Astronomy_GES

From **Big Bang to Galaxies** According to the Big Bang Theory, the universe began about 15 billion years ago when it exploded out of nothing. In the beginning, the Universe was about the size of an atomic nucleus, but in a fraction of a second, the universe expanded to about the size of the earth. In the first few seconds, billions of particles were created. The temperature in the Universe began to gradually fall. In the first few seconds, quarks formed together to become protons and neutrons. Matter and Antimatter particles were created. After 1 second, the temperature has fallen to 10 billion degrees and electrons and positrons are created. After 3 minutes, 1/4 of the protons and neutrons have formed together to become helium nucleus. For the next 300,000 years after the initial explosion, very little happens until, once the temperature reaches 300,000 degrees K, it is cold enough for atoms to come together. From then on, the universe began to create galaxies and solar systems. About two billion years after the Big Bang, Galaxies began to form. In places where there is a lot of matter, gravity causes clumps to grow and get denser. Galaxies and Galaxy clusters are clumped around large voids. Some stars formed in the middle of the clusters. The rest settled into a disk around the center. Galaxies can be classified as one of three shapes, Elliptical, Spiral, or Irregular. In the early universe, galaxies were clumped close together and often mixed with each other, but now they are more spread out. In conclusion, Galaxies were created long ago out of clouds of gas. ** Structure of the Galaxy ** The Milky Way Galaxy has a very defined structure. The Galaxy has many parts. The first of which is a flat disk of stars that is about 100,000 light years across. There is a very thin layer of gas and dust in the center of the disk. At the center, there is a large bulge that is about 20,000 light years across. In this bulge, lies billions of old stars packed in close. In the very center of our Galaxy, it is theorized that there is a supermassive black hole. Our sun lies in the disk of our Galaxy. Globular star clusters are sprinkled around the galaxy. In conclusion, the Milky Way was formed about 3 billion years ago and has a basic spiral structure. ** Rotation of the Galaxy ** The Milky Way Galaxy rotates. Four rotating spiral arms extend from the disk, which contain bright blue young stars and pinkish clouds of hydrogen. These arms move in a steady, counterclockwise motion. The individual gas clouds and stars, however, move in their own orbit around the arms. The movement is kind of like a person in a wave pool because the waves move against you, but you are still moving in your own direction. ** Formation of the Planets ** In the beginning of our solar system, planets were formed. Planets were formed when planetessimals, or small asteroids, collided gently together enough times to form massive spherical planetessimals called planets. The outer four planets were formed first because there were less hard collisions occuring. The inner, or terrestrial, four planets were formed in the same way, but later on because of the high collision rate of planetessimals near the sun. ** Formation of Moons, Rings, & Comets ** Moons, Rings, and Comets are all forms of planetessimals. A moon is when a planet captures a plantessimal in its orbit and the planetessimal continues to orbit the planet. An example of one of these moons in Ganymede, one of Jupiter's moons. A ring is when a planetessimal is shredded apart by the force of gravity and the small parts orbit around the planet and appear as a ring. Comets are when cold planetessimals from the outer solar system enter the inter solar system and become hot, and as a result, burn up in a spectacular flare. Return to Grey S web page
 * Structure of Galaxies**