The Milky Way is our home in the Universe on a grander scale than just our planet or solar system. Practically everything we see with the unaided eye in the night sky is a part of the Milky Way, except for a few visible extragalactic objects such as M31, the Andromeda galaxy.
The Milky Way is an island of hundreds of billions of stars, gas, and dust held together by gravity to form a gigantic disk that is surrounded by a halo of globular clusters, which are smaller spherical groups of stars.
On a clear night we can see the band of the Milky Way in the sky, the fuzzy light that stretches from one horizon to the other. This band of light is made up of a vast number of stars that telescopes can pick out as individuals but our eyes cannot. This is our view of the billions of stars that make up the disk of our galaxy, from a viewpoint inside the disk, about two- thirds of the way out from the center of the galaxy to the edge of the disk. There are dark patches in the band of light, due to dark clouds of interstellar matter. Radio telescopes can see through these clouds of dust and have shown that the material in our Galaxy is distributed in a disk with spiral arms of material trailing through. Our Galaxy has a bulge of stars at the center, wherein there may lie a giant black hole at the center. The galactic center lies in the direction of the constellation Sagittarius.
The stars in the disk of the galaxy orbit around the center in a way similar to the way in which planets orbit around the Sun. The speed of each star depends on its distance from the center. The Sun moves at about 250 km/s in its orbit, and it takes 225 million years to complete one revolution around the center. Studying the way in which the stars orbit gives details of the gravitational field of the Galaxy and ultimately reveals its mass. This total mass is about 1 trillion times the mass of our Sun, which is about 10 times the mass of all the visible stars in the Milky Way put together. This mass that is unaccounted for is strong evidence that there is dark matter in our galaxy, matter that is affecting the gravitational field of the Galaxy but is not visible to us.
- Where does the name "Milky Way" come from? (Beginner)
- Which hemisphere has the best view of the Milky Way? (Beginner)
- How can we know that there are other galaxies if we cannot leave ours? (Beginner)
- What will happen to the Milky Way in the future? (Beginner)
- Is the Milky Way a collision of two galaxies? (Beginner)
- Is Andromeda part of the Milky Way? Where can I find them in the sky? (Beginner)
- Has anyone made a map of the Milky Way? (Beginner)
- How many stars are born and die each day? (Beginner)
- How many stars are there in our Galaxy (Milky Way)? (Intermediate)
- What class of spiral galaxy is the Milky Way? (Intermediate)
- Where, in relation to the entire universe, is the Milky Way located? (Intermediate)
- Would a planet near the galactic center have a brighter night sky? (Intermediate)
- How can we see the Milky Way if we are inside it? (Intermediate)
- Could there be life in the galaxies nearest to the Milky Way? (Intermediate)
- What is happening just around the black hole in the center of the Milky Way? (Intermediate)
- What's going to happen on December 21st 2012? (Intermediate)
- What is the best way to see the Milky Way? (Intermediate)
- Are names given to black holes? (Advanced)
- Are the planes of solar systems aligned with the plane of the Galaxy? (Intermediate)
- How do we know the Milky Way is a spiral galaxy? (Intermediate)
- Can we make a movie of the Milky Way galaxy's rotation? (Beginner)
- Does the Milky Way spin counter-clockwise? If so, do all spiral galaxies spin in this direction and why? (Beginner)
- Will we get sucked into the black hole at the center of the Milky Way? (Beginner)
- How do stars move in the Galaxy? (Intermediate)
- Do supermassive black holes cause galaxy rotation? (Advanced)
The Ask an Astronomer team's favorite links about The Milky Way:
- SEDS Milky Way Page: SEDS (Students for the Exploration and Development of Space) have tons of good informational pages. This is the one they have on the Milky Way
- NASA's Multiwavelength Milky Way Education Page. I think the title says it all. It has these really neat maps of the Milky Way in all different wavelengths.
- Gene Smith's Astronomy tutorial on the structure of the Milky Way: some really nice diagrams and pictures, and a lot of information. A good first read on the subject.
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