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.
Buldge:
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.
Disk:
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.
Galaxy Cluster:
See Cluster of Galaxies
Globular Cluster,
Globular Star Cluster:
HRD, Hertzsprung-Russell Diagram:
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)
Nova:
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.
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.
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.
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:
Strictly speaking, every star is physically variable over timescales of its evolution.
Last Modification: 25 Sep 1999, 13:35 MET