Galaxies
Introduction | Hubble Classification | Spirals | Ellipticals | Irregulars
ISM | Clusters of Galaxies | Superclusters | a
In the early years of the 20th Century, the prevailing view appears to have been that
there was only one
galaxy - our Milky Way. Although M31 and other galaxies had appeared on
Messier's List of nebulas ever since the 18th Century,
they appear not to have been widely recognized as galaxies as such in their own right.
In 1923, the 'Great Debate' was staged to try and solve the problem as to whether other
'Island Universes' did actually exist, as some astronomers were coming to believe. The
outcome of this debate appears to have been inconclusive.
It was the work of Hubble that finally brought the matter to a definite conclusion, in 1924 - he made use of Cepheid variables to deduce that M31 was external to our galaxy.
Today, in contrast, the Universe is assumed to contain around 100 billion
galaxies, each having typically 100 billion stars.
The above diagram is from Hubble's own book, The Realm of the Nebulae.
Broadly speaking, Hubble classified all galaxies into Spirals, Ellipticals and Irregular
Galaxies. Although originally there appeared to be a tendency to view
spirals and ellipticals as being different stages of a galaxy's evolution, spirals
and ellipticals now tend to be viewed as totally separate types of galaxies (there are however theories that
purport to make a connection between the two types, although these are coming from a 'different
direction' from
before).
Ellipticals are classified according to their degree of ellipticity
Spirals are first divided into Barred and Normal (plus a class called S0).
Each of these
sub-classes is then split into three types, as shown above.
The S0 is considered a 'spiral' with no arms.
Spirals contain both young and old stars, and a fair amount of dust and gas.
The adjacent image shows the Milky Way but its basic features are typical of all spiral galaxies
The spiral arms actually delineate regions of bright objects and stars rather than areas
of increased number of stars. The density of stars is the same throughout the entire disk
The nature of the arms is still a bit of a mystery. They cannot be rotating as
independent entities
because they would soon 'wind-up'.
Present-day theories commonly attempt to describe a pressure wave maintaining the spiral arm pattern as it appears.
The bulge is populated primarily by older Population II stars.
The halo is a low density spherical region.
The younger population I stars are to be found predominantly in the disk
The first spiral to be actually recognized as a spiral was M51 (the Whirlpool
Galaxy) in 1845,
although it was
not known to be a galaxy at that time.
It was seen from the 1.83m reflector at Birr Castle in Ireland (run by the 'Earl of Rosse')
M31 (NGC 224) is similar to the Milky Way.
It is 2.2 million light years away and is the most distant object we
can see with the unaided eye.
Barred spiral are much less numerous than 'conventional' spirals.
Elliptical Galaxies contain mostly Population I stars. They have
little dust and gas, as expected.
The range in size is large - from 100 thousand solar masses to 10 million solar masses. Their dimensions range from about a tenth of the diameter of that of the Milky Way to ten times the diameter
of the Milky Way.
Introduction
Hubble Classification
Spirals
Elliptical Galaxies
| Cold | Hydrogen Molecules | 0.39 | |
| Warm | Hydrogen Atoms | HI | |
| Hot | Hydrogen Ions | HII |
ISM is the raw material for new stars.
It is seeded by supernovae with heavier elements
It also contains organic molecules