System of two stars which are bound together by their mutual gravity.
Luminous cloud or mass of gas or dust in space (Nebula) which either shines by its own light (emission nebula) or by reflecting light of nearby stars ( reflection nebula). Besides diffuse nebulae, Planetary Nebulae and Supernova Remnants are special types of bright emission nebulae.
Elliptical or spheroidal component of Disk galaxies, with most properties of elliptical galaxies: Consisted basically of old stars (Population II) filling an ellipsoidal volume
Cluster of Galaxies:
Group of physically neighbored and gravitationally bound galaxies. At least almost all galaxies are members of small groups (like our Local Group) or large clusters of galaxies (like the Virgo Cluster of Galaxies). Clusters of galaxies tend to form superclusters.
Deep Sky Object (DSO):
Celestial object beyond the solar system. In a closer sense, the term applies to nonstellar objects only, i.e., star clusters, nebulae, and galaxies.
Two stars situated close together in the sky, so that they may appear as one star with the naked eye, or under bad viewing conditions. These may be physically related binary stars or optical chance alignments of unrelated stars with different distances.
See Cluster of Galaxies
Globular Star Cluster:
HRD, Hertzsprung-Russell Diagram:
Various regions in galaxies are composed of different populations of stars: Young stars of second or third generation, enriched with heavy elements gained from earlier generation stars, form population I which is usually found in the disks and spiral arms of galaxies. Old stars of the first generation, populatiion II are typically located in the core and halo of galaxies. Elliptical galaxies are often made totally of population II stars, irregulars like the Magellanic Clouds of pure population I.
Galaxies (mostly spiral) with extremely bright small nuclei which show broad emission lines in their spectra. In Type I Seyfert galaxies, permitted lines have bright cores which are as broad as forbidden lines, and very wide wings indicating velocities of 5,000 to 10,000 km/s. In type II Seyfert galaxies, these wings are absent and lines are Doppler broadened corresponding to velocities of 500 km/s; type II Seyferts are often strong and variable X ray sources. The brightest Seyfert galaxy, of type II, is M77. Find more information in the Seyfert Galaxy Scholar page.
A group of stars, bound together by their mutual gravity, occupying a certain volume of space and showing common proper motion. Presumably the stars of a cluster have formed together at about the same time and within the same area of space from a diffuse nebula. Their HRD's are thus isochrones (lines, surfaces or states of constant time) of stellar evolution. One distinguishes open and globular star clusters.
Supercluster (of galaxies):
Stellar explosion which causes a star to flash up rapidly (hours) to the brightness of a whole galaxy (up to absolute magnitudes of about -19 to -20), to fade again slowly (over months) after some time. Classification from spectral analysis as Type I (no H lines) and II (contains H), where type I is further subdivided into Type Ia (spectrum contains Si lines), Ib (no Si, but Helium), and Ic (no Si, no He). While all supernovae of all 3 subtypes of type I have similar light curves, the light curves of type II give rise to classification of subtypes IIL (linear decrease) and IIP (brightness stays on a constant plateau for some time), and peculiar light curves like that of SN 1987A. Rare subtypes of Type II are II-b which has only little hydrogene in spectrum, and type II-n which has narrow emission lines on top of broad ones, and a slowly and lately declining light curve. There are two causes for supernova explosions:
Supernova Remnant (SNR):
Last Modification: 27 Mar 1998, 22:15 MET