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Deep Sky Glossary

This glossary is to provide short definitions of terms related to Deep Sky astronomy. If you don't find the desired information here, or want further informations, please refer to the following online resources:

The Terms

0-9, A, B, C, D, E, F, G, H, I, J, K, L,
M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z



See Stellar Association


Barred Spiral Galaxy:
Special type of a Spiral Galaxy with a bar structure in the central region of the galactic disk of the galaxy. There are also barred lenticular (S0) and barred irregular galaxies. Like spiral arms, bars are comparatively short-living structures which are triggered to occur by interaction of the galaxy with its environment (neighbor galaxies).

Binary Star:
System of two stars which are bound together by their mutual gravity.

Bright Nebula:
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


See Star Cluster or Cluster of Galaxies

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.


Dark Nebula:
Dark cloud of dust which is visible only because it absorbs the light of celestial objects behind it.

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.

Diffuse Nebula:


Disk Galaxy:

Double Star:
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.


Elliptical Galaxy:

Emission Nebula:



Galactic Cluster:
Other name for Open Cluster


Galaxy Cluster:
See Cluster of Galaxies

Globular Cluster, Globular Star Cluster:


H II Region:

HRD, Hertzsprung-Russell Diagram:


Irregular Galaxy:




Lenticular Galaxy, S0 Galaxy:

LINER, Low-Ionization Nuclear Emission Region:
Galactic nucleus with a characteristic emission line spectrum, dominated by low-ionization states (O II, N II, S II) and only weak emission lines from higher-ionization states (He II, O III, N III). The spectrum indicates Seyfert-like activity in the nucleus, probably not related to stars, but either the massive central object in the nucleus, or shock waves generated by supernovae; the observed linewidths are similar to those observed in Seyfert galaxies and indicate rapid motion (Spectra as Seyfert 2, except for stronger low-ionization lines). Like Seyfert nuclei, LINERs are more abundant in disk galaxies (spirals and lenticulars) of early types S0, Sa, and Sb than in other types, but much more common. More on LINERs (NED Level 5)


Multiple Stars:





Open Cluster, Open Star Cluster:
Star Cluster of several dozen to several hundred, rarely few thousand stars filling a volume of several light years diameter. Mostly rather loose and composed of rather young stars.


Peculiar Galaxy:

Planetary Nebula:

Population, Stellar:
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.





Reflection Nebula:
Diffuse Nebula which shines by the light of nearby stars which is reflected by the dust particles the nebula contains. The brightest, most famous and earliest discovered reflection nebula is M78.


S0 Galaxy:
other name for Lenticular Galaxy

Seyfert Galaxy:
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 Text page.

Spiral Arm:

Spiral Galaxy:

Star Cluster:
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.

Starburst Galaxy:
A galaxy which experiences a current, or has experienced a recent burst, or outburst, of star formation, with star formation rates of up to about 100 times the normal rate. Consequently, starbursts produce large numbers of young stars, including high mass stars of spectral types O and B. Frequently these stars are obscured by interstellar dust, which is heatened by their radiation to a temperature of about 100 K, and therefore shines brightly in the infrared light. Starbursts are probably triggered by gravitational perturbations in encounters with neighboring galaxies. The most prominent example of a Starburst galaxy is M82.

Stellar Association:

Supercluster (of galaxies):

Supernova (SN):
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. The term "Supernova" was coined by Baade and Zwicky 1934. 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:

  1. The explosion of a massive star at the end of its life; these occurs only for young (population I) stars, and thus in disk galaxies, and generates types Ib, Ic, and II supernovae; Ib supernovae occur if the progenitor has been stripped off (by stellar winds [Wolf-Rayet] or binary interaction) its H mantle layer, Ic if also the He layer has been removed.
  2. The explosion of a white dwarf as it acquires too much mass in a binary system (occur in all types of galaxies), supposed reason for type Ia.
An older classification, proposed by Zwicky, also contained types III, IV and V which are now obsolete, where types III and IV are now regarded as variations of type II, and type V probably not as supernovae at all. Only one SN has ever been classified as type IV, SN 1961 in NGC 3003, which had a unique light curve. Classified as type V have been Eta Carina and a SN in NGC 1058.

Supernova Remnant (SNR):




Variable Star:
A star which varies notably in brightness. For some stars, this is caused by occultations in a binary system (eclipsing binaries) or rotation, while others vary phyically by pulsation, cataclysmic events like explosions, or other reasons.

Strictly speaking, every star is physically variable over timescales of its evolution.





Hartmut Frommert (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)

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Last Modification: 25 Sep 1999, 13:35 MET