To learn about the galaxies and the Milky Way -- our own galaxy.
New Concept:
Terms used in this unit:
light year, observable universe, black hole
To us, the Sun is a very special star, for without it, there would never have been any life on Earth. However, our Sun is just an ordinary star among countless others in the universe. In fact, our Sun is relatively small in size compared to the other stars in the universe. A giant star can have a diameter 500 times larger than our Sun.

Fig. 1 A snap shot of our universe by Hubble Space Telescope. Nearly every object in the photograph is a galaxy. These bluish, irregular galaxies are up to 8 billion light years away.
The stars do not usually stay alone; instead, the force of gravity holds them together to form a group of stars called a galaxy.
Galaxies are usually measured in light years. A galaxy might be 10,000 light years across -- and the nearest neighbouring galaxy might be 15 million light years away. So, if a galaxy were the size of Vancouver, its nearest neighbouring galaxy could be as far away as Halifax. The observable universe contains about 10 to 100 billion galaxies.
Fig.2 A far infrared edge-on view of our own Milky Way galaxy taken by the COBE satellite. The distance from the satellite to the center of the galaxy is about 30,000 light years.
The Milky Way -- our own galaxy

Our solar system is part of a spiral galaxy called the Milky Way. It is easy to observe the Milky Way. In a clear, moonless night sky, you may see a hazy band sweeping across the sky from horizon to horizon, like a pathway, or milky way. This is our Milky Way, and when you see it, you will know how it got its name.

If you look at the Milky Way through a telescope, it will seem as though the hazy band has dissolved into hundreds of thousands of stars. These stars are actually our close neighbors in astronomical terms. The hazy band we see is, in fact, a side view of the Milky Way. The photo above is a complete side view of the Milky Way scanned by the COBE satellite. If we could view our own Milky Way galaxy from a far distance above, it would look most like the galaxy M51.

The Milky Way is 100,000 light years across and 1,000 light years thick. Our Sun inhabits a spiral arm at about two thirds of the way between the Milky Way's center and its edge. A galaxy may contain millions or trillions of stars. Our Milky Way galaxy contains about 100 billion stars, making it one of the largest galaxies in the universe.

Galaxy Gallery

To the naked eye, most galaxies appear to be no more than dim patches of light. However, they are stunningly beautiful if you have chance to look though a powerful telescope. Here are some examples:

The galaxies can be roughly divided into three categories: elliptical, spiral and irregular. A typical elliptical galaxy has a ball shape, like the galaxy M87. Viewed from above, the spiral galaxy may look like a rotating dual-jet water sprinkler, like the galaxy M31 or M51. The irregular galaxy -- as its name suggests -- has no regular form.
Galaxies Compared

NameDistanceDiameterStellar MassType
- (million light years) (light years) (billion solar masses) -
Milky Way - 100,000 200 spiral
Andromeda Galaxy 2.2 150,000 300 spiral
Whirlpool Galaxy 25 60,000 200 spiral
Sombrero Galaxy 40 110,000 600 spiral
M82 10 25,000 30 irregular
M87 60 75,000 4,000 elliptical
The figures in the table are only estimates. Stellar mass estimates do not include the "dark matter" mass.
Figure Credits: Fig.1 NASA, HST; Fig.2 NASA, COBE.

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