Click the icon to view galaxies of the Messier catalog
The icon shows M51, the Whirlpool Galaxy.
Spiral galaxies usually consist of two major components: A flat, large disk
which often contains a lot of interstellar matter (visible sometimes as reddish
diffuse emission nebulae, or as dark dust clouds) and
young (open) star clusters and associations, which
have emerged from them (recognizable from the blueish light of their hottest,
short-living, most massive stars), often arranged in conspicuous and striking
spiral patterns and/or bar structures, and an ellipsoidally formed bulge
component, consisting of an old stellar population without interstellar matter,
and often associated with globular clusters.
The young stars in the disk are classified as stellar population I, the old
bulge stars as population II.
The luminosity and mass relation of these components seem to vary in a wide
range, giving rise to a classification scheme. The pattern structures in the
disk are most probably transient phenomena only, caused by gravitational
interaction with neighboring galaxies.
Our sun is one of several 100 billion stars in a spiral galaxy, the
These are, in short, "spiral galaxies without spiral structure", i.e. smooth
disk galaxies, where stellar formation has stopped long ago, because the
interstellar matter was used up. Therefore, they consist of old population II
stars only, or at least chiefly. From their appearance and stellar contents,
they can often hardly be distinguished from ellipticals observationally.
Elliptical galaxies are actually of ellipsoidal shape, and it is now quite safe
from observation that they are usually triaxial (cosmic footballs, as
Paul Murdin, David Allen, and David Malin put it). They have little or no
global angular momentum, i.e. do not rotate as a whole (of course, the stars
still orbit the centers of these galaxies, but the orbits are statistically
oriented so that only little net orbital angular momentum sums up). Normally,
elliptical galaxies contain very little or no interstellar matter, and consist
of old population II stars only: They appear like luminous bulges of spirals,
without a disk component.
However, for some ellipticals, small disk components have been discovered, so
that they may be representatives of one end of a common scheme of galaxy forms
which includes the disk galaxies.
Often due to distortion by the gravitation of their intergalactic neighbors,
these galaxies do not fit well into the scheme of disks and ellipsoids, but
exhibit peculiar shapes. A subclass of distorted disks is however frequently
From their appearance, galaxies are classified in types as given above, as
spiral, lenticular, elliptical, and irregular galaxies, where spirals may be
further classified for the presence of a bar (S: spirals, SAB: Intermediate,
SB: Barred spirals). More precisely, ellipticals are sub-classified for
ellipticity from E7 (strongly elongated) to E0 (circular), and spirals for
prominence of bulge versus spiral arms from Sa (or SABa, SBa) to Sc or Sd.
This so-called Hubble Classification Scheme can well be illustrated by
Galaxies of all types, though of a wide variety of shapes and appearances, have
many basic common features. They are huge agglomerations of stars like our Sun,
counting several millions to several trillions.
Most of the stars are not lonely in space like our Sun, but occur in pairs
(binaries) or multiple systems.
The most massive galaxies are giants which are a million times more massive than
the lightest: Their mass range is from at most some million times that of our Sun
in case of the smallest dwarfs, to several trillion solar masses in case of
giants like M87 or M77.
Accordingly, the number of stars in them varies in the same range.
The linear size of galaxies also scatters, ranging from small dwarfs of few
thousands of light years diameter (like M32) to
respectable several 100,000 light years. Among the biggest Messier galaxies are
the Andromeda galaxy M31 and the bright active
Seyfert II galaxy M77.
Our Milky Way Galaxy, a spiral galaxy, is
among the massive and big galaxies with at least 250 billion solar masses (there
are hints that the total mass may even be as large as 750 billion to 1 trillion
times that of the Sun) and a disk diameter of 100,000 light years.
Besides very many individual stars, most galaxies contain the following
Galaxies normally emit light of every wavelength, from the long radio and
microwave end over the IR, visual and UV light to the short, high-enregy
X- and gamma rays. Interstellar matter is coolest and therefore best
visible in radio and IR, while supernova remnants are most conspicuous in
the high-energy part of the electromagnetic spectrum.
- Globular star clusters, large but quite
compact agglomerations of some 100,000 to several million stars. These
large clusters have about the same mass as the smallest galaxies, and are
among the oldest objects in galaxies. Often, they form conspicuous systems,
and occur at galaxies of every type and size. The globular cluster systems
vary in a wide range in richness between the individual galaxies.
