Galaxies

Click the icon to view galaxies of the Messier catalog

>>Links;

>>References;

>>Galaxy Clusters

The icon shows M51, the Whirlpool Galaxy.

Spiral

Spiral galaxies usually consist of two major components: A flat, large disk

which often contains a lot of interstellar matter (visible sometimes as reddish

emission nebulae, or as dark dust clouds) and young

(open) star clusters and

associations which have emerged from them, 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

Milky Way.

Lenticular (S0)

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

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.

Irregular

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

occuring.

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

Messier’s galaxies:

Hubble Scheme with Messier galaxies
for HTML browser supporting tables (e.g., Netscape, IBM Webexplorer)
Hubble Scheme for HTML browsers
not supporting tables (Lynx, Mosaic)

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

typical objects:

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.

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.

Some galactic nuclei are remarkably distinguished from the average: These

so-called active galactic nuclei are intensive sources of light of all

wavelengths from radio to X-rays. 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 of them, 3C273 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 catalog.

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.

References

Imagery and atlasses:

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

observatories.

Of course, fine galaxy photos can be found in many more general astronomy books

also.

Special observing Guides:

Kenneth Glyn Jones (editor).
Webb Society Deep-Sky Observer’s Handbook, Volume 4,

Galaxies, 1981; Volume 6, Anonymous Galaxies, 1987.

Enslow Publishers, Hillside, NJ.

Most general Deep Sky Observing Guides are good as well.

Textbooks:

Dimitri Mihalas and James Binney. Galactic Astronomy.
W.H. Freeman, 1981 (probably out of print).

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.

Historical Review:

Richard Berendzen, Richard Hart, and Daniel Seeley.
Man Discovers the Galaxies. Science History Publications,

Neale Watson Academic Publications, New York 1976.