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



Absorption Nebula:
Another name for Dark nebula.

See Stellar Association.

Pattern of apparently neighbored, but physically unrelated stars, formed by a chance alignment of stars at different distances which happen to be situated in about the same direction.


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 thought to be 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. Also see Double Star.

Bipolar Nebula:
Nebula consisting of two rather symmetrical bright lobes with a star between them. They are thought to have been ejected along the star's polar axis, typically during the process of star formation.

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. It is sometimes thought to be divided in a central galactic nucleus and an outer galactic halo.


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.

Cometary Globule:
Small dark nebula (globule) appearing like a comet.

Cometary Nebula:
Small fan-shaped reflection nebula appearing like a comet.


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:
Bright nebula of irregular shape. Sometimes, arbitrary irregularly-shaped nebulae are called diffuse, such as fragments of and very old supernova remnants. But typically, the term describes a nebula consisting of interstellar matter, i.e. gas and dust, from which stars are forming, resulting in a young open star cluster. When some of the stars are hot enough, the nebula is shining from the emission of light by the excited atoms of its gas (emission nebula), and and also referred to as H II region. Otherwise, it reflects the light of the stars involved (reflection nebula).

The disk component of disk galaxies results from the overall angular momentum of these galaxies and shows overall rotation; the disk is typically very thin and about circular in shape. It is often consisted of young stellar population (Population I) and Interstellar Matter, and contains diffuse nebulae and young open star clusters.

Disk Galaxy:
Galaxy that contains a more or less pronounced disk component besides an ellipsoidally shaped bulge. The disk is a result of a significant overall andular momentum of these galaxies.

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.


Disk Galaxies oriented in space so that they are seen from near their equatorial plane are called "edge-on;" the contrary orientation is called "face-on." Edge-on galaxies look flat and elongated, often with an equatorial dark band or lanes of dust. Messier galaxies M65, M82, M98, M102 (NGC 5866), M104, and M108 are examples for galaxies oriented at least nearly edge-on.

Elliptical Galaxy:
Ellipsoidally-shaped galaxy, generally tri-axial, consisting primarily of an old stellar population (Population II); with properties similar to the bulges of disk galaxies. They have at best little overall angular momentum, and contain at best a small disk component.

Emission Nebula:
Nebula of arbitrary nature which shines from the emission of light by the atoms of its gas, which is excited by high energy radiation of stars involved.


Disk Galaxies oriented in space so that they are seen from near one of their poles are called "face-on;" the contrary orientation is called "edge-on." Face-on galaxies look round and often show structures in their disks prominently, e.g. as "Grand Design" spirals. Examples for at least almost face-on galaxies from Messier's catalogue are M33, M51, M61, M74, M77, M83, M91, M95, M99, M100, and M101.


Galactic Cluster:
Other name for Open Cluster.

Galactic Halo:
Outer region of a galaxy of any type; typically containing old Population II stars and globular star clusters, sometimes mixed up with somewhat younger stars from disrupted, "cannibalized" dwarf galaxies.

Galactic Nucleus:
Central region of a galaxy, typically composed of an old stellar population (Population II).

Largs system of stars and interstellar matter, typically containing several millions to some trillions of stars, with masses a few million to several trillion times that of our sun, and dimensions of a few 1,000s to several 100,000s lightyears. They come in a variety of flavors: Spiral, Lenticular (S0), Elliptical and Irregular Galaxies, and besides stars, typically contain various star clusters and nebulae.

Galaxy Cluster:
See Cluster of Galaxies

Globular Cluster, Globular Star Cluster:
Star Cluster of about spherical shape, containing some 10,000s to millions of stars of common origin, spread over a volume of severel tens to 100s lightyears. Typically of old age (Population II). Globular clusters are apparent in all types of galaxies; in disk galaxies, they typically populate the galactic halos.

Spiral Galaxies exhibiting prominent spiral patterns (spiral arms) are sometimes called Grand-Design spirals. Grand-design spirals are typically seen close to face-on orientation. The heavy spiral structure is typically caused by recent bursts of star formation, following interaction with neighboring galaxies. Examples from Messier's catalog include M51, M74, M83, M100, and M101.

