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Facts About the Milky Way

Here are ten interesting facts about your home – well, at least your home galaxy – the Milky Way.

1. It’s warped.

The Milky Way is a disk about 120,000 light years across (see the Guide to Space article on the diameter of the Milky Way for more), with a central bulge that has a diameter of 12,000 light years. The disk is far from perfectly flat though, as can be seen in the picture below. What warped it? Two of the galaxy’s neighbors – the Large and Small Magellanic clouds – have been pulling on the dark matter in the Milky Way like in a game of galactic tug-of-war. The tugging sets up a sort of oscillating frequency that pulls on the hydrogen gas (of which the Milky Way has lots of). Here’s a more in-depth article on Universe Today about How the Milky Way got its Warp.

The warped Milky Way. Image credit:NASA/APOD

The warped Milky Way. Image credit:NASA/APOD

2. It has a halo, but you can’t directly see it.

The Milky Way has a halo of dark matter that makes up over 90% of its mass. Yes, 90%. That means that all of what we can see (with the naked eye or telescopes) makes up less than 10% of the mass of the Milky Way. Now, it doesn’t have a halo like those old cartoon characters that die, sprout wings and play a harp in the clouds. The halo is actually invisible, though we know it exists by running simulations of what the Milky Way would look like and how fast stars inside the galaxy’s disk orbit the center. The heavier it is, the faster they should be orbiting. If you assume that the galaxy is made up only of matter that we can see, then you get a rotation rate for the stars that is well below what it should be, so the rest is made up of what is elusively called “dark matter,” or matter that only interacts gravitationally (so far as we know) with “normal matter”.

To see some images of the probable dark matter density distribution in our galaxy, check out The Via Lactea Project.

3. It has over 200 billion stars

As galaxies go, the Milky Way is a middleweight. The largest galaxy known, IC 1101, has over 100 trillion stars, and other large galaxies can have more than a trillion stars. Smaller galaxies like the aforementioned Large Magellanic Cloud, have about 10 billion stars. The Milky Way has between 200-400 billion stars, but when you look up into the night sky the most you can see from any one point on the Earth is about 2,500. We aren’t stuck with this many stars forever, though, because the Milky Way is constantly losing stars – through supernovae – and producing stars, netting about seven stars per year.

sig05-025_small4. It’s really dusty and gassy.

You may not think so by looking at it, but the Milky Way is full of dust and gas. And when I say full of dust, I mean that we can only see out about 6,000 light years into the disk of our own galaxy in the visible spectrum, and the galaxy is about 100,000 light years across! The dust and gas makes up a whopping 10-15% of the “normal matter” in the galaxy, with the remainder being stars. The thickness of the dust deflects visible light, as is explained here, but infrared light can pass through the dust, which makes infrared telescopes like the Spitzer Space Telescope extremely valuable tools in mapping and studying the galaxy. Spitzer can peer through the dust to give us extraordinarily clear views of what is going on at the heart of the galaxy and in star-forming regions.

5. It’s made up of other galaxies.

The Milky Way wasn’t always as it is today, a beautiful barred spiral. It became its current size and shape by eating up other galaxies. It’s still doing so today – the Canis Major Dwarf Galaxy is the closest galaxy to the Milky Way because its stars are currently being added to the Milky Way’s disk, and our galaxy has consumed others in its long history, such as the Sagittarius Dwarf Galaxy.

6. Every picture you’ve seen of the Milky Way from above is either another galaxy or an artist’s interpretation.

We can’t take a picture of the Milky Way from above (yet) because we are inside the galactic disk, about 26,000 light years from the galactic center. This means that any pretty pictures you see of a spiral galaxy with elegant arms that is supposedly the Milky Way is either a picture of another spiral galaxy, or the rendering of a talented artist. Imaging the Milky Way from above is a long, long way off; however, this doesn’t mean that we can’t take breathtaking images of the Milky Way from our vantage point!

7. There is a black hole at the center.

Most galaxies have a supermassive black hole at the center. Ours is no exception. The center of our galaxy is called Sagittarius A* (pronounced “A-star”), and it houses a black hole with a mass of 40,000 Suns that is 14 million miles across (about the size of Mercury’s orbit). But this is just the black hole itself. All of the mass trying to get into the black hole – called the accretion disk – forms a disk that has a mass of 4 million Suns, and would fit inside the orbit of the Earth. Though like other black holes, Sgr A* tries to consume anything that happens to be nearby, star formation has been detected near this black hole behemoth.

8. It’s almost as old as the Universe itself.

The most current estimate for the age of the Universe is about 13.7 billion years. Our Milky Way has been around for about 13.6 billion of those years, give or take 800 million years. The oldest stars in our the Milky Way are found in globular clusters, and the age of the galaxy is determined by taking the age of these stars, and then extrapolating the age of what preceded them. Though some of the constituents of the Milky Way have been around for a long time, the disk and bulge themselves didn’t form until about 10-12 billion years ago, and the bulge may have formed earlier than the rest of the galaxy.

9. It’s part of the Virgo Supercluster, a grouping of galaxies within 150 million light years.

As big as it is, the Milky Way is part of an even bigger structure called a supercluster. Superclusters are groupings of galaxies on very large scales (100s of millions of light years). In between these superclusters are large voids of space where any space traveler would encounter very little in the way of galaxies or matter. Our close neighbors include the Large and Small Magellanic Clouds, and the Andromeda Galaxy (the closest spiral galaxy to the Milky Way), and along with about 30 other galaxies this group of galaxies makes up what is called the Local Group. But as you get further on out, on the scale of hundreds of millions of light years, the Milky Way can be seen to be just a small part of a large grouping of galaxies 150 million light years in diameter called the Virgo Supercluster.

10. It’s on the move

The Milky Way, along with everything else, is moving through space, and puts to shame anything from everyday life that one could compare its speed to. The Earth moves around the Sun, the Sun around the Milky Way, and the Milky Way is part of the Local Group, which is moving relative to the Cosmic Microwave Background radiation – the radiation left over from the Big Bang, which is a convenient reference point to use when determining the velocity of things in the Universe. The Local Group is calculated to move relative to the CMB at about 600 km/s (2,200,000 km/h), which is pretty darn fast!

For many more facts about the Milky Way, visit the Guide to Space, listen to the Astronomy Cast episode on the Milky Way, or visit the Students for the Exploration and Development of Space at seds.org.. Here are some black hole facts.

Source: SEDS.org

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