The Life Cycle of Stars


Stars are said to be the father of all planets and organisms. When super giants collapse under the pressure of gravity, an explosion known as supernova occurred and the elements within the star’s core spread out into the universe, thus created Earth and other planets. But just what are stars? Why is it so important? How does it form? Well, long before the explosion, even before a star, there was a mist of clouds known as nebula. Although it may seem insignificant, it was the sign of a new born star…..

Nebulae are clouds of gas and dusts in space. It was a term originally for any celestial objects, including the Milky Way along with other galaxies. There are many types of nebulae; emission nebulae, reflection nebulae, planetary nebulae, and dark nebulae would be some examples.


As more and more nebulae fuses together, a part of compressed matter would start to heat up and form a protostar. At this time, the temperature of the protostar should be around 15 million degrees Celsius, and nuclear fusion can begin. Nuclear fusion is the fusion of two elements. In this case, hydrogen would fuse to become helium. As the process continues, the star would begin to release energy and become more stabilize. Our sun right now is in the main sequence phase; it had burned for about 5 billion years, and would continue to for another 5 billion years.

A massive star went through a process similar to an average star. The only difference occurs in the main sequence phase, during nuclear fusion. The massive star, unlike an average one, it would keep on shining until all the hydrogen had fused to become helium.

Scientists notice an interesting relationship between the size of a star and the rate of nuclear fusion. They found out that the smaller the size of a star, the longer the nuclear fusion, and the bigger the star, the shorter the nuclear fusion. In other words, as the size of a star increase, the shorter it takes for all hydrogen to fuse and become helium. An average star usually last up to billions of years, while a massive one would only last millions of years.


After all the hydrogen had fused to became helium, reactions began to occur in a shell around the core. Helium would fuse to become carbon, just like when hydrogen fused to form helium. The outer layer expands, becoming ten to a thousand times the diameter of the sun. Here’s a picture of a red giant.

A massive star, on the other hand would evolve into a super giant. It starts off as a normal red giant, but radiation are released by the fusion of helium into carbon, therefore caused the red giant to expand into a super-giant. A super-giant is at least five hundred times the sun’s size.

All super-giants, after the sudden expansion of mass would began to shrink because of gravity. As the size of the super-giant decrease, the density and temperature would increase. Nuclear fusion would again take effect, preventing the collapse of the core. However, when the core are left with only iron, it could not fuse*. Nuclear fusion could not begin, therefore the core starts collapse. Temperature raised over a hundred billion degrees as elements within the core crashed. The repulsive force between the elements overcame the force of gravity, causing an explosion known as supernova.

* Iron’s molecular structure does not allow it self to fuse with other heavier elements.


White dwarfs are considered to be a dead star, and it is a step that would only occur in the average star cycle. As a sun-size star reaches the red giant phase, the outer layers of the star would continue to expand, while the core starts to contrast. Helium fuses to become carbon; this would temporarily reprieve the star for a few minutes. Eventually the core looses materials to fuse and form carbon; resulting a star without fuel and energy. Without energy, there is no heat. The core would slowly cool down becoming a white dwarf. After several billion years, the color would start to change, from white to black. When it turned black, it would be then known as a Black dwarf. A white dwarf has a density equal to sun, and it is only a little bit bigger than Earth, making it the second densest object known, after neutron stars.


This part of the cycle occurs only with massive stars, after the explosion of supernova. Although very rare, after supernova, the core of the star may still survive. The survived core would now either turn into a black hole (if its mass is 5 times the sun or more) or a neutron star (if its mass is 1.5 to 3 times the mass of the sun).

Black Holes are unknown matters that are so dense that not even light can escape their gravitational fields. The theory of relativity, by Albert Einstein, states that light is the fastest thing in the world, and no matter how fast you travel, you can’t travel faster than the speed of light. If Einstein is correct about light, then we know that absolutely nothing can escape the gravitational force of a black hole. No one knows what would happen after you are pulled in. However, we do know that with the technology we have now, know one could survive to tell the tale.

Neutron stars are the densest object known. They are about ten miles in diameter and have about 1.4 times the mass of our sun. Because neutron stars are so small yet so high in density, the gravitational force of an average neutron star is about 3000,000 times the force on Earth.