- As the stars develop, many of them leave nebulous remnants
(planetary nebulae or
supernova remnants) which then populate the galaxies.
- While the older stars, including the globular clusters, tend to form an
ellipsoidal bulge, the interstellar gas and dust tends to accumulate
in clouds near an equatorial disk, which is often conspicuous (i.e.,
in spiral and lenticular galaxies).
- The interstellar clouds are the places of star formation.
More acurately, huge diffuse nebulae
are places where crowded
(open) clusters and associations
of stars are formed.
- A rather dense galactic nucleus, which is somewhat similar to a
"superlarge" globular cluster. In many cases, galactic nuclei contain
supermassive dark objects, which
are often considered as Black Hole candidates.
Some galactic nuclei are remarkably distinguished from the average: These
so-called Active Galactic Nuclei (AGNs) are intensive sources of light
of all wavelengths from radio to X-rays. The activities seen in the AGNs are
caused by gaseous matter falling into, and interacting with, the supermassive
central objects mentioned above, according to the current consensus of most
Sometimes, the spectra of these nuclei indicate enormous gaseous masses in
rapid motion; galaxies with such a nucleus are called
Seyfert galaxies (for their discoverer, Karl Seyfert).
M77 is the brightest Seyfert galaxy in the sky.
Few galaxies have even more exotic nuclei, which are extremely compact
and extremely bright, outshining their whole parent galaxy; these are called
quasars (an acronym for QUAsi-StellAR objects). From their properties,
quasars resemble extremely active Seyfert galaxy nuclei.
However, quasars are so rare and the nearest is so remote that the brightest
about 2 billion lightyears away in the constellation Virgo, is only of
magnitude 13.7, and none of them is in Messier's or even in the NGC or IC
Sometimes, at irregular intervals given by chance, in any type of galaxies,
a supernova occurs: This is a star suddenly brightning to a high luminosity
which may well outshine the whole galaxy; the maximal absolute magnitude of
a supernova may well reach -19 to -20 magnitudes. This remarkable phenomenon
has attracted the attention of many astronomers (equally both professionals
and amateurs), who observe galaxies regularly as they "hunt" supernovae.
Supernovae have been observed in
several Messier catalog galaxies.
Imagery and atlasses:
Of course, fine galaxy photos can be found in many more general astronomy books
- Allan Sandage. The Hubble Atlas of Galaxies.
Carnegie Institution of Washington, 1961.
185 superb black & white photographs of galaxies of all types,
obtained by the Mt. Palomar and Mt. Wilson Observatory telescopes, with
captions and data, and a technical and scientific introduction.
- James D. Wray. The Color Atlas of Galaxies.
Cambridge University Press, 1988. 3-color (UBV) images of 616 galaxies
(including all Messier galaxies but M89), taken with telescopes at the
McDonald Observatory, Texas, and the Cerro Tololo Interamerican Observatory,
Chile, with data and captions.
- Timothy Ferris. Galaxies. Sierra Club Books, San Francisco, 1980.
Superb book (look to get the more expensive full-size edition) with
color and b/w photographs of galaxies and some other objects, from various
Special observing Guides:
Most general Deep Sky Observing Guides are good as well.
- Dimitri Mihalas and James Binney. Galactic Astronomy.
W.H. Freeman, 1981 (probably out of print).
Now replaced by: James Binney and Michael Merrifield.
Galactic Astronomy. Princeton University Press, 1998.
This is a good introduction and review especially for the observational
properties of galaxies (as they were known at the time of publication).
- James Binney and Scott Tremaine. Galactic Dynamics.
Princeton Series in Astrophysics, Princeton University Press, 1987.
In-depth treatment of the physics of galaxies.
Some mathematical and physical background is required for this book.
- Richard Berendzen, Richard Hart, and Daniel Seeley.
Man Discovers the Galaxies. Science History Publications,
Neale Watson Academic Publications, New York 1976.
Some galaxies are isolated "island universes" which float lonely through an
otherwise empty region of the universe. But usually, space is too densely
crowded with them, so that they form groups of some galaxies (or some dozens
of galaxies), or even large clusters of up to several thousands of galaxies.
The galaxies of these groups are in mutual gravitational interaction which
may have significant influence on their appearance.
Last Modification: 25 Jan 1998, 14:08 MET