|
When you look
up at the night sky at night, every star you see is part of the
Milky Way galaxy. On a clear night you can see a narrow band
of stars stretching across the sky. Through a telescope or pair
of binoculars it becomes clear that this band is a dense region
of the Milky Way. The band of light we see as the Milky Way is
the light that originates from stars and other material that lie
within the galactic plane (Note: the galactic plane is
the actual mid-plane – the flat part – of our galaxy’s huge spinning
disk of stars). The disk of the Milky Way galaxy spans a distance
of about 100,000 light years (one light year is about 9.5 x 1015
meters). The Milky Way galaxy though is only about 2000 light
years thick at the edge of a spiral arm where the sun is located.
Our Galaxy
contains about 200 billion stars. Most of the stars are located
in the disk of our galaxy, which is the site of most of the star
formation because it contains lots of gas and dust.
 |
|
The
Andromeda galaxy is a spiral galaxy much like our own Milky
Way galaxy but is approximately 2.5 million light-years
away.
The
Milky Way and the Andromeda Galaxy are a binary system of
giant spiral galaxies belonging to a group of 50 closely
bound galaxies known as the Local Group.
Andromeda
is one of the few blue-shifted galaxies (moving toward us)
at a speed of 100 to 140 kilometers per second and is expected
to collide in about 4.5 billion years with our own Milky
Way galaxy.
| An
object moving toward us will have light shifted to the
blue end of the spectrum. Objects moving away will be
red-shifted. This is attributable to the Doppler effect. |
|
|
Photo:
NASA
|
|
|
Where is the earth in the Milky Way galaxy?
On a clear
night you can see about 3,000 stars. This is only a fraction of
the stars in the Milky Way galaxy -- there are approximately 200-400
billion stars in the Milky Way galaxy.
 |
|
Our
sun and solar system is at the edge of 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.
See
image to left -- sun is in the cross-hairs
Image
from Wikipedia
|
Just as the
earth orbits around the sun, our solar system orbits the galaxy's
center of mass -- but each rotation takes 225 million Earth years.
This period of time is called a "cosmic year".
What are the parts of the Milky Way galaxy?
A spiral galaxy
like the Milky Way has 3 basic components to its visible matter:
(1) the disk (containing the spiral arms), (2) the halo, which
extends beyond the disk, and (3) the nucleus or central bulge.
The halo of the galaxy is rather spherical in shape and contains
little gas, dust, or star formation. The clusters found in the
halo are globular clusters, so the halo contains very old stars.
(See reference for more information).
What is in the center of the Milky Way galaxy?
The center
of the Milky Way galaxy lies in the direction of the constellation
Sagittarius, and it is here that the Milky Way looks brightest.
From Sagittarius, the Milky Way appears to pass westward through
the constellations of Scorpius, Ara, Norma, Triangulum Australe,
Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis
Major, Monoceros, Orion and Gemini, Taurus, Auriga, Perseus, Andromeda,
Cassiopeia, Cepheus and Lacerta, Cygnus, Vulpecula, Sagitta, Aquila,
Ophiuchus, Scutum, and back to Sagittarius. The fact that the
Milky Way divides the night sky into two roughly equal hemispheres
indicates that the Solar System lies close to the galactic plane.
(See Reference from Wikipedia)
The center
of the galaxy looks awfully bright from the image above. Perhaps
millions of stars are in the center. But, radio telescopes tell
us more. Observational data suggests that a massive black hole
exists at the center of the central star cluster.
The galactic
center harbors a compact object of very large mass as determined
by the motion of material around the center. The intense radio
source named Sagittarius A*, thought to mark the center of the
Milky Way, is newly confirmed to be a supermassive black hole.[See
reference below] Most galaxies are believed to have a supermassive
black hole at their center.
How old is the Milky Way galaxy?
Astronomers
have known that the Milky Way is among the oldest of galaxies.
A recent study puts the age of the Milky Way at approximately
14.5 billion years, plus or minus more than 2 billion years. (See
reference below)
 |
|
You
can see an example of a Hertzsprung-Russell diagram to the
right. This diagram is based on measurements from 23,000
stars in our Milky Way galaxy. The Hertzsprung-Russell diagram
is named after its creators, astronomers Ejnar Hertzsprung
and Henry Norris Russell.
These
collections of stars by brightness and temperature are important
when talking about stellar evolution. In general, stars
are created in the main sequence. (Of course, when we say
"in the main sequence" we really mean "having a brightness
and a temperature that causes them to be plotted within
the main sequence on a Hertzsprung-Russell diagram.") After
billions of years, they expand into red giants. Then, after
another one or two billion years, they shrink into white
dwarfs.
See:
Wikipedia -- Hertzsprung-Russell
Diagram
|
|
|
|
|
What's beyond the Milky Way galaxy?
 |
|
The
Milky Way galaxy is just one of probably several hundred
billion galaxies in the observable universe. The image on
the left shows the part of the universe that is within 5
million light years of earth. The Milky Way is shown as
one of three large galaxies that belong to the Local Group
(Milky Way galaxy, Andromeda galaxy and Triangulum galaxy).
A visit
to the site atlasoftheuniverse
will let you zoom in and out and give you a better feel
for where earth lies in the universe.
|
|
|
|
|
TEST
YOUR KNOWLEDGE
OF THE MATERIAL
|
