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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.
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.
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.
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.
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 Great Attractor. Generally superclusters are not bound to each other gravitationally and are moving away from each other due to the expansion of the universe.
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 Harlow Shapley 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.