant stars losing their outer layers for some reason. What causes the loss of these outer layers is uncertain but one possible reason is a giant star having its outer layers stripped off by a binary star companion.

Related Terms:

• B-type Star
• Binary Star
• Galactic Halo
• Giant Star
• Hertzsprung–Russell (HR) Diagram
• Magnitude
• Main Sequence
• Milky Way
• Solar System
• White Dwarf
• Metal
• O-type Star
• Luminosity Class

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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