In 1920, a Great
Debate raged between Harlow Shapley and Heber Curtis; what was
the scale of the Universe?. Considering the ``spiral nebulae", what
we now know to be external galaxies, Shapley
argued that these were merely nearby gas clouds in a Universe that
existed wholly within our own Galaxy. Curtis disagreed; he expounded the
view that the Sun was but a member of a small Galaxy of which there
were many.
The true nature of our Galaxy and the Universe was resolved by Edwin
Hubble. Using the 100 m Hooker telescope he was able to gauge the
distance to Cepheids in M31, the Andromeda Galaxy; this distance was
greater than that proposed by Shapley for the dimensions of our Galaxy
and thus M31 must lie beyond its boundaries, a galaxy in its own
right.
Hubble is perhaps better known for his observations which demonstrated
that the Universe is expanding (borne out in the by now infamous
Hubble Law), but he was also responsible for another important
development of extragalactic astronomy - the Hubble
Sequence. It should not be surprising that Hubble spent many years
surveying thousands of galaxies, and during the course of his work, he
noticed a trend - galaxies could be ordered into a sequence based on
their morphology, the so called Hubble Sequence.
On the left hand side of the sequence, which is commonly called a
Tuning Fork diagram, we have the ellipticals which range from E0 up to
E7 based on their ellipticity. E0s are spherical and ball like while an
E7 is shaped like a rugby ball. At the vertex we have the S0s,
lenticulars - galaxies which have disks like spirals but unlike
spirals are bulge dominated. Then we branch out into the spirals, Sa, Sb and
Sc and the barred spirals, SBa, SBb and SBc.
The obvious question to ask is why the dichotomy of shapes among
bulge dominated versus disk dominated galaxies? This was a problem of
nature versus nurture - initial guidance was sought in the formation of galaxies,
and indeed, the ELS (Eggen, Lynden-Bell, Sandage) hypothesis seemed to offer a solution. Stars in ellipticals
formed far earlier and more quickly during the initial collapse of a
primordial gas cloud than their
counterparts in spirals. However, this now introduced the question,
why should this be so? It is at this point that the importance of
galaxy mergers becomes clear.