Galaxy

The Milky Way is the galaxy that contains our Solar System.[12][13][14][nb 1] Its name “milky” is derived from its appearance as a dim glowing band arching across the night sky in which the naked eye cannot distinguish individual stars. The term “Milky Way” is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, "milky circle").[15][16][17] From the Earth, the Milky Way appears like a band because its disk-shaped structure is viewed from within the Galaxy. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. In the past, astronomers thought that all of the stars in the universe were contained inside of the Milky Way. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis,[18] observations by Edwin Hubble definitively showed that the Milky Way is just one of many galaxies.[19] The Milky Way is a barred spiral galaxy some 100,000–120,000 light-years in diameter which contains 100–400 billion stars. It may contain at least as many planets as well.[20][21] The Solar System is located within the disk, about 27,000 light-years away from the Galactic Center, on the inner edge of a spiral-shaped concentration of gas and dust called the Orion Arm. The stars in the inner ≈10,000 light-years form a bulge and one or more bars that radiate from the bulge. The very center is marked by an intense radio source named Sagittarius A* which is likely to be a supermassive black hole. Stars and gases at a wide range of distances from the Galactic center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been given the name “dark matter”.[22] The rotational period is about 240 million years at the position of the Sun.[9] The Galaxy as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest known star in the Galaxy is at least 13.6 billion years old and thus must have formed shortly after the Big Bang.[6] Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a subcomponent of the Virgo Supercluster.

The Milky Way Galaxy

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Structure and Composition

The galaxy we live in, called the Milky Way Galaxy, is a barred spiral galaxy composed of at least 100 billion stars. It is approximately 100,000 light years across and about 1000 light years thick. It has a central bulge that is about 10,000 light years in diameter. Our solar system is about a third of the way towards the edge of the Galaxy from the central bulge. If the Solar System were inside the bulge, at night we would be able to see a million stars as bright as Sirius (the brightest star in our night sky). The night sky would be so bright, that it would not seem much different than day. The Sun and Solar System are within the 1,000 light year thick disk, and we are only about 95 light years from the central plane.

The disk of our galaxy appears blue because it has a large proportion of young, hot O and B main sequence stars. The disk contains gas and dust from which stars can form. The central bulge of our galaxy appears yellow or reddish because it contains many red giants and red super giants, but not the short lived blue O and B stars. This shows that the central bulge does not have active star formation going on. The stars in the disk of the Galaxy are generally younger, population I stars, which orbit the central bulge along paths within the disk. The stars and globular clusters in the halo of our galaxy are very old population II stars. They orbit the Galaxy along paths tilted at random angles to the disk. Many of the single stars in the halo orbit the galaxy at very high speeds, relative to the sun and are called high-velocity stars.

At the center of the Milky Way Galaxy is a supermassive black hole. The region where the black hole is located is called Sagittarius A* (prounounced "A star"). The black hole itself cannot be observed partly because it emits no light, and partly because there is too much gas and dust between us and that region for us to be able to observe it. The stars around Sagittarius A* move at such great speeds, that astronomers know that it must be incredibly massive. Estimates show that it must be at least 3.7 million times more massive than our sun. However, it is very compact and at most 45 AU (6.7 billion km) across.

Astronomers believe that only about 10% of the mass of our galaxy comes from stars, gas and dust. They suspect that there must be more matter than we can see because of the way the galaxy rotates. If all of the stars in our galaxy were orbiting a massive object in the center, the way the planets orbit the Sun in the Solar System, then the stars closer to the edge of the galaxy should be orbiting more slowly than stars closer to the center, the same way the outer planets orbit more slowly than the inner ones. Instead, stars near the edge of our galaxy orbit at nearly the same speed as stars nearer the center. To produce this kind of motion, the galaxy must contain much more mass than we can see. Astronomers theorize that this extra mass is dark matter. This matter is not visible, emits no electromagnetic radiation, and has so far eluded detection.

There are some alternative theories to dark matter being investigated. These theories propose that there is no extra matter, but that our understanding of gravity is incomplete or inadequate on large scales, and the motion of the stars within our galaxy can be explained this way. So far none of the alternative theories have been able to explain the observations as cleanly as dark matter and dark matter is the more widely accepted theory.

