Galaxies
| M101, 25 million light years (C9.25 f/5/ST10 Lum, 5 x 5 min) |
| M65 and M66, 35 million light years (C9.25 f/5/ST10 lum, 8 x 5 min) |
Galaxies are massive systems of stars, gas, dust, and dark matter. They range in size from dwarfs with
as few as ten million stars up to giants with one trillion stars or more, all orbiting a common center of
mass. In the case of the larger galaxies, this mass is usually a super massive black hole. The fact that
galaxies consist of immense "islands" of stars in the vast, largely empty universe, was not established
until the late 1920s with Edwin Hubble's work which determined the distance of some galaxies from the
Milky Way. Prior to this work, many thought these structures (then called white nebula) were part of our
own galaxy, the Milky Way.
Edwin Hubble also published a classification system for galaxies based on their apparent shape. The
system has three main types: elliptical, spiral and irregular. Elliptical galaxies were so named because
they look like elliptical blobs of light. In general, they show no obvious structural features other than a
smooth concentration of light to the center. They account for about 60% of known galaxies, although
this estimate varies considerably in the literature. For a good example of an elliptical galaxy, see M105
below. Spiral shaped galaxies, which are visually much more interesting, appear to account for about
20% of the total. Galaxies with irregular or unusual shapes make up the rest. Many of the irregulars are
small dwarf galaxies that are satellites of other larger galaxies. However, some large irregulars result
from the disruptive effects of the gravitational pull of a neighboring galaxy, or result from the collision of
two galaxies. The classic example of two galaxies interacting is M51 and NGC5194 (the Whirlpool
Galaxy) shown below. An example of an irregular galaxy can be seen in the unusual shape of M66
which is under the gravitational influence of the nearby galaxy M65. The same process is thought to be
responsible for the unusual shape of M82, which is thought to have interacted with the nearby galaxy
M81 (also shown below). Some astronomers believe that spiral and irregular galaxies evolve over time
to ellipticals. Others propose that most galaxies are about the same age and that they do not "evolve"
with time other than through collisions and other gravitational interactions with nearby galaxies.
There are probably more than 100 billion galaxies in the observable universe. Most galaxies are 3,000
to 300,000 light years in diameter and are usually separated by distances on the order of millions of
light years. The majority of galaxies are organized into clusters, which in turn can form larger groups
called superclusters. The Milky Way, our spiral galaxy, is part of a cluster known as the Local Group
which contains over 45 galaxies, most of which are small, irregular galaxies. However, including our own
galaxy, the Local Group contains three large spiral galaxies, one of which is larger than the Milky Way
(Andromeda Galaxy or M31). Our local group ranges out to more than 4 million light years.
as few as ten million stars up to giants with one trillion stars or more, all orbiting a common center of
mass. In the case of the larger galaxies, this mass is usually a super massive black hole. The fact that
galaxies consist of immense "islands" of stars in the vast, largely empty universe, was not established
until the late 1920s with Edwin Hubble's work which determined the distance of some galaxies from the
Milky Way. Prior to this work, many thought these structures (then called white nebula) were part of our
own galaxy, the Milky Way.
Edwin Hubble also published a classification system for galaxies based on their apparent shape. The
system has three main types: elliptical, spiral and irregular. Elliptical galaxies were so named because
they look like elliptical blobs of light. In general, they show no obvious structural features other than a
smooth concentration of light to the center. They account for about 60% of known galaxies, although
this estimate varies considerably in the literature. For a good example of an elliptical galaxy, see M105
below. Spiral shaped galaxies, which are visually much more interesting, appear to account for about
20% of the total. Galaxies with irregular or unusual shapes make up the rest. Many of the irregulars are
small dwarf galaxies that are satellites of other larger galaxies. However, some large irregulars result
from the disruptive effects of the gravitational pull of a neighboring galaxy, or result from the collision of
two galaxies. The classic example of two galaxies interacting is M51 and NGC5194 (the Whirlpool
Galaxy) shown below. An example of an irregular galaxy can be seen in the unusual shape of M66
which is under the gravitational influence of the nearby galaxy M65. The same process is thought to be
responsible for the unusual shape of M82, which is thought to have interacted with the nearby galaxy
M81 (also shown below). Some astronomers believe that spiral and irregular galaxies evolve over time
to ellipticals. Others propose that most galaxies are about the same age and that they do not "evolve"
with time other than through collisions and other gravitational interactions with nearby galaxies.
There are probably more than 100 billion galaxies in the observable universe. Most galaxies are 3,000
to 300,000 light years in diameter and are usually separated by distances on the order of millions of
light years. The majority of galaxies are organized into clusters, which in turn can form larger groups
called superclusters. The Milky Way, our spiral galaxy, is part of a cluster known as the Local Group
which contains over 45 galaxies, most of which are small, irregular galaxies. However, including our own
galaxy, the Local Group contains three large spiral galaxies, one of which is larger than the Milky Way
(Andromeda Galaxy or M31). Our local group ranges out to more than 4 million light years.
| M82, 12 million light years (C9.25 f/5/ST10 Lum 8 x 5 min) |
| M81, 12 million light years (C9.25 f/5/ST10 Lum 8 x 5 min) |
| M109, a barred spiral galaxy, 40-50 million light years (C9.25 f/5/ST10 Lum, 10 x 5 min) |
| M51, the Whirlpool Galaxy, 31 million light years (C9.25 f/5/ST10 Lum 8 x 5 min) |
| M106, 25 million light years (C9.25 f/5/ST10 Lum 8 x 5 min) |
| NGC2403, 12 million light years (C9.25 f/5/ST10,13 x 5 min) |
| M105 (near center) with NGC3394 and NGC3389; 38 million light years for M105 (C9.25 f/5/ST10 Lum 12 x 5 min) |
| NGC4565, 30 million light years (C9.25 f/5/ST10 lum, 11 x 5 min) |
| NGC3628, 32 million light years (C9.25 f/5/ST10; lum, 10 x 5 min, 2 x 5 min R,B and G) |
| M63 (Sunflower Galaxy), 35 million light years (C9.25 f/10/ST10 lum, 2:2 10 x 5 min, RGB f/5 x 3 min) |
