A galaxy is an enormous collection of a few million to
trillions of stars, gas, and dust held together by gravity. Galaxies can
be several thousand to hundreds of thousands of light-years across.
|2. What is
the name of our galaxy?
The name of our galaxy is the Milky Way. All of the
stars that you see at night and our Sun belong to the Milky Way. When
you go outside in the country on a dark night and look up, you will see
a milky, misty-looking band stretching across the sky. When you look at
this band, you are looking into the densest parts of the Milky Way: the
disk and the bulge.
is Earth in the Milky Way galaxy?
Our solar system is in a spiral arm called the Orion
Arm, and is about two-thirds of the way from the center of our galaxy
to the edge of the starlight. Earth is the third planet from the Sun in
our solar system of nine planets.
What is the closest galaxy like our own, and how far away is it?
The closest spiral galaxy is Andromeda, a galaxy much
like our own Milky Way. It is 2.2 million light-years away from us. Andromeda
is approaching our galaxy at a rate of 670,000 miles per hour. Five billion
years from now it may even collide with our Milky Way galaxy.
|5. Why do
we study galaxies?
By studying other galaxies, astronomers learn more about
the Milky Way, the galaxy that contains our solar system. Answers to such
questions as "Do all galaxies have the same shape?," "Are
all galaxies the same size?," "Do they all have the same number
of stars?," and "How and when did galaxies form?" help
astronomers learn about the history of the universe. Galaxies are visible
to vast distances, and trace the structure of the visible universe with
their collections of billions of stars, gas, and dust.
|6. What are
the parts of a galaxy?
A galaxy contains stars, gas, and dust. In a spiral galaxy like the
Milky Way the stars, gas, and dust are organized into a bulge, a disk
containing spiral arms, and a halo. Elliptical galaxies have a bulge-shape
and a halo, but do not have a disk.
Bulge A round structure made primarily of old stars, gas, and dust.
The bulge of the Milky Way is roughly 10,000 light-years across. The
outer parts of the bulge are difficult to distinguish from the halo.
Disk A flattened region that surrounds
the bulge in a spiral galaxy. The disk is shaped like a pancake. The
disk of the Milky Way is 100,000
light-years across and 2000 light-years thick. It contains mostly young
stars, gas, and dust, which are concentrated in spiral arms. Some
stars are also present.
Spiral arms Curved extensions beginning at the bulge of a spiral galaxy,
giving it a "pinwheel" appearance. Spiral arms contain a lot
of gas and dust as well as young blue stars. Spiral arms are found only
in spiral galaxies.
Halo The halo primarily contains individual old stars and clusters of
old stars (globular clusters). It may be over 130,000 light-years across.
The halo also contains dark matter, which is material that we cannot
see but whose gravitational force can be measured.
Stars, gas, and dust Stars come in a variety of types. Blue stars,
which are very hot, tend to have shorter lifetimes than red stars, which
are cooler. Regions of galaxies where stars are currently forming are
therefore bluer than regions where there has been no recent star formation.
Spiral galaxies seem to have a lot of gas and dust, while elliptical
galaxies have very little gas or dust.
|7. How are
galaxies classified? What do they look like?
Edwin Hubble classified galaxies into four major types:
spiral, barred spiral, elliptical, and irregular (see also Question 8
and Question 10). Most galaxies are spirals, barred spirals, or ellipticals.
A spiral galaxy consists of a flattened disk containing spiral
(pinwheel-shaped) arms, a bulge at its center, and a halo. Spiral galaxies
have a variety of shapes, and they are classified according to the size
of the bulge and the tightness and appearance of the arms. The spiral
arms, which wrap around the bulge, contain many young blue stars and lots
of gas and dust. Stars in the bulge tend to be older and redder. Yellow
stars like our Sun are found throughout the disk of a spiral galaxy. These
galaxies rotate somewhat like a hurricane or a whirlpool. (See
a side view of a spiral galaxy, below.)
A barred spiral galaxy is a spiral that has
a bar-shaped collection of stars running across its center.
An elliptical galaxy does not have a disk
or arms; rather, it is characterized by a smooth, ball-shaped appearance.
Ellipticals contain old stars and possess little gas or dust. They are
classified by the shape of the ball, which can range from round to oval
(baseball-shaped to football-shaped). The smallest elliptical galaxies
(called dwarf ellipticals) are probably the most common type of galaxy
in the nearby universe. In contrast to spirals, the
stars in ellipticals do not revolve around the center in an organized
way. The stars move on randomly-oriented orbits within the galaxy like
a swarm of bees.
An irregular galaxy is neither spiral nor elliptical. Irregular galaxies
tend to be smaller objects without definite shape, and they typically
have very hot newer stars mixed in with lots of gas and dust. These
galaxies often have active regions of star formation. Sometimes their
irregular shape is the result of interactions or collisions between
galaxies. Observations such as the Hubble Deep Fields show that irregular
galaxies were more common in the distant (early) universe.