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Milky Way – Our Galaxy
If you're lucky enough to have a clear dark sky, you can see a faint milky-white band dividing the sky in half. This is the Milky Way and it was a mystery for thousands of years. But in 1610 Galileo turned his telescope on it and discovered that it was made of stars.
Our Galaxy was named after this milky band and the word galaxy itself comes from a Greek word meaning milky. When we capitalize Galaxy, it refers to our own galaxy. Although the Galaxy and the milky band of stars share a name, all of the stars we can see when we look at the sky are part of the Galaxy, not just those in the band. The Milky Way Galaxy is a large galaxy containing from 200-400 billion stars.
The shape of the Milky Way
It's hard to tell the shape of a galaxy from inside it, but astronomers have concluded that the Milky Way is a barred spiral galaxy. They've worked this out by comparing extensive observations and measurements of the Milky Way with the galaxies which we can see from the outside. Here is a simple diagram of the main galaxy types. The SB galaxies are the barred spirals.
The Milky Way has a disk, bulge and nucleus, and a halo, shown in this edge-on diagram of the Galaxy.
The disk, which contains the spiral arms, is at least 170,000 light years in diameter and, on average, around 1000 light years thick. (A light year is the distance light travels in a year – it's equal to about ten trillion kilometers or six trillion miles.)
This is one possible diagram with the spiral arms labeled. You may see diagrams labeled differently, as there isn't universal agreement about interpreting the spiral arms. They are the trickiest bits to work out if you can't see them from outside. Generally, four major arms are listed plus a number of shorter ones. However some astronomers think that there are only two major arms, which is typical of barred spiral galaxies.
We are about 26,000 light years from the center of the Galaxy on the inner rim of the Orion Spur between the Perseus and Sagittarius arms. When we see the milky band of stars we're getting an edge-on view of the Milky Way's disk from inside the disk. We're seeing stars at varying differences over tens of thousands of light years.The stars and constellations that we recognize are close enough to resolve as stars.
There is a good deal of star formation going on in the spiral arms. This makes the disk very bright in infrared and gives it a bluish color in optical images. Most of the Galaxy's gas and dust is contained in the disk and the dust lanes are prominent. The dust provides the material for star formation, but it obscures our view of the Galactic Center.
The bulge and the nucleus
The bulge is a slightly squashed sphere of stars about 13,000 light years in diameter with the nucleus in the center. The young blue stars that color the disk are rare. Most of the stars are at least seven billion years old, much older than those in the disk. (The Sun is five billion years old.) Our understanding of this part of the Galaxy has been hindered by by its being hidden behind the dust in the disk. However infrared radiation penetrates the dust, so infrared space telescopes have increased our knowledge of the bulge.
In the center of the Galaxy lies a supermassive black hole which is over four million times the mass of the Sun. Yet it's invisible. The evidence that it exists is its gravitational effect on the motion of objects that we can see. Since there are also black holes at the center of other galaxies, they may represent an important part of galaxy formation.
The visible part of the halo is a sphere of stars and globular clusters that formed in the early Galaxy. They're at least 12 billion years old. A globular cluster is a large collection of stars which are held together by gravity in a spherical shape. Globular clusters orbit the galaxy and can have hundreds of thousands of stars in them. The halo is filled with hot gas, but no dust.
However, beyond the visible halo astronomers think there is a greatly extended halo of "dark matter". From the rotation of the visible matter in the Galaxy there seems to be invisible matter accounting for up to 95% of the total mass. It's embarrassing for astronomers that most of the Galaxy is made of an unknown invisible material.
The Milky Way's neighbors
People used to think that the Milky Way was the entire Universe, but it isn't even alone in our part of the Universe. It's one of a group of over fifty galaxies which form the Local Group. The two giant spiral galaxies, the Milky Way and Andromeda, dominate the group. A third spiral galaxy, the Triangulum galaxy, is much smaller than the other two, and there are no large elliptical galaxies. The rest of the group members are dwarf galaxies.
Most of the Local Group dwarf galaxies are satellites either of the Milky Way or Andromeda. The Milky Way has 14 known satellite galaxies. The best known are the Large Magellanic Cloud and the Small Magellanic Cloud. There is also evidence that in the past some dwarf galaxies have been pulled apart and their stars added to the Milky Way.
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