Astronomy_smr

=**From the Start of Our Universe to the Start of Galaxies**= =**The Universe began 15 billion years ago in an event called the Big Bang. The Universe started small and expanded rapidly. At the beginning, the Universe was made up of just matter, anti-matter, and radiation. These were the only particles that could survive the heat, but when the temperature cooled down enough, protons and neutrons formed. After the Universe got even cooler, atoms were able to be formed without being ripped apart. Throughout all of this, the Universe was still expanding and matter clumped together. Eventually gravity collapsed on the matter to make galaxies. Galaxies can be elliptical, spiral, or irregular. Our Galaxy, the Milky Way Galaxy, is a spiral galaxy that is 3 billion years old. Some day in the future our galaxy will merge with the Andromeda Galaxy as merger are very common in space.**=

= How Galaxies Form = = Our Galaxy, the Milky Way Galaxy, started off as a giant sphere of gas. Some of the stars formed in globular clusters that were scattered around the sphere. These stars are now the halo of the galaxy. The rest of the gas settled into a disc around a central bulge. Finally, spiral arms developed around the central bulge. That is how our galaxy was formed. =

=**The Structure of the Galaxy **= = In the night sky, our galaxy has an arch like structure, but when you look at our galaxy from thousands of light years away, edge on, you see something else. Our galaxy has a disc on either side of the central bulge. Both of those sides are about 100,000 light years across. In between the disc, there is a thin layer of gas and dust. At the center of the disc, there is what is called a central bulge. That bulge was flattened when gravity came down upon it. This is the center of the entire galaxy and is where the old stars are located and it is home to a black hole. Our galaxy is normally sized and is classified as a spiral galaxy. =



**The Rotation of Galaxies **
== Our galaxy, like all other galaxies, does not stay still. The whole galaxy and the spiral arms turn one way as where each star and gas cloud is in its own orbit. Stars like our sun take 250 million years to orbit once even though they go up to 250 kilometers per second. The spiral arms are not where stars like the sun are located but the arms only represent where matter temporarily piles up on its orbit path. They should not be confused with these pieces of matter being a part of the spiral arms. From outside of our galaxies, it is obvious to tell it is surrounded by a huge corona. == =**Formation of the Planets **= = When the sun was first formed, small objects orbited it. Those objects clumped together to become Planetecimals. Over time, these Planetecimals either collided and stack together or they collided and broke down. On the inner part of the solar system, there were too many collisions so the four remaining Planetecimals are the ones that are still here today because they were spread out enough that the did not collide. In the outer part of the solar system, the four remaining Planetecimals grew so large that they were able to shoot off other Planetecimals or keep them in their outer rings. Now, we just call these remaining Planetecimals Planets. =
 * [[image:smr_moon.jpg align="left"]]The Formation of Moons, Rings, & Comets **
 * Every planet there is has a moon. Earth's moon was formed when Earth collided with a Mars like structure and after merging, Earth became bigger and the moon was slung off and formed. On other planets, the Planetecimals were captured as moons for the bigger planets. Comets are formed when icy Planetecimals from the outer part of the solar system reach the warmer vicinity of the of the sun. The rings around certain planets were formed by stray Planetecimals and comets being torn apart by gravity when they get to close to the planet. The planet then pulls these pieces in and makes their rings from them. **

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