when stars are viewed through a telescope, they appear as bright points
of light without any apparent size or structure. However, there are
some objects in the sky that, viewed through a telescope, look like "fuzzy"
clouds. Some of these are the star-forming regions called nebulae.
Others, like those shown in the Hubble Space Telescope image to the right,
are actually islands of stars that are much farther from us than the individual
stars we see in the night sky. Although Immanual Kant had first advanced
the idea of "island universes" to explain the observed compact clouds during
the eighteenth century, is wasnt until this century that astronomers began
to develop an understanding of the nature of galaxies.
Edwin Hubble developed a galaxy classification scheme consisting of four
types: elliptical, spiral, barred spiral, and irregular. Three of
these types are represented in his famous "tuning fork" diagram below.
Hubble thought that this scheme might well represent an evolutionary sequence
for galaxies. Today, we know that this is not the case. Nonetheless,
his classification scheme is still used by astronomers today.
An elliptical galaxy shows no spiral structure and can vary from
almost round (which Hubble called E0) to almost cigar shaped (E7).
This classification is based on our perspective from Earth and not
on the actual shape.
As their name implies, spiral galaxies have outstretched, curving
arms suggestive of a whirlpool or pinwheel. Hubble distinguished
different sub-classes according to the tightness of the arms and
the size of the nucleus. He called these Sa, Sb,
and Sc. In terms of the arms, Sa is the tightest wound
while Sc is the most open. In terms of the size of the nucleus,
Sa has the largest while Sc has the smallest. The galaxies
that appear to have a spiral disc, but no visible arms, are called
Barred spirals show the same spiral structure as normal spirals,
and also a prominent bar through the nucleus. The spiral arms
emerge from the end of the bar. The sub-classifications are
the same as for normal spirals.
Certain galaxies lack either an obvious spiral structure or nuclear
bulge, appearing instead as a random collection of stars with no
obvious order. They are distinguished from ellipticals by
their lack of symmetry.