Above: a side-view of a spiral galaxy (type Sa). Notice the central bulge and the thin disc. Your Milky Way
Galaxy is also a spiral galaxy (type Sb/Sc) with a spiral disc of about 20 thousand million times the Sun's mass
and 5 thousand million times the Sun's luminosity! Your galaxy contains some 100 thousand million stars (10^11
stars) of which your Sun is but one. The Milky Way galaxy has a disc some 15 000 parsecs (15 kiloparsecs,
kpc. One parsec is 3.26 light years or about 3 x 10^13 kilometres (30 000 000 000 000 km).) The central bulge
of the Milky Way Galaxy is several kpc across. The disc is every thin relative to its diameter, and is similar in
proportions to an old vinyl record. The central bulge gives these galaxies a 'fried-egg' shape. The stars orbit
the galactic centre - the Sun is about 8 kpc from the centre of the Milky Way Galaxy at about 200 kilometres per
second (carrying its planets with it) and takes about 250 million years to complete one orbit. The known
Universe contains something like 125 billion galaxies, but the Universe is much bigger than that part which we
can ever see!
The picture above shows a (type Sa) spiral galaxy in plan view (rendered using Steve Colefax's Galaxy.inc for
Pov-Ray). The density of stars increases toward the galactic centre, making the bulge very luminous. At the
centre of the bulge is the galactic nucleus. The galactic nucleus contains a super-massive black hole, with
about one million times the Sun's mass in the case of the Milky Way Galaxy. Notice that the thin disc is made up
of spiral arms rich in massive young blue stars. Since these hot and massive blue stars are short-lived, such a
galaxy has undergone recent star formation, as is typical of the spiral arms of a spiral galaxy. Indeed, the disc is
rich in gas and dust (which also fills the spaces between the arms, but is cold here and not very luminous and
essentially invisible to optical light). A spiral pressure wave has been set up in the disc, compressing the gas in
the spiral arms and triggering this cold gas to collapse under its own gravity, heating up as it does so, and
fragmenting into newly formed stars. This spiral wave probably travels very slowly through the disc, triggering
new waves of star formation as it goes. Notice also the dark dust lanes of cold dust clouds. These dust lanes
are also characteristic of spiral galaxies.
Types of Galaxies
Spiral galaxies are divided into a series of classes ranging from Sa to Sd. As we move along the series in
the order Sa, Sb, Sc, Sd the spiral arms become more open (less tightly wound) and the central bulge
becomes less prominent and the fraction of gas and young stars in the disc (and hence blueness)
increases. Spiral galaxies have rapidly rotating discs. About 50% of spiral galaxies possess a central bar
from which the arms extend, and these are classed SBa, SBb, SBc and SBd, with the properties changing
along the series in the same way as for non-barred spirals. The Milky Way Galaxy is generally classified as
a type Sc (or intermediate Sbc) non-barred spiral galaxy, but it appears to have a small bar. This
classification scheme is shown in the diagram below (notice the central bars in SB type galaxies) along with
additional galaxy types described below. Sm galaxies (and their barred equivalents SBm galaxies) often
have the spiral reduced to a single stubby arm and are named after the Magellanic Spirals which are of
These are smooth, round and almost featureless galaxies. They have little cool gas and lack cool dust
lanes and have few young blue stars, but are instead populated mostly by older red stars. These galaxies
predominate in rich galaxy clusters, occurring in the densest regions of these clusters. The largest are the
cD galaxies, which have elliptical cores and enormous diffuse envelopes hundreds of kpc across. They
are up to 100 times more luminous than the Milky Way Galaxy.
Normal or giant ellipticals have luminosities a few times that of the Milky Way Galaxy and are tens of kpc
across. The stars in elliptical galaxies have little organised motion - their orbits are oriented randomly
(rather than near to a common plane as in spiral galaxies). Less luminous elliptical galaxies show less
random stellar motions and some appear to have embedded discs. The faintest are less than 10% of the
Milky Way Galaxy's luminosity. These less luminous ellipticals fall into two groups:
1. The rare compact ellipticals (e.g. M32).
2a. Faint diffuse dwarf elliptical (dE) galaxies.
2b. Less luminous dwarf spheroidal (dSph) galaxies, which are very diffuse.
'Lenticular' literally means 'lens-shaped' and lenticular (SO) galaxies have a rotating disc and a central
elliptical bulge, but no spiral arms and no extensive dust lanes. These galaxies are transitional between
ellipticals and spirals and occur mostly in regions of space that are densely populated with galaxies. SBO
galaxies have a central linear bar.
These are irregularly shaped small blue galaxies that lack spirals and other structures. Dwarf irregulars
have ample gas and are rich in young blue stars. Dwarf spheroidals are dwarf irregulars which have lost
or used-up all of their gas. These galaxies appear to have been disturbed, perhaps by frequent supernova
shock waves and recent star formation.
Coming soon: interacting galaxies, dark matter ...
Above: a spiral galaxy of type Sa (rendered by Bot in Pov-Ray) - the same galaxy as the one
shown edge-on at the top.