The Stars and Galaxies
A stars Life-Cycle
A star begins in a part of space where there are gas clouds. The gas particles inside of the clouds are pulled together to overcome the random movements that a gas has. This is all due to gravity. Gravity also causes the cloud to contract and warm up, specifically in the core, due to the strong compression. The core reaches millions and millions of degrees celsius, causing nuclear reactions to occur, thus birthing a star.
The mass of the star determines the size, temperature, and color. As the stars get hotter, the colors change from red, to yellow, to white, to finally blue (if they are excessively hot). The hotter that the star is, the more energy the star will have. For instance, a blue star will have more energy than a white one. This was all determined by Albert Einstein with his famous formula, E=mc².
The mass of the star determines the size, like I previously said, but the size of the star also determines the life span of the star. An average sized star, such as the sun, will have a much longer lifespan than that of a massive star. This happens because the bigger that the star is, the faster that it uses up all of its energy. The amount of energy that a star has is determined by a reaction of fusing together atomic nuclei, which is the basic nuclear fusion reaction (4 hydrogen atoms into 1 helium atom). The reaction causes the star to lose a small amount of mass as a large amount of energy. No need to worry though, our sun will keep fusing hydrogen and helium for roughly 10 billion years.
When a star can no longer fuse hydrogen because it has run out, it will start to fuse together helium and create much heavier elements. These heaver elements cause the star to heat up and expand much larger than its original size. As the star gets bigger, the outer portions of a star get much larger and cool down, causing the star to turn red. The result of this is what we call a “Red Giant.” The star begins to throw material out into space because of its periods of instability. After all of this happens, the star collapses and forms a “white dwarf”. These are very dense stars that are about as big as the Earth. However, if the star is much bigger than a star about the size as our sun, the end could be much more dramatic, such as a supernova. And if a star is so small and dense, it could become a black hole. They can also become black dwarfs. This happens after a white dwarf. It is a very dark, dense, decaying ball. This takes trillions of years. In fact, there are no known black dwarfs in our universe because stars take so long to get to that stage.
The Milky Way Galaxy
The Milky Way galaxy, like many other galaxies, is very old and is made up of billions of stars. The Milky Way is not only made up of stars, its also made from planets and gas, all bound together by gravity. Like the biggest Galaxy in our little cluster of galaxies, the Milky Way is a barred spiral. This just means that it looks like a big spiral with an elliptical looking shape in the center of it. The reason that we know this is because whenever we look at the Milky Way, from Earth (we cannot see it from the outside), we do not have to look through a telescope. We can see a group of stars that is visible from the naked eye, and it shows us that our galaxy is basically flat.
Something else that astronomers look at when they try to determine the shape of the Milky Way is the amount of gas and the color of it. They are able to look at this and see that it matches the other barred spiral galaxies.
Like I recently said, the Milky Way is made up of billions of stars, approximately 100 billion. The Galaxy is about 100,000 lightyears across, which means it takes light 100,000 year to get across. It is also only 1,000 light years thick.
Our solar system is around 25,000 lightyears away from the center of the galaxy, and just like our Earth rotates around the sun, the sun rotates around the center of the galaxy. Although, the Earth only takes about 365 days to revolve around the sun, it takes the sun about 250 million years to rotate around the center of the galaxy. This is interesting because the closer that you are to the center of the galaxy, the less time it takes to get all the way around the Milky Way, rather than if you are a star on the outer perimeter, it will take a substantially longer time to get all the way around.
There are billions of other galaxies in our universe, but there are only a few that can be seen from the Earth with a naked eye and a very dark sky. Two of these galaxies are the large and small Magellanic clouds, they are both satellite galaxies of the Milky Way and appear to be around 160 thousand lightyears away. The largest galaxy in our universe can also be seen from the naked eye, even though it is about 2.5 million lightyears away. The Andromeda Galaxy (the biggest galaxy in our universe) can be seen getting closer. This is because in about 4 billion years, the Milky Way and the Andromeda are expected to collide and form an even bigger galaxy.
Something cool to note about the Milky Way is how it got its name. The reason for its name is because when you look up at the night sky in a very dark area, it appears to be a milky strip of light.
One other cool thing about different galaxies is that there are three different types of galaxies. The obvious one is ours, and that is the spiral. The other two are called an irregular and elliptical. The elliptical shape just looks kind of like an egg shape if you look at it from a far, and rather than being flat like the spiral, it has a more spherical shape. The irregular is basically in the name, it is irregular or random looking, one example that you can even see from Earth is the small Magellanic clouds.
