Alachua Astronomy Club: Deep Sky Glossary

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	Deep Sky Glossary	Member Astronomical League	Member International Dark-Sky Association
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Terms Associated with Deep Sky Objects

The following is a brief list of terms associated with deep sky (or space) objects. For more details, click on the name of the term to link to SEDS, Students for the Exploration and Development of Space (hosted by the University of Arizona Chapter at the Lunar and Planetary Lab).

absorption nebula See dark nebula.

barred spiral See sprial galaxy.

bright nebula An emission nebula or reflection nebula.

clusters and groups of galaxies A grouping of galaxies in space, linked by mutual gravitation. Their distribution in space in not uniform but they clump together on scales that may be millions of light years across. Shapes may be spherical and symmetrical, or quite irregular. Note: The term "galactic cluster" would seem logical for naming these groupings of galaxies. However, use of galactic cluster to signify a "group of galaxies" would conflict with its former usage— which designates an open cluster of stars.

dark (absorption) nebula A dark cloud of interstellar material, often in the form of a diffuse nebula. These clouds, if very cold, may contain simple molecules (molecular clouds). Although containing both gas and dust, the micro dust (10-100 nm) component can dim and often block the passage of short wave radiation (e.g., light) through the cloud though longer waves (infrared and radio) can usually pass through.

deep sky (Messier) objects A loose term referring to objects not belonging to our solar system. These objects include star clusters, nebulae and galaxies. Messier objects belong this category. Other catalogs of deep sky (or space) objects include the New General Catalog (NGC) and Index Catalogs (IC). Many other deep sky catalogs exist.

diffuse nebula Diffuse or "fuzzy" appearing clouds of interstellar gas and dust. Diffuse nebulae can include both bright (emission and reflection) and dark nebular components.

elliptical galaxy (E) A type of galaxy with an ellipsoidal shape and no spiral structure. Categorized from E0 (no disk or round to E7 (most elliptical). [The ellipticity is calculated from 10(a-b)/a where a = major axies and b = minor axis.] Other designations include dwarf elliptical (dE), giant ellipticals (D) and supergiant ellipticals (cD).

emission nebula A type of bright nebula consisting of glowing gas, primarily hydrogen, which can be excited or ionized by the action of ultraviolet radiation from nearby or embedded hot stars. Intense hydrogen emission can produce a pinkish color while other glowing gases can produce other hues (e.g., oxygen often gives rise to a greenish tinge).

extragalactic nebula A galaxy located beyond the Milky Way Galaxy. Term is no longer used. Because galaxies can look like diffuse clouds ("nebulae") if their stars are unresolved, early observers confused these objects with true galactic nebulae.

galactic cluster An open star cluster. Term no longer used since easy to confuse with clusters of galaxies. Originally named galactic clusters since these objects are often confined to the plane of the Milky Way Galaxy.

galaxy A typically huge group of hundreds of thousands of stars with many groups containing hundreds of billions or more stars. Stars are gravitationally bound and often distributed over many thousands of light years. Includes several types with a wide variety of forms including spirals (normal, S and barred, SB), lenticular (S0), elliptical (E) and irregular (I). (Other classifications schemes exist.) Formerly galaxies were confused with nebulae. Subsequently the term extragalatic nebula was used for galaxies but this term has become obsolete. (Galaxies have also been called "Island Universes.")

globular cluster A roughly spherical, moderately dense star cluster of hundreds of thousands or millions of stars often many hundreds of light years across. Most are moderately old (five to fifteen billion years.) Concentration Classes: I (most compact) to XII (least compact).
irregular galaxy (I) A type of galaxy that does not fit the form of a spiral galaxy or elliptical galaxy.

IC See Index Catalog

Index Catalog (IC) Either of two supplements to the New General Catalog of Nebulae and Star Clusters (NGC) compiled by J.L.E. Dreyer. The First Index Catalog (IC), published in 1895, added 1,529 newly discoverd object. The Second Index Catalog (IC II), published in 1908, added another 3,857 objects.

island universe A galaxy.

lenticular galaxy (S0) A type of galaxy intermediate between elliptical and spiral types showing a hint of a disk and with a shape resembling a convex lens.

Messier Catalog A catalog of about one hundred deep sky objects compiled by the French astronomer Charles Messier (1730-1817) with additions by his colleague, Pierre Mechain. Most Messier objects, identified by the prefix 'M' and their catalog number, are good objects for small telescopes. Most Messier objects now also carry NGC catalog numbers.

