Unlike a regular spiral, a barred spiral contains a bar across its center region, and has two major arms. The Milky Way also contains two significant minor arms, as well as two smaller spurs. One of the spurs, known as the Orion Arm, contains the sun and the solar system. The Orion arm is located between two major arms, Perseus and Sagittarius. The Milky Way does not sit still, but is constantly rotating. As such, the arms are moving through space.
The sun and the solar system travel with them. Even at this rapid speed, the solar system would take about million years to travel all the way around the Milky Way. As material passes through the dense spiral arms, it is compressed and this triggers more star formation," said Camargo. Our galaxy is surrounded by an enormous halo of hot gas that extends for hundreds of thousands of light-years. The gas halo is estimated to be as massive as all of the stars in the Milky Way.
Like the galaxy itself, the halo is spinning rapidly. This hot gas reservoir is rotating as well, just not quite as fast as the disk. Curled around the center of the galaxy, the spiral arms contain a high amount of dust and gas. New stars are constantly formed within the arms. These arms are contained in what is called the disk of the galaxy. It is only about 1, light-years thick. At the center of the galaxy is the galactic bulge.
The heart of the Milky Way is crammed full of gas, dust, and stars. The bulge is the reason that you can only see a small percentage of the total stars in the galaxy. It is very difficult to count the number of stars in the Milky Way from our position inside the galaxy.
Our best estimates tell us that the Milky Way is made up of approximately billion stars. These stars form a large disk whose diameter is about , light years. Our Solar System is about 25, light years away from the center of our galaxy — we live in the suburbs of our galaxy. It takes million years for our Sun and the solar system to go all the way around the center of the Milky Way.
We can only take pictures of the Milky Way from inside the galaxy, which means we don't have an image of the Milky Way as a whole. Why do we think it is a barred spiral galaxy, then? There are several clues. The first clue to the shape of the Milky Way comes from the bright band of stars that stretches across the sky and, as mentioned above, is how the Milky Way got its name. This band of stars can be seen with the naked eye in places with dark night skies.
That band comes from seeing the disk of stars that forms the Milky Way from inside the disk, and tells us that our galaxy is basically flat. The concentration of stars in a band adds to the evidence that the Milky Way is a spiral galaxy. If we lived in an elliptical galaxy, we would see the stars of our galaxy spread out all around the sky, not in a single band. Another clue comes when astronomers map young, bright stars and clouds of ionized hydrogen in the Milky Way's disk.
The Milky Way is a relatively thin, flattened disk. This explains why it appears as a band in our sky. When we are looking in the direction of the disk, Earthlings see the combined light of all the stars in the galaxy. When we look in a direction away from the disk, we see only the stars close to our solar system. The core isn't spherical; it's elongated into the shape of a bar anywhere from 5, to 20, light-years long.
Up to a quarter of all the stars in the Milky Way reside in the core; the density of stars there is up to a million times greater than it is in the neighborhood of the sun, according to the Space Telescope Science Institute. The stellar disk of the Milky Way has a radius of 75, to , light-years, but it is only about 1, light-years thick. Within the disk sit several major spiral arms, according to NASA , where the density of stars and gas is higher than average and star formation occurs at a higher rate, making these arms stand out in visual observations.
Our solar system sits in the disk, about 27, light-years from the galactic center, near the inner rim of the Orion Arm. Beyond the disk of the Milky Way is its halo, which is a spherical region with a radius of about , light-years. The halo contains old stars and globular clusters, all orbiting the galactic center in random directions.
The Milky Way has two major satellite galaxies — the Large and Small Magellanic Clouds — and dozens of smaller satellites. Our nearest neighbor is the Andromeda galaxy, located about 2. Together with Andromeda and about 80 smaller galaxies, the Milky Way is a part of the Local Group, which is a group of galaxies, about 10 million light-years across, bound together by their common gravity, according to Swinburne University.
The Local Group is one member of a larger structure called the Virgo Supercluster, which is surrounded by several great intergalactic voids, according to Durham University.
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