| Active Galactic Nucleus (AGN) | Galactic nucleus emitting enormous amounts of energy. |
| adiabatic | Following the normal gas-laws. |
| astrometric binary | Binary system where only one star can be seen telescopically, but its oscillatory motion in the sky reveals that it is accompanied by an unseen companion. |
| baryonic matter | Matter as we know it. It consists of protrons, electrons and neutrons |
| binary system | System of two stars bound together by gravity that revolve around a common center of mass |
| bulge | The bright, dense star formation in the center of a galaxy. |
| Chandrasekhar-limit | The limiting mass for a white dwarf : 1.44 Msun. A star whose mass exceeds this limit will be forced to undergo further gravitational collapse to become a neutron star (or even a black hole), because its material will be unable to support itself against the force of gravity. |
| cluster | (star cluster), see Stellar system |
| collision time | Average time in which a star will collide once with another star or object with comparable mass. |
| core | The central region of a star, such as the sun, in which energy is generated by thermonuclear reactions. |
| dark matter | Matter around galaxies, not emitting light, which is the reason little is known about this matter. |
| degenerate matter | Matter in a highly dense form that can exert a pressure as a result of quantum mechanical effects. Degenerate matter occurs in white dwarfs and neutron stars. |
| density cusp | Increasing mass density towards a single point following an inverse power law (1/rn) with a finite value in such a single point. |
| dust | (interstellar dust), all kinds of matter existing between stars, mostly hydrogen. |
| dyadosphere | The sphere extending from the Schwarzschild radius to the Dyadosphere radius in which annihilation processes take place. |
| evaporation time | The time it takes before all stars in a cluster have vanished. |
| exotic matter | An unknown type of matter with unknown properties. |
| fuel | (here) All matter disappearing into a black hole making it heavier. |
| galactic core | See Galactic nucleus |
| galactic nucleus | The core of a galaxy, usually this means the bulge. |
| gamma-ray | A very high energy photon with a wavelength shorter than that of x-rays. |
| gamma-ray burst | An event in which gamma-rays are excited. |
| gas cloud | See Dust |
| halo | (Glowing) ring/sphere enclosing an object larger than this ring/sphere. |
| host galaxy | (here) Galaxy containing an MBH. |
| hydrostatic equilibrium | A state of equilibrium in which the inwardly directed gravitational force in a star just balances the outwardly directed gas and radiation pressure. The star is thus held together but supported against collapse. |
| hypernova | The collapse of the nucleus of a very large neutron star to a black hole, during which the outer layers are spinning around it. |
| isotropic | Having no preferred direction in space. |
| loss cone | The region for which the pericenter of the stellar orbit around a black hole lies inside the tidal radius. |
| Low-Mass Black Hole (LMBH) | Black hole with a mass comparable to Msun. |
| mass density | Mass divided by volume (m/V) |
| Massive Black Hole (MBH) | Black hole with a mass varying between 106 to 109 Msun. |
| massive entity | An extremely heavy object. |
| Msun | Solarmass = 1.99 x 1030kg |
| neutron star | A star that has undergone gravitational collapse to such a degree that most of its material has been compressed into neutrons. |
| non-baryonic matter | Matter that doesn't consist of neutrons electrons and protrons, but of other elementairy particles, like neutrinos. |
| nuclear black hole | MBH located exactly in the center (of rotation) of a galaxy. |
| optical counterpart | The optical images of a scenario that at first was discovered in the gamma-ray spectrum. |
| planetary nebula | An expanding and usually symmetrical cloud of gas that has been ejected from a dying star, possibly in the form of a red giant. |
| primordial black hole | Black hole having existed before galaxies existed. |
| proto-galaxy | Disk of dust in which the first stars of a full- grown galaxy are born. |
| pulsars | A rotating star that lightens up with a certain frequency. |
| quasars | An intense, pointlike source of light and radio waves that is characterised by large red shifts of the emission lines in its visible spectrum. |
| redshift | Cosmological star clusters are moving away from us with a certain speed; The wavelenght of the 'light' that reaches us is thus (Doppler) shifted to the red. |
| relaxation time | Time during which a cluster remains stable. |
| Schwarzschild radius | The radius of a black hole. |
| self consistent | Mathematic expression, following a closed, independent set of rules. |
| singularity | Object with extreme properties, (here) a possible black hole. |
| spectroscopic binary | Binary system that can be identified by periodic variations in the Doppler shift of the lines of their spectra |
| stellar system | A certain (stable) configuration of multiple stars. |
| triaxial | Only symmetric when looking at one axis at a time. |
| white dwarf | A star that has a mass below the Chandrasekhar-limit and has undergone gravitational collapse. White dwarfs are the final phase in the evolution of a low-mass star and result from gravitational collapse following the exhaustion of nuclear fuel. |