Milky Way Galaxy The name of our own galaxy in which the Sun resides.

nebula or galactic nebula (pl. nebulae) A cloud of interstellar gas and dust. (This term was also formerly applied to objects now called galaxies.) There are two basic types: bright (which include both emission nebulae and reflection nebulae) and dark nebulae (absorption). Nebulae are also categorized by form including diffuse nebulae (often "protostellar nebulae"), planetary nebulae and supernova remnants.

NGC See New General Catalog

New General Catalog of Nebulae and Star Clusters(NGC) A catalog of 7,840 non-stellar objects (galactic nebulae, star clusters and galaxies) compiled by J.L. E. Dreyer (publ. 1888). Two supplements, the Index Catalog (IC, publ. 1895) and the Second Index Catalog (IC II, publ. 1908) added 1,529 and 5,386 additional objects respectively. Many of the NGC objects were discovered by F.W. and J.F.W. Herschel and the NGC is thus a revised and expanded version of their General Catalogue of Nebulae and Galaxies (publ. 1864). Althought the NGC is no longer "new," objects in the Dreyer catalogs are still referred to by their NGC and IC numbers. Most Messier objects are included in these catalogs.

open (galactic) cluster A moderately loose star cluster of several hundred to several thousand stars distributed in a region a few hundred light years across. Most open star clusters are moderately young with ages approximated by their main sequence stars of earliest spectral class. (O5 = 2; B0 = 8; B5 = 70; A0 = 400; A5 = 1,000; F0 = 3,000; F5 = 10,000 million years.) Alternate name: galactic cluster (no longer in common use). Several classification schemes exist: Shapley: c (very loose and irregular); d (loose and poor); e (intermediately rich); f (fairly rich); g (considerably rich and concentrated). Another more complex scheme with three parts is due to Trumpler (1930): Concentration: I (Detached; strong concentration toward center); II (Detached; weak concentration toward center); III (Detached; no concentration toward center); IV (Not well detached from surrounding star field). Range in Brightness: 1 (Small range in brightness); 2 (Moderate range in brightness); 3 (Large range in brightness). Richness: p (Poor: Less than 50 stars); m (Moderately rich: 50 to 100 stars); r (Rich: More than 100 stars). An "n" following the Trumpler class indicates there is nebulosity associated with the cluster.

planetary nebula A type of emission nebula containing an expanding shell of gas surrounding very hot stars in a late stage of stellar evolution. Shapes include ring-like or circular structures, dumbbell-like and irregular. Planetary nebulae are of the emission type. Planetary nebulae are often typed by appearance (Vorontsov-Velyaminov System): 1 (Stellar Image); 2 (Smooth disk with a, brighter toward center; b, uniform brightness; c, traces of a ring structure); 3 (Irregular disk with a, very irregular brightness distribution; b, traces of ring structure); 4 (Ring structure); 5 (Irregular form, similar to a diffuse nebula); 6 (Anomalous form). More complex structures are designated as , "4+2" (ring and disk), "4+4" (two rings), and "3a+2" (irregular disk with irreg. brightness distribution plus smooth disk).

reflection nebula A type of nebula that is cool and shines because the dust within scatters the light from nearby or embedded stars. Hence, the cloud is not self luminous. Spectra of reflection nebulae mimic the spectra of the nearby stars since the dust within scatters ("reflects") starlight, though the blue starlight is scattered more effectively than the redder colors.

spiral galaxy A type of galaxy with spiral arms. Includes both normal spirals (S) and barred spirals (SB, spirals with a "central bar"). Both types are categorized into subtypes such as Sa and SBa (tightly wound arms and relatively large central bulge); Sb and SBb; and Sc and SBc (loose arms with a small central bulge). (Type Sd is even more extreme while Sm refers to no nuclear bulge.) Spiral galaxies are also typed by coherence and strength of the spiral arms: I (well formed globular spiral) to V.

star cluster A group of physically associated stars bound by gravitation and presumably having a common origin. Associations (loose collections of usually young stars), open (or galactic) clusters and globular clusters belong to this group.

supernova remnant (SNR) A type of emission nebula consisting of expanding shells of material caused by the ejection of huge amounts of stellar material ejected from catastrophic stellar explosions (supernova).

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