Astronomy_rm

__From Big Bang to Galaxies__
=== When our Universe formed it started slowly then expanded out of nothing in a tiny fraction of a second. Very little happens for the next 3 million years. Our Universe began about 15 million years ago. Our Universe is very big and contains many galaxies. It is basically a thick soup of radiation and particles. It also contains billions of trillions of stars. Our Universe is huge and we have discovered much of it, but there is still more to be found out. ===

__Formation of Galaxies__
=== Galaxies began forming when the Universe was about 3 billion years old. They are classed by the shapes. A long time ago galaxies were closer than they are now. They collided a lot of times with other galaxies when they were that close. Now it is very rare for them to collide. Galaxies are made of billions of stars. Our galaxy is medium sized and our solar system is located inside the galaxy. ===

__Structure of the Galaxies__
=== Galaxies are made up of billions of stars. A galaxy is a large flattened disk that is about 100,000 light years across. There is a large bulge in the center of the flattened disk that is about 20,000 light years across. Inside of the bulge there are red and orange stars, those stars are old stars. Below, is a side view of our galaxy. ===

__Rotation of the Galaxy__
=== All galaxies spin. Each gas cloud and each star has its own orbit. The sun takes 250 million years to go around once. The way that our galaxy rotates shows that outside of us is a corona. The corona contains 10 times more material than we actually see. It is in the form of stars, gas, and dust. ===

__ Formation of the Planets __
=== Our sun was formed when gravity pulled together interstellar gas and dust. In the outer solar system four planets formed; Jupiter, Saturn, Uranus, and Neptune. Later, the four terrestrial planets formed; Mercury, Venus, Earth, and Mars. The planets were heated by constant bombardments. Inside radioactivity also created heat. ===

__Formation of Moons, Rings, and Comets__
=== Moons, rings, and comets were all formed at different times and they were all formed long after the Big Bang. The moon was created in a catastrophic collision between the newly formed Earth and another planted about the size of Mars. The surface of the moon was cratered by heavy bombardments for millions of years. A few of the moons created were captured as the moons of planets. Icy planetecimals from the outer solar system became comets considering they reach the warm area of the sun. Rings around the large planets are compiled of stray planetecimals and comets being torn apart by gravity. ===