The Outer Galaxies
B
eyond the Local Group we come to other clusters of
galaxies, millions of light-years away. Not surprisingly,
it was Edwin Hubble who devised the first really useful
system of classification. The diagram which he produced
is often called the Tuning Fork, for obvious reasons.
There are three main classes of systems:- Spirals, from Sa (large nucleus, tightly wound
spiral arms) to Sc (small nucleus, loosely wound arms).
Our own Galaxy is of type Sb, while M51 in Canes
Venatici (the Hunting Dogs: Map 1) is Type Sa, and M33,
the Pinwheel in Triangulum, is a typical Sc galaxy. - Barred spirals, where the arms issue from the ends
of a sort of bar through the nucleus; they range from SBa
through to SBc in order of increasing looseness. It seems
that stars in a large rotation disk sometimes ‘pile up’ in a
bar-like structure of this sort, but it does not last for very
long on the cosmic scale, which is presumably why SB
galaxies are much less common than ordinary spirals. - Ellipticals, from E0 (virtually spherical) through
to E7 (highly flattened). Unlike the spirals, they have
little interstellar material left, so that they are more
highly evolved, and star formation in them has practically
ceased. Giant ellipticals are much more massive than any
spirals; for example, M87 in the Virgo cluster of galaxies
(Map 6) is of type E0 and is far more massive than our
Galaxy or even the Andromeda Spiral. On the other hand
many dwarf ellipticals, such as the minor members of our
Local Group, are very sparse indeed. It is not always
easy to decide upon the type of an elliptical; for example
a flattened system, which really should be E7, may be
end-on to us and will appear round, so we wrongly class it
as E0.- Irregulars. These are less common than might be
thought; they have no definite outline. M82 in Ursa Major
(Map 1) is a good example. The Magellanic Clouds were
formerly classed as irregular, but it now seems that the
Large Cloud, at least, shows definite signs of some faint
spiral structure.
The first spiral to be recognized as such was M51, the
Whirlpool – by Lord Rosse, who looked at it in 1845 with
his giant home-made telescope at Birr Castle in Ireland.
It is 37 million light-years away, and is not hard to find,
near Alkaid in the tail of the Great Bear. As with all
spirals, it is rotating; the arms are trailing – as with a
spinning Catherine-wheel. Apparently the arms of a spiral
are due to pressure waves which sweep round the system
at a rate different from that of the individual stars. The
added pressure in these waves triggers off star formation;
the most massive stars evolve quickly and explode as
supernovae, while the pressure waves sweep on and leave
the original spiral arms to disperse. If this is correct,
it follows that no particular spiral arm can be a permanent
feature.
Recent photographs of the Whirlpool taken with
ATLAS OF THE UNIVERSE
Hubble classification
of galaxies. There are
elliptical galaxies (E0 to E7),
spirals (Sa, Sb and Sc) as
well as irregular systems
which are not shown here.
There are many refinements;
for instance, Seyfert galaxies
(many of which are radio
sources) have very bright,
condensed nuclei.▲ Type E6. NGC205,
photographed in red light.
Its system appreciably
more elongated than
with NGC174, it is the
smaller companion of the
Andromeda galaxy and is
made up of Population II
objects.▲ Type E4. Dwarf galaxy
NGC147 in Cassiopeia,
magnitude 12.1. Typical
of the relatively small
systems, and made up
entirely of Population II stars,
so that there are no very
luminous main sequence
stars, and star formation
has ceased.▲ Type E0.M87 in Virgo,
magnitude 9.2, distance
41 million light-years. It is
a powerful source of radio
emission and seems to
contain a massive black
hole at its core.SBaE0
E4
E7
SaSBb SBcSb ScF Atl of Univ Phil'03stp 3/4/03 5:42 pm Page 200