ESO Astronomical Glossary - B
Be star
Be stars are stars of spectral type B that have atypical spectra, suggesting the presence of a disc of material around the star at the location of its equator. The material is thought to be ejected from the star's atmosphere by its rotation, which is very rapid. In the Milky Way galaxy one in five B-type stars are observed to be of the Be class. Why some B stars become Be stars is unknown.
Big Bang
The Big Bang theory describes how the Universe emerged from a dense, hot state in a single giant explosion (a 'big bang'). The principle of the theory was first suggested as an explanation for the fact that the Universe appears to be expanding. The Cosmic Microwave Background Radiation that fills the entire Universe is thought to be the remaining energy from the explosion. While other theories about the origins of the Universe do exist, the Big Bang theory is the most widely supported by models and observations.
Big Bang Nucleosynthesis
Big Bang nucleosynthesis refers to the production of nuclei other than those of 1H (i.e. the normal, light isotope of hydrogen, whose nucleus consists of a single proton) during the early phases of the universe.
Binary star
Two stars that orbit around a common centre of gravity are called binary or double stars. At least half of all stars are in a pair. Systems containing more than two stars have also been observed.
Black hole
A black hole is a massive object or region in space that is so dense that within a certain radius, known as the event horizon, its gravitational force is so strong that not even light can escape from it. Although from their very nature black holes do not emit radiation for us to detect, material falling towards them can emit large amounts of energy. They can form either from the death of very massive stars following a supernova explosion, or from the collapse of large amounts of gas in the centre of galaxies.
Blazar
A blazar is a type of active galaxy, with the central region emitting huge amounts of energy thought to be powered by a supermassive black hole. Typical for blazars is the variability of the energy they emit. Blazars are thought to be among the most energetic objects in the Universe.
Blue giant star
Blue giants are stars weighing the equivalent of at least ten Suns. Usually of spectral type O or B, they are very hot and luminous, giving them a distinct 'blue' colour. Contrary to the regular use of the term giant star, they are not necessarily reaching the final stages of their life; the term 'giant' in this case was merely coined to indicate their extreme luminosity. The biggest blue giants are also referred to as supergiants.
Blueshift
See Doppler Effect.
Brown dwarf
Brown dwarfs are gaseous objects that form like stars but lack the necessary mass to sustain nuclear fusion in their core. Their mass is typically in the range between the masses of stars and planets. Brown dwarfs are not very luminous and continue to cool down and contract until they turn into compact dark objects that are not easily detected. They can be seen orbiting other stars or free-floating in space.
B-type star
See Spectral Type.