Small dark nebula which occurs in or near starforming regions in diffuse nebulae.


H II Region:
Diffuse Nebula, typically with active star formation, in which hot stars cause the emission of light (emission nebula) by ionized hydrogen (H II).

Outer region around an object. In particular, see Galactic Halo.

Herbig-Haro Object, HH Object:
Small emission nebulae with high velocities (several 100 km/s), formed by bow shocks when fast-flowing jets of ejected material from a young star collides with interstellar matter. The matter is typically ejected along the rotation axis of the young star, and the emission occurs from recombination of ions behind the bow shock. They are named for American astronomer George Howard Herbig (1920-) and Mexican astronomer Guillermo Haro (1913-88).

HRD, Hertzsprung-Russell Diagram:
Diagram for stars after their absolute brightness (luminosity) versus their spectral class (color, surface temperature), named after its inventors, Hertzsprung and Russell.


Interstellar Matter, ISM:

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)


Milky Way:
Our Galaxy. A spiral galaxy, it consists of a flattened disk component, about 100,000 lightyears in diameter and about 5,000 light years thick, and an ellipsoidally-shaped bulge

Milky Way Patch:
Portion of the Milky Way that appears as a connected nebulous patch; consisted of stars, clusters and nebulae. This may be simply a lesser obscurred hole in the foreground dust, or a star cloud. A prominent example is M24.

Multiple Stars:
System of closely neighbored stars held together by their mutual gravity. In case of two stars, these are also called binaries or physical double stars, in case of three, triple stars, etc.


At older times, "Nebula" referred to any kind of non-stellar Deepsky Object. The current usage of the term describes an object consisting of Interstellar Matter, i.e. gas and/or dust. From their observational properties and spectra, they are sometimes classified as emission, reflection and absorption or dark nebulae. From their nature, they are better classified as star-forming or pre-stellar (typically diffuse nebulae or H II regions) and post-stellar nebulae (typically planetary nebulae and supernova remnants).


Central, dense core of an object. In particular, see Galactic Nucleus.


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:
Gaseous nebula ejected by a sunlike star at the end of its nuclear life, before becoming a White Dwarf star. Named so because they reminded William Herschel to the appearance of planet Uranus. Planetary Nebulae are short-living objects and dissipate into space in a few 10,000s of years.

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.

Post-Stellar Nebula:
Nebula that was created by one or more aged stars, typically at the end or late stages of their nuclear life, when the star ejected matter into the interstellar space. Such matter is typically enriched with heavier elements ("metals") created during the nuclear life of the parent star. Typical examples are Planetary Nebulae and Supernova Remnants, or Wolf-Rayet Nebulae.

Regularly pulsating radio sources, with periods of order seconds or less. They are actually fastly neutron stars with a hot spot, which emits radiation in a cone as the star rotates, and are observable if Earth and our solar system happens lie within the cone.




Red Giant:

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, the first to be identified as reflection nebula is the reflection nebula associated with the Pleiades M45.


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.

Star Cloud:
Region of a galaxy which is populated by stars, clusters and nebulae. Star clouds frequently occur in the arms of spiral galaxies, and in irregular galaxies. A prominent example is M24 in the Milky Way or NGC 206 in the Andromeda Galaxy M31.

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.

Star-forming Nebula:
Also Pre-stellar Nebula. Nebula, consisted of Interstellar Matter, from which stars are forming; typically H II regions or Diffuse Nebulae. Special processes in star formation also lead to particular classes like Bipolar Nebulae and Herbig-Haro Objects.

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 hydrogen 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):

Synchrotron Nebula:
Nebula emitting synchrotron radiation, i.e. radiation emitted by electrons accelerated in strong magnetic fields. Typically found in supernova remnants and cosmic jets.




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.


White Dwarf:

Wolf-Rayet Nebula:
Nebula ejected by a hot (35,000-100,000 K) massive star, called Wolf-Rayet star (after the discoverers of this type), at its later evolutionary stages. Wolf-Rayet stars are recognized from broad, bright emission lines in their spectra.




Hartmut Frommert
Christine Kronberg

[SEDS] [MAA] [Home] [Indexes]

Last Modification: May 11, 2003