Spiral Galaxies (3) ...
The image shown adjacent right
is of the spiral galaxy ESO 269-57, which is in the southern constellation
Centaurus.
The galaxy is a variation on
type Sa and lies at a distance of
about 155 million
light-years.
Its angular size implies a diameter of
about 200,000 light-years, twice as large as the Milky Way.
This spiral has a complex structure, with an inner ring-like set of tightly wound arms surrounded by outer
arms that may be split into branches. Notice the
prominent blue star-forming regions in the spiral arms.
Properties of Spiral Galaxies
Spiral galaxies have masses
~109-1012
solar masses
(~1011 solar masses is average).
The typical range of luminosities corresponds to an
absolute blue magnitude of from -16
to -23, and the diameter of the visible disk is typically 5-100 kpc.
Because spiral galaxies tend to have high luminosities, they are prominent. The most
luminous spirals and ellipticals are of comparable brightness, but
the least luminous spirals are much brighter than the fainter ellipticals.
Spiral galaxies contain large amounts of dust in their disks. This is particularly
evident when a spiral
is seen edge-on. The adjacent figure illustrates.
This is the spiral galaxy UGC 03214, which is in the
constellation
Orion at a distance of approximately 70 Mpc.
The image is dominated by
the bright nuclear bulge and the dark and dusty disk.
