From Big Bang to Galaxies
The big bang was the event in which all of matter and energy was created 15 billion years ago. Before the big bang, all the matter was energy, and it was all focused into a point smaller than an atomic nucleus. When it exploded, it expanded rapidly. It expanded from the size of an atomic nucleus to the size of the Earth in less than a second. While the Universe expanded, it also began to cool, and particles began to form from the radiation. The particles that formed were quarks and antiquarks. When the quarks and antiquarks touch each other, they turn into energy again. In one second, it was 10 billion degrees, and the universe was made up of lighter particles such as electrons. However, three minutes into the big bang, 1/4 of the protons and neutrons created turned into hydrogen or helium nuclei. This same process goes on for the next 300,000 years. However, when the big bang is 300,000 yeas old, the temperature of the universe is 3000°K. When the temperature gets that low, the electrons that were hindering the radiation start orbiting the hydrogen and helium nuclei, and the radiation is free to travel. All of the matter and energy in the Universe was created by the big bang.


Formation of Galaxies
When the universe was 2 million years old, galaxies started to form. Our galaxy started to form 3 billion years ago. Galaxies begin as a huge cloud of gas and dust. Gravity begins to pull the gas and dust together. The sphere of gas collapses into a disk, orbiting a central bulge. The cloud forms stars on the way in. Those stars make the halo around the galaxy. Inside the disk, stars begin to form. The galaxies are classed according to shape, which are either elliptical, spiral, or irregular. Collisions and merges of galaxies can cause spiral arms to grow, as well as causing the actual galaxy to grow. In the future, we may collide with the Andromeda galaxy.


Structure of the Galaxy
Our galaxy is a spiral galaxy. Looking at it horizontally from 500,000 light years away, there is a long disk, with a central bulge in the middle. The disk is 100,000 light years apart, and 1,000 to 2,000 light years thick. Across the middle of the disk, there is a thin layer of gas and dust. The central bulge is 20,000 light years across. A dwarf galaxy is merging with the milky way on the other side of the galaxy. There is also a halo of stars around our galaxy, about 130,000 light years from the center of our galaxy. If you are looking down on the galaxy from 500,000 light years away, you would see a central bulge with four arms winding out of the central bulge. There are bluish-white young stars in the arms of the galaxy, and there are red and yellow old stars in the central bulge of the galaxy. At the center of our galaxy, there is a supermassive black hole.


Rotation of the Galaxy
Each star and gas cloud is in its own orbit around the center of the galaxy. To go around once, the sun takes 250 million years, going 250 kilometers per second. Some of the matter in the spiral arms are not permanent structures moving with the galaxy. The way our galaxy rotates tells us that we have a huge, invisible, corona surrounding us. We call the material in this corona dark matter. Because of this, our galaxy can be five times bigger than it appears. We can tell this because the way the stars in the milky way move, we can tell something is pulling on it. No one can define what this dark matter is yet.


Formation of Planets
Planets were formed by solid materials orbiting clumping together. This was caused by gravity. They eventually grew to be a few kilometers across, and were called planetecimals. Icy planetecimals were created further away from the sun. These planetecimals collided, some gently enough to merge and grow bigger, and some so hard that they broke up again. In the outer solar system, four large masses became the giant planets, Jupiter, Saturn, Uranus, and Neptune. They pulled disks around themselves, out of which moons formed. They pulled in their own atmosphere from the surrounding gas because they have ten times the Earth's gravity. There were too many collisions in the inner solar systems for the giant planets to form. Mercury, Venus, Earth, and Mars were the four planets that formed in the inner solar system. Their surfaces were molten for a while, kept hot by the constant bombardment and by the radioactive metals inside. The denser metals sunk to the center of these planets while the lighter rock rose to the top. The surfaces eventually cooled off, and solidified.


Formation of Moons, Rings, and Comets
Moons, rings, and comets are the objects in our solar system that aren't planets or stars. Our moon was probably created by a catastrophic collision between the Earth, and a planet the size of Mars. The moon, as well as the other rocky planets, have craters created by a million years of bombardment. Some planetecimals were captures as the moons of the giant planets. If an icy planetecimal comes too close to the sun, then it becomes a comet. The rings around the giant planets are made from planetecimals being torn apart by gravity when they get too close to a giant planet.


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