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What is a galaxy?
• A collection of gas, dust and billions of stars
held together by gravity
(e.g. Earth and our solar system are part of
the Milky Way Galaxy)
• They are scattered throughout the universe
• They vary greatly in size and shape
The “Discovery” of Galaxies
At the beginning of the 20th century, what
we now call spiral galaxies were referred to
as “spiral nebulae” and most astronomers
believed them to be clouds of gas and stars
associated with our own Milky Way. The
breakthrough came in 1924 when Edwin
Hubble was able to measure the distance to
the “Great Nebula in Andromeda” (M 31, at
right) and found its distance to be much
larger than the diameter of the Milky Way.
This meant that M 31, and by extension
other spiral nebulae, were galaxies in their
own right, comparable to or even larger
than the Milky Way.
Edwin P. Hubble (1889-1953)
Types of Galaxies
In 1926 Edwin Hubble classified galaxies according
to their shape:
• Elliptical (shape similar to a football)
– Most common
– Composed mainly of old stars and have very little interstellar gas or dust
• Spiral (shape similar to a flat pinwheel)
– Arms are composed of a lot of gas and dust and young
blue stars (I.e. star formation is ongoing)
Types of Galaxies (Cont’d)
• Irregular (no particular shape)
– Mixture of young and old stars embedded in gas and dust
– Smaller and less common than spiral or elliptical
Lenticular (intermediate between an elliptical galaxy and a spiral
– Lenticular galaxies are disc galaxies (like spiral galaxies) which
have used up or lost most of their interstellar matter and therefore
have very little ongoing star formation.[2] As a result, they consist
mainly of aging stars (like elliptical galaxies). The dust in most
lenticular galaxies is generally found only near the nucleus and
generally follows the light profile of the galaxies' bulges. Because
of their ill-defined spiral arms, if they are inclined face-on it is
often difficult to distinguish between them and elliptical galaxies.
Types of Galaxies I. Spirals
Spiral galaxies are so-named because of the
graceful shapes of arms emanating from a bright
central nucleus. Spirals are classified according to
how tightly or loosely wound the arms are, and it
turns out that the brightness of the central nucleus
is correlated to the tightness of the arm. The
galaxies M 104 (below) and M 51 (right)
respectively show tightly and loosely wounds.
Notice the effects of dust in both galaxies.
(NOAO/AURA Photos)
Region of
the Spiral
M 51
(Hubble Space Telescope
Barred Spiral Galaxies
The spiral galaxies M 91 (left) and M 109 (right) have bars across their nuclei from which spiral arms
unwind. In virtually all spirals (barred or not) the galaxies rotate such that the spiral arms trail behind in
the rotation. The Milky Way is thought to be a barred spiral galaxy.
(NOAO/AURA Photos)
Types of Galaxies II. Ellipticals
Elliptical galaxies lack spiral arms and dust
and contain stars that are generally
identified as being old. The elliptical galaxies
M 32 (below) and M 110 (right) show
varying degrees of ellipticity.
(NOAO/AURA Photos)
Types of Galaxies III. Irregulars
Irregular galaxies lack any specific
form and contain stars, gas and dust
generally associated with a youth.
The irregular galaxy at right is the
Large Magellanic Cloud, a satellite
of the Milky Way located about
180,000 light years from the sun.
The LMC is about 60,000 light years
across. The bright reddish feature in
the upper right is the “Tarantula
Nebula” a region of star formation
in the LMC. (NOAO/AURA Photo)
Galaxy Clusters
• Most galaxies are not alone in the vast
expanse of space, but are connected to one
or more other galaxies by gravity
• These collections of galaxies are known as
galaxy clusters and they too appear to be
organized into larger “superclusters”
Unusual Galaxies
• Quasars
– Brightest objects in the
– Lie at the edge of the
observable universe
– Give off 100 times more
energy than Milky Way
– Energy provided by black