The Life Cycles of Stars

The Life Cycles of Stars

In the universe, there are hundreds of billions of stars. They are formed when gas and dust from space "clump" together. As the mass grows larger, gravity takes over and a core begins to form. The core becomes extremely dense and this compaction causes temperatures within it to rise to the point where nuclear fusion (the burning of hydrogen in a chain reaction) can begin. The heat generated by fusion can reach 24 million degrees F (Fahrenheit). "That amount of heat keeps the outer gas shell expanding while the ultra-dense core keeps from collapsing under its own gravity. The star is said to be in "equilibrium" or on the "main sequence". This is the point where a star spends most of its life." (Aspire.Cosmic-Ray.org, Stars, Main Sequence).

If in the formation of a star not enough heat is generated in the core to begin a fusion reaction, it is known as a Brown Dwarf. The stars core burns different gases or materials, but soon dies off to be a cold mass in space which is very difficult to detect since they give off no luminosity.

Another category of star is the Yellow Dwarf. The most known star to all of us in this group is our Sun. With a diameter of 864 thousand miles, it is a comparatively small star. The surface temperature is 11 thousand degrees F., with a core temperature of 24 and one-half million degrees. Our Sun is currently halfway through its 10 billion year life. The Sun is brighter than 85% of the stars in the galaxy, most of which are Red Dwarfs.

Red Dwarf stars make up the majority of stars in the universe. They are still "main sequence" stars, or stars in equilibrium, but they have a much cooler surface temperature than the Sun around five thousand nine hundred degrees F. Due do the relatively cool surface temperatures, the star glows red. Since Red Dwarfs are smaller in size, they can burn more fuel than larger, more massive stars and are much more stable. This gives them a much longer life, sometimes as long as several trillion years. Currently, research is being done to find other planets in the galaxy and the focus is on Red Dwarfs since they are so common. Several planets have been found recently but habitability on these planets that orbit Red Dwarfs is debatable. Since the star is cooler, the planet would have to be very close to the star." Most likely it would be "tidally locked" where the planet has one side perpetually light and the other eternally dark." (Wikipedia.org, Red Dwarf, Habitability). Our own moon is tidally locked to the earth. Also, Red Dwarfs show some variability in their brightness, sometimes doubling in a matter of minutes or cooling the same rate just as quickly. These factors would make life very tough!

As a star moves off of the "main sequence" and begins to deplete its supply of hydrogen, certain things begin to happen depending on the stars mass. A star like our Sun will lose equilibrium and the outer gas layers will expand as the core contracts. This expansion will cause the Sun to expand to a size which engulfs the nearest planets including the Earth. It will then shed is outer layers and only the hot core will remain which will take billions of years to cool off. It will be known as a White Dwarf because it will glow white hot.

Other stars which are larger than the Sun, and therefore more massive, have different deaths. Since these stars are so large, when the fuel is gone, the core contracts in an instant and the outer shell explodes into space in an event known as a Supernova. Temperatures in the core can reach 100 billion degrees F. Depending on the mass of the star, the core can remain incredibly dense to the point where it weighs three times as much as the Sun, but only be six miles in diameter. Stars rotate very quickly, (the Sun rotates four times faster than the Earth), and as the core collapses on itself, it spins even faster. Rotation speeds of some stars can be as fast as 1 seven hundredth of a second and as long as 30 seconds. This remaining rapidly spinning core is called a Neutron Star." Its matter is so dense that one teaspoon full weighs 100 million tons." (Wikipedia.org, Neutron Star, Structure).

The star Betelgeuse in the constellation Orion is one which is close to supernova. Currently, the star is 500 million miles in diameter, quite large compared to the Sun because it is in its dying phase where its hydrogen fuel is running out and is beginning to burn Helium which is a much hotter process. The star becomes bigger because it is still trying to maintain equilibrium by expanding