Orion Glossary

An historical, logarithmic scale devised to represent the observed brightness of astronomical objects such as stars.

The portion of the lifetime of a star during which it produces energy via the fusion of hydrogen to helium in its core. A star will remain on the Main Seqeunce for the majority of its total lifetime.

Extra-terrestrial object that is observed by the electromagnetic radiation it emits as it enters the Earth's atmosphere. 'Shooting stars' are visible meteors.

An extra-terrestrial rock (generally a fragment of an asteroid) that has fallen to the Earth’s surface.

An asteroid that has an orbit that brings it close to the orbit of the Earth (defined as any asteroid that comes within 1.3 astronomical units from the Sun). Abbreviated to NEA.

The process by which massive stars fuse a neon and helium nucleus together to produce a magnesium nucleus, releaseing energy in the process.

When a high-mass star runs out of nuclear fuel, it will undergo a type II supernova explosion and its core will collapse to form a neutron star. A neutron star has a mass similar to that of the Sun, but a radius of only about 10 km. A neutron star is supported against gravity by an effect of quantum physics known as neutron degeneracy pressure. See also white dwarf, black hole.

The fusing together of two atomic nuclei to make a larger nucleus. There is a decrease in the total mass of the nuclei in a fusion reaction, so this process results in the release of large amounts of energy, according to Einstein’s equation E = mc2.

Processes in which two or more nuclei are involved, resulting the creation of different nuclei from those originally present. See also nuclear fusion.

The process by which the nuclei of elements (other than hydrogen) are formed. There are believed to be three sites (or epochs) where (or when) nucleosynthesis occurs (or has occurred). Light nuclei, such as deuterium, helium and lithium, were formed in the early Universe when the Universe was between about 100 s and 1000 s old. Nuclear fusion inside the cores of stars is responsible for the formation of more helium nuclei, and also for the formation of other nuclei up to those with a mass around that of iron. Supernovae explosions are responsible for the formation of more massive nuclei.