supermassive black hole - A black hole with a mass much much greater than the most massive stars. It is thought that the central regions of almost every galaxy contain a supermassive black hole of a million solar masses (A solar mass is the mass of our Sun = 1.989 x 1030 kg) or more.
supernova - Massive outburst, signifying the death of a star. One of the most energetic events of the universe. At its peak, a supernova may become 15 000 000 times as luminous as the Sun. If it occurs in our Galaxy, a very dim star may suddenly (and temporarily) out-shine the rest of the stars. The type I is caused by the sudden onset of huge thermonuclear explosion. This usually occurs when the material from a companion star falls (accretion) at a slow rate onto the surface of a white dwarf. The gradual build up of materials causes the white dwarf to collapse and heat up as a whole until the entire star blows up.
A faster rate may cause the infalling materials to explode all at one and hence a nova occurs instead. In type II supernova, the core of an old star no longer be able to sustain a fusion reaction. Consequently, the core becomes colder and the star collapse under its own gravity. This results in super-dense core to a point the massive gravitaional energy is released in the form of heat in a few seconds. The massive explosion pushes the stellar materials outward near to the speed of light.
supernova remnant - The expanding glowing remains from a supernova.
variable star - A star whose luminosity changes with time.
white dwarf - A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; such a star is near its final stage of life.
X-ray burster - X-ray source that radiates thousands of times more energy than our Sun, in short bursts that last only a few seconds. A neutron star in a binary system accretes matter onto its surface until temperatures reach the level needed for hydrogen fusion to occur. The result is a sudden period of rapid nuclear burning and release of energy.