A life cycle of a star

written by Mohammed Idrees Bhatti

The study of stars is the oldest science. We have recently understood what made the stars to shine. Today we know, like every thing else, that all poor stars go through a cycle of stages called a life cycle of a star. Stars are born, they radiate energy, and then they expand, contract, possibly explode, and eventually die out-- a painful death.

We like to answer following questions:

1. What is Universe?

2. What are its composition, structure, size and age?

3. What is the origin of the Universe, and what is its future?

4. What are galaxies and what are stars?

The birth of a star, according to accepted theory, begins with the condensation of interstellar material, mostly hydrogen clouds, as a result of gravitational attraction, radiation pressure from near by stars, and supernova shock waves. The birth of our star took place about 5 billion years ago. Our Sun is the brightest star in the sky which has magnitude or brightness -26.7. Note the greater the negative number, the brighter the star. Closest to earth, there are three more stars which are 4.3 light years (ly) away. They are Alpha Centauri A, Centauri B, and Proxima Centauri -- a faint red dwarf star. Now, I am going to refer you to Hertzsprung-Russell diagrams which illustrates the evolution of a star with same mass as our Sun --- see any astronomy book.

We obtain H-R diagrams when the absolute magnitudes of stars are plotted against their surface temperatures or colors. If you look at H-R diagrams you will notice that younger and brighter star stay on the diagonal and form a main sequence. On the main sequence the star continues to burn hydrogen into helium. It is called nuclear burning not a chemical fire. The burning of the hydrogen takes place for billions of years--- may be 10-15 billion years for a star like our Sun. Note this time scale is small on astronomical scale.

As the time passes in star's life the hydrogen in the core of the star continues to burn into helium, the core begins to contract and heat up. The resulting heat causes the fusion of the hydrogen to proceed at a more rapid rate. At this point a star quietly leaves the main sequence above the diagonal and start expanding to become a red giant and may be super giant. Eventually the core gets so hot that helium can fuse into carbon, nitrogen, oxygen--essential elements for life -- and soon other nuclear reactions occur in which all elements up through iron are created. This process for creating nuclei of elements inside stars is called nucleosynthesis. Wait a minute this isn't end of it.

The greater the mass of a star, the faster it moves through its life cycle. Then the star spends part of its life as super red giant and moves down the diagonal or main sequence. We believe at this stage it become a supernova--this stage comes after red giant stage. A very dangerous stage. Occasionally, a star explodes and throws off large amount of material that may be so great that the star is destroyed. Such a gigantic explosion is known as supernova. Only few supernovae have been observed in our galaxy. One of them is the Crab nebula in the constellation Taurus. What are Nebulae?--They are clouds of interstellar gas, dust or both that shine by reflection of light from stars. Note we may be made of star dust or remains of the star debris. Also note that the supernova stage does not occur for most stars.

No more nuclear fusion reactions are possible and the star's nuclear energy has been spent. Our own star might go through this process of supernova explosion. What becomes of the ejected material due a supernova explosion? We believe that it is thrown into space and eventually becomes the seed for the future stars. As a mater of fact, we believe our own Sun is a second generation star. because the universe is believed to be 15-20 billion years old and our Sun is only about 5 billion years old. Anyhow, the left over of the star may begins to collapse gravitationally and go to its end stage. The end stage of a star depends on the star's mass at the end of its active life. Believe me its not over yet. Small-mass stars become white dwarfs and it is about the size of the Earth, so dense that a single teaspoonful can weigh up to 5 tons. Large-mass stars ( between 1.5-3 times the our Sun's mass) have tremendous gravitational attraction and may collapse to the size of a city. These stars are called neutron stars. A teaspoonful of a neutron star may weigh up to 1 billion tons. They can become Black Holes depending on their gravitational collapse. Oh its scary now even the light can not escape from them. Well folks that's it. What a life of a star.

Some say the world will end in fire

Some say ice.

From what I have tasted of desire

I hold with those who favor fire. ----Robert Frost.