We live in the Milky Way Galaxy. If you were looking down on the Milky Way, it would
look like a large pinwheel rotating in space. Our Galaxy is a spiral galaxy that formed
approximately 14 billion years ago. Contained in the Milky Way are stars, clouds of dust
and gas called nebulae, planets, and asteroids. Stars, dust, and gas fan out from the
center of the Galaxy in long spiraling arms. The Milky Way is approximately 100,000
light-years in diameter. Our solar system is 26,000 light-years from the center of the
Galaxy. All objects in the Galaxy revolve around the Galaxy's center. It takes 250 million
years for our Sun to pull us through one revolution around the center of the Milky Way.
When you look up at the night sky, most of the stars you see are in one of the Milky
Way arms. Before we had telescopes, people could not see many of the stars very clearly.
They blurred together in a white streak across the sky. A myth by the ancient Greeks said
this white streak was a "river of milk". The ancient Romans called it the Via
Galactica, or "road made of milk". This is how our Galaxy became known as the
Milky Way.
It is interesting to note that astronomers capitalize the "G" in galaxy when
talking about our Milky Way!
Today, astronomers have been able to observe the Milky Way in all regions of the
electromagnetic spectrum. They have had to be clever in making the observations since they
are having to look through the disk of the Galaxy from our location in one of the arms!
Our Sun is a star in the Milky Way Galaxy. If you were looking down on the Milky Way,
it would look like a large pinwheel rotating in space. Our Galaxy is a spiral galaxy that
formed approximately 14 billion years ago. Contained in the Milky Way are stars, clouds of
dust and gas called nebulae, planets, and asteroids. Stars, dust, and gas fan out from the
center of the Galaxy in long spiraling arms. The Milky Way is approximately 100,000
light-years in diameter. Our solar system is 26,000 light-years from the center of the
Galaxy. All objects in the Galaxy revolve around the Galaxy's center. It takes 250 million
years for our Sun to pull us through one revolution around the center of the Milky Way.
The stars we see over our head every night are also members of the Milky Way family.
It is interesting to note that astronomers capitalize the "G" in galaxy when
talking about our Milky Way!
Click to enlarge.
How do we know the
Milky Way is 100,000 light years across?
Because we can measure the
distances to objects such as globular star clusters to figure out the distance to the
Galactic center at 8.5 kiloparsecs, and by using radio astronomical measurements of
distant hydrogen clouds, we can detect such objects outside the 'solar circle' at
distances of 10 klioparsecs or more. The 100,000 light year number is an aproximation, but
10 kiloparsecs plus the sun-center distance of 8.5 kiloparsecs gives you a maximum
Galactic distance to some of these hydrogen clouds of nearly 20 kiloparsecs or 70,000
light years. Double this and you get nearly 150,000 light years for the approximate
diameter of the Milky Way.
What is in the middle
of the Milky Way?
The 'galactic center' contains
many millions of stars, dense gas clouds a hundred light years across, the dead bodies of
old stars that went supernova thousands of years ago, and at least one very large black
hole that is about 1 million times the mass of our sun, but only about 6 million
kilometers across. There is lots of activity down in there as matter flows in, and is
expelled along with lots of energy which comes out in the form of intense radio waves and
x-rays.
What is the Great
Annihilator?
This is a very powerful high
energy x-ray source near the center of the Milky Way which seems to be a massive black
hole...perhaps 10 - 1000 times the mass of the Sun, which is producing an enormous flux of
positrons ( anti-electrons). This anti-matter is interacting with ordinary matter nearby
to produce electron annihilation radiation at an energy of 511 keV, which is how it was
detected.
Click to enlarge.
The Andromeda galaxy is
about 2 million light years from Earth.
The Andromeda galaxy can be seen with the naked eye in the constellation of Andromeda.
It looks like a bright cookie in the sky several degrees across. In the sense that one
does not need an optical aid to see it, one could say that the earliest people to look up
at the night sky "discovered" it.
In terms of when it was realized that it is a galaxy, and in fact that there are such
things as galaxies, each "island universes" in and of themselves, that was only
about 50 to 60 yrs ago. Up until that time, telescopes were not powerful enough to be able
to discern individual stars in galaxies outside our own, so the objects which we now know
to be galaxies were referred to as "nebulae" - or bright gas clouds. In the
early 1920s there was a debate between Curtis and Shapley - two famous astronomers of the
day - as to the nature of the "nebulae". Curtis argued for the island universe
interpretation whereby each nebula is a galaxy consisting of many individual stars;
Shapley argued the traditional view - namely that all that exists in the Universe is
within a few thousand light years of us, and our galaxy IS the Universe.
Although Shapley was generally acknowledged to be the "winner" of the debate by
virtue of having more correct and supportable arguments, Curtis turned out to be right in
the end.
Click to enlarge.
The Triangulum galaxy M33 is another prominent member of the Local Group of galaxies.
This galaxy is small compared to its big apparent neighbor, the Andromeda galaxy M31, and
to our Milky Way galaxy, but by this more of average size for spiral galaxies in the
universe. One of the small Local Group member galaxies, LGS 3, is possibly a satellite of
M33, which itself may be a remote but gravitationally bound companion of the Andromeda
galaxy M31.
M33 was probably first found by Hodierna before 1654 (perhaps together with open
cluster NGC 752) and independently rediscovered by Messier in 1764. Nevertheless, William
Herschel, who otherwise carefully avoided to number Messier's objects in his survey,
assigned it the number H V.17. Also because of the cataloging of Herschel, the brightest
and largest HII region (diffuse emission nebula containing ionized hydrogene) has obtained
a NGC number of its own: NGC 604 (William Herschel's H III.150); it is situated in the
northeastern part of the galaxy; apparently the bright knot near the top of our image.
This is one of the largest H II regions known at all: it has a diameter of nearly 1500
light years, and a spectrum similar to the Orion nebula M42. Hui Yang (University of
Illinois) and Jeff J. Hester (Arizona State University) have taken a photograph of NGC 604
with the Hubble Space Tepescope, resolving over 200 young hot massive stars (of 15 to 60
solar masses) which have recently formed here. |
NEARBY GALAXIES