Galactic Coordinates

The galactic coordinate system is a way of describing where an object is in the Milky Way Galaxy, relative to the Sun. In the diagram below, l is glactic longitude and b is galactic latitude. l is measured in degrees counterclockwise from the direction towards the galactic center and goes from 0 to 360°. b is measured in positive degrees above or negative degrees below the galactic plane and goes from 0 to 90° above and o to -90° below the plane. This coordinate system doesn't give information about the distance to an object, so many objects may have the same galactic coordinates by being along the same line of sight, but be at different distances from us.

The Milky Way Galaxy's disk rotates, with all of the stars and dust in the disk traveling at a fairly uniform speed. Because of this, stars inside the Sun's orbit complete trips around the bulge more quickly than we do. Stars outside the Sun's orbit complete the journey more slowly. Our galaxy is not like a rotating CD or DVD, where different points on the CD travel at different speeds, but always complete a rotation in the same amount of time. In our galaxy, stars in the disk all travel at nearly the same speed, so stars closer to the edge will take longer to orbit the galaxy since they have farther to travel.

The spiral arms in our galaxy may be density waves, similar to the ripples that form when a stone is dropped into a pool of water. The spiral arms are areas of greater density of gas, dust and stars, and are the regions where star formation happens.

Neighborhood

The Milky Way Galaxy is the second largest member of a cluster of over 30 galaxies called the Local Group. The largest member of the local group is the Andromeda Galaxy, and the 3rd largest is the Triangulum galaxy. Most of the other galaxies in the Local Group are much smaller dwarf spheroidal and dwarf elliptical galaxies.

Our Local Group is a member of the Virgo Supercluster, which contains over 100 galaxies and clusters, and is over 100 million light years across. Our supercluster is traveling at about 600 km/sec towards a very massive supercluster called the . Generally superclusters are not bound to each other gravitationally and are moving away from each other due to the expansion of the universe.

History of Discovery

In the 1700s, astronomers began to suspect that the Milky Way is a disk of stars that completely encircles us. However, for a long time astronomers believed that our sun was at the center of the Galaxy. Astronomers including Herschel and Kapteyn counted the stars in the Milky Way, and there seemed to be equal numbers in every direction, which led them to the conclusion that we are at the center. What they didn't account for was interstellar extinction; dust and gas throughout our galaxy was obscuring their view. This dust acts like a fog, and they were actually only observing the very nearest stars. The view through our galaxy is obscurred by dust and gas, but our view out of the plane of the Galaxy has very little interstellar extinction.

In 1920 and astronomer named was studying RR Lyrae variable stars in the globular clusters that orbit our galaxy. He was able to use the RR variable stars to determine the distances to 93 globular clusters, and realized that they were much farther than had been thought, with some as far away as 100,000 light years. He also realized that almost all of the globular clusters he was observing were in the direction of Sagittarius. He came to the conclusion that the globular clusters were orbiting the center of our galaxy, which was not where the Earth and Sun are, but rather in the direction of Sagittarius. He estimated the distance to the center of our galaxy, but because interstellar extinction was not well understood at the time, he was off by a factor of 2. We now know the distance to the center of the Galaxy to be about 26,000 light-years + or - 3000 light years.

Around this time, the astronomical community was divided about the nature of what they called spiral nabulae. Harlow Shapley was a member of a group that said these were structures within our galaxy. Others proposed that these nebulae were "island universes," rotating systems of stars much like our galaxy. It was not until Edwin Hubble discovered Cepheid variables in some of these spiral nebulae that the question could be answered. Cepheid variables, like RR Lyrae variables, can be used to measure distances in space. It turned out that these spiral structures were many millions of light years away, and certainly not smaller structures within our galaxy.

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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 — milkyway_apod-2

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.

4. 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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The Milky Way Galaxy

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Structure and Composition

Galactic Coordinates

Neighborhood

History of Discovery

It's enough to keep you awake at night: Photographer captures beauty of the Milky Way on midnight trek to beat insomnia

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Facts About the Milky Way

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