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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:
- Chris Kronberg’s
- Our small Basic Astronomical Terms list
- Bill Arnett’s
Web Nebula Glossary
- Chris Kronberg’s
of Astronomical Teaching Texts
- or our (soon to come)
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,
0-9
A
B
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).
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
C
See Star Cluster or
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.
D
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,
Disk:
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.
E
F
G
Other name for Open Cluster
Globular Star Cluster:
H
HRD, Hertzsprung-Russell Diagram:
I
J
K
L
S0 Galaxy:
M
N
Nova:
O
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.
P
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.
Q
R
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.
S
other name for Lenticular 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 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:
- 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.
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.
T
U
V
W
X
Y
Z
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Last Modification: 27 Mar 1998, 22:15 MET