The icon shows M51, the Whirlpool Galaxy.
Physically, it is not necessary so clear (at least in the opinion of the present author) if this classification is real, because there exist intermediate types even between ellipticals and spirals, i.e. spiral galaxies often have an ellipsoidally formed "bulge" which may be very luminous (as in case of the Sombrero galaxy M104) or rather inconspicuous; some spirals seem to lack this component at all. A heavy bulge is often connected with the presence of a big ellipsoidal core. On the other hand, at least some ellipticals seem to house a disk component also; ths most conspicuous example of such a galaxy is probably Centaurus A (NGC 5128), a prominent galaxy in the Southern hemisphere which is not a Messier object because of its southern declination, but forms a group with the beautiful spiral M83. Centaurus A is regarded as peculiar. One may speculate that e.g. the disk around the center of M87, which is often regarded as the accretion disk around the supermassive object in that galaxy's nucleus, may be a smaller manifestation of the same disk phenomenon.
Focussing on the disk (or disk dominated) galaxies, these often show beautiful and conspicuous patterns in the form of spiral arms and/or luminous bars. These structures have been a mystery for a long time, it was thought that there may be physically different classes of disks (e.g., "normal" and "barred" disks), but now it seems as if they are all the consequences of gravitational interactions with neighboring galaxies. Encounters with neighbors cause inhomogenities and un-symmetries in the gravitational field within the disk, which tends to compress the gas in some regions. If the density of the gas in these regions exceeds a certain critical value (which depends on parameters as the temperature), star formation can take place, resulting in the formation of red diffuse emission nebulae and blue clusters of hot young stars, which slowly change their color to the yellow when they come to age, and their hottest stars have disappeared (i.e., exploded as supernovae).
The star forming regions tend to be aligned along spiral arms, as the denser regions in the interstellar matter apparently prefer to form such patterns. When getting older, they sometimes stay conspicuous as yellowish "fossil arms", which can be traced in several galaxies.
Galaxies are classified, according to their appearance, in the so-called Hubble scheme (after its inventor, Edwin Powell Hubble; see e.g. our illustration of the Hubble Scheme with Messier galaxies). This scheme defines the classes listed above, i.e. spiral, elliptical and irregular galaxies, and is especially interesting for spirals: Those with pronounced bar structures are called "barred spirals" and classified "SB", while normal spirals are simply called "S" or sometimes "SA"; some authors take "SAB" or "S(B)" for mixed types. Spiral galaxies, "normal" and barred, with conspicuous bulges (especially near their center) are classified "Sa" or "SBa", those which have prominent bulges and pronounced arms are clssified "Sb" or "SBb", and those which are dominated by the arms are "Sc" or "SBc". If the core and bulge seems to be lacking, a galaxy is classified "Sd" or "SBd", and those which have no pronounced core and show irregularities are classified as "Sm"; these represent a type between disk and irregular galaxies.
Some of the galaxies, mostly those who had no closer encounters for a longer period of time, and those who have lost most of their interstellar matter for some reason, do not show any conspicuous pattern within their disks; these are often called "S0" or "lenticular" galaxies. Although they are disks, they can often hardly be distinguished from ellipticals from their appearance, and have often been misclassified in the past. This misclassification happened to all the four Messier lenticulars in the past, and to many other galaxies of this type.
When undergoing a heavy interaction, or collision, with a massive neighbor, disk galaxies may be distorted very peculiarly, and then are often classified as irregular; this is the case for the only Messier irregular, M82.
All disk galaxies have a very different appearance, depending from what direction they are seen, or under which angle toward the line of sight (to us) their disk is inclined. According to this situation, they are either seen from their edge (or "edge-on"), or from near their equatorial plane, as thin, flat, linear and elongated patches, often with dusty structures along their equators, or almost from their poles so that we can see their disks "face-on". Tom Polakis has featured some edge-on galaxies.
Our Milky Way is one of the big and more massive spiral galaxies, and is of Hubble type Sbc, or perhaps SBbc if it should have a bar.
(Probable) S0 galaxies in Messier's catalog: M84, M85, M86, NGC 5866 (M102 ?).
The irregular galaxy M82 is also a distorted disk galaxy.
Last Modification: 25 Jan 1998, 16:26 MET