The life cycle of stars

There are several hundred billion galaxes in our universe. Here on Earth we are just one star. Our galaxy is in a group called local group which is part of Vergo great galaxy. Andromeda galaxy is biggest in our galaxy. Earth is 2/3 away from the perseus arm that is apart of the Milky Way. Sometimes it is difficult for us to see stars because of air pollution, street lights, and buildings. They are up to two to four hundred billion stars. Now let us examen the life cycle of a star.

Imagine an enormous cloud of gas and dust many light years across. Gravity, as it always does, tries to pull materials together. A few grains of dust collect a few more, then a few more, then more still. Eventually. enough gas and dust has been collected into a giant ball that, at the center of the ball, the temperature reaches 15 million degrees or so. Now nuclear fusion begins and the ball of gas and dust starts to glow. A star has begun its life in our universe.

After millions to billions of years, depending on their initial masses, the star runs out of their main fuel hydrogen. Once the ready supply of hydrogen in the core is gone, the nuclear process there stops. Without the outward pressure generated from these reactions to counteract the force of gravity, the outer layers of the star begins to collapse inward toward the core. Just as during the first stages, when the materials contracts, the temperature and pressure increases. The star expands to larger than it ever was during its lifetime. The star is now a red giant.

What happens next in the life of the star depends on its intial mass. Whether it was five times bigger than the sun, or if it was a medium star only .4 to 3.4 times the mass of the sun. The next steps after the red giant phase are very different.

Once the medium size star has reached the red giant phase, its outer layers continue to expand, the core contracts inward, and helium atoms in the core contracts together to form carbon. The star will begin to shed its outer layers as a diffuse cloud called a planetary nebula. The star spends the rest of its days cooling and shrinking until its only a few miles in diameter. It has become a white dwarf . With no fuel left to burn, the hot star radiates its remaining heat into the coldness of space for many billions of years. In the end it will just sit in space as a cold, dark mass sometimes referred to as a black dwarf.

For stars five times or more the mass of the sun there is a different outcome. After the outer layers have swollen into a supergiant, the core yields to gravity and starts to shrink. As it shrinks it becomes hotter and denser. In less than a second, the star begins its final phase in life. The core temperature rises to over 100 billion degrees as the iron atoms crushes together. The replusive force between the nuclei overcomes the force of gravity, and the core recoils out from the heart of the star there is an explosive wave called a supernova.

In conclusion, stars have a very unique life and their life spand is longer than life spand of earth. In the fastest space craft we still would not be able to reach the closest star. We would have to combine our lifetimes together just to reach the nearest star. Star have some importance. They can tell you a location in the night's shy. For example, the Polaris star better know as the north star can tell you what direction to move if you are lost. People have been interested in stars since the beggining of time and will stay interested for many more years to come.