OB star
OB stars are hot, massive star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s of spectral types O or B which form in loosely organized groups called OB associations. They are short lived, and thus don't move very far from where they were formed within their life. During their lifetime, they will emit copious amounts of ultraviolet radiation. This radiation rapidly ionizes the surrounding interstellar gas of the giant molecular cloud, forming an H II region
H II region
An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...

 or Strömgren sphere
Strömgren sphere
In theoretical astrophysics, a Strömgren sphere is the sphere of ionized hydrogen around a young star of the spectral classes O or B. Its counterpart in the real world are the H II-regions, a type of emission nebula, the most prominent of which is the Rosette Nebula...


In lists of spectra the "spectrum of OB" refers to "unknown, but belonging to an OB association so thus of early type".

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.