Galaxies come in a
variety of shapes, with the shapes depending in a way
not yet completely understood on the
evolution of the galaxies.
More than half of all observed galaxies are spiral galaxies.
Examples of Spiral Galaxies
The figure below right shows a
nice spiral galaxy, M100,
which is in the
Another beautiful example of a spiral galaxy is
M83. Presumably our own galaxy would resemble these galaxies in appearance
if we could view it from the outside.
left image shows a class Sc spiral galaxy M101 (NGC 5457; also called the Pinwheel Galaxy),
which lies at a distance of about
7 Mpc or 22 million light years
The image shown adjacent left
is of the spiral galaxy ESO 269-57, which is in the southern constellation
The galaxy is of
type is Sa(r), with a recessional
velocity of just over 3100 km/sec, implying by the Hubble Law a distance of about 155 million
light-years. Its angular size is over 4 arcmin, corresponding to to a diameter of
about 200,000 light-years.
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. It has prominent blue star-forming regions in the spiral arms.
Properties of Spiral Galaxies
The range of masses for spiral galaxies is ~ 109 -
1012 solar masses,
with the typical mass being ~ 1011 solar masses. The typical
range of luminosities corresponds to absolute blue magnitude
-16 to -23, and the
typical diameter of the visible disk is 5-100 kpc.
The Spiral Structure
The spiral structure
is associated with active star-forming regions. As already noted in
conjunction with the Milky Way, This is in
fact why the spiral arms of these galaxies are so prominent: because they are
regions of active star formation, there are many hot young blue and blue-white
stars there, making the spiral arms extremely visible.
The adjacent image shows a star-forming region in the spiral arms of the galaxy M31 (Andromeda) called
(Ref). Notice the diffuse
blue glow associated with many young stars.
In constast the nucleus of
a spiral galaxy is typically much more red in color, often resembling elliptical
galaxies; this indicates the presence of many old stars in the nucleus of spiral
Here is a movie sequence of a galaxy
morphing its perceived shape from spiral to
elliptical as the wavelength is varied.
The image shown below illustrates rather clearly the
spiral structure for several spiral galaxies classified according to their Hubble class. These images were
taken in UV light with the Ultraviolet Imaging Telescope aboard the Space Shuttle Columbia. Because the
spiral arms are dominated by hot young stars, they stand out particularly well when photographed at UV
wavelengths, since only hot young stars can produce much UV
In these images we can see clearly the progression to more tightly wound spirals as we move from Sc through
Gas and Dust
Spiral galaxies are rich in gas and dust, which is often visible as lanes of
dust when viewed from the "top" or "bottom", and as layers of dust when viewed
from the side. In our own Milky Way
the center of the galaxy is invisible from our vantage point
because the interstellar dust between us and the center (in Sagittarius) is