All Stars

Life Cycle of Star - A story

The life and death of star in the universe is a question that remains unanswered.

We certainly know that our universe exists, however, this knowledge alone has not satisfied mankind's quest for further understanding. Our curiosity has led us to question our place in this universe and furthermore, the place of the universe itself. Throughout time we have asked ourselves these questions: How did our universe begin? How old is our universe? What is the story about life and death of star in the universe? Obviously, these are not simple questions and much of what we know is still only speculation.

We have, however, come a long way from the mystical beginnings of the study of cosmology and the origins of the universe. Through the understandings of modern science we have been able to provide firm theories for some of the answers we once called hypotheses. True to the nature of science, a majority of these answers have only led to more intriguing and complex questions. It seems to be inherent in our search for knowledge that questions will always continue to exist.  Although in this short article an attempt is made to address certain fundamental questions of life and death of star in the universe.

Life of stars

Stars are large balls of hot gas, they look small because they are a long way away, but in fact many stars are bigger and brighter than the sun. The heat of the star is made in the centre by nuclear fusion reactions. There are lots of different colors and sizes of star.

There are two main life cycles for stars, and almost all stars can fit into one or the other. The determining factor for categorizing the star is its mass. Any star less than about three solar masses (one solar mass is the mass of our sun ) will spend almost all of its life transiting what is called the "Main Sequence." About 90% of all stars are like this. If a star is above the crucial value of three solar masses when it is born, it will spend much less time on the Main Sequence, have a much shorter life span, and it will die much more violently, creating either a neutron star or black hole (1).

During the major part of their lives, most stars on the Main Sequence will create their energy by the process of hydrogen fusion - the process of fusing two hydrogen atoms to create one helium atom. Energy is created because a helium atom weighs slightly less than the two hydrogen atoms, and the excess mass is converted into energy, as related by Einstein's famous equation E=mc2. Our sun is currently in this stage of converting hydrogen to helium.

During nuclear fusion of the star two protons of hydrogen ionsjoin together to form a deuterium nucleus, i.e. "heavy water." A positronand a neutrinoare released as by-products. The deuterium nucleus is bombarded by another proton, creating a helium-3 nucleus. The by-product of this is a photonin the form of a gamma ray (a very high-energy form of light). Then, the helium-3 nucleus in bombarded by another helium-3 nucleus, creating a normal helium-4 nucleus. The by product of this are two protons, which are free to start the whole process over again. The positron will be destroyed and form another gamma ray; the energy from this in the form of gamma rays is radiated out of sun's core. Since our sun is currently in this stage, the numbers here are for it alone, although since it is like most other stars, they are representative of how all stars work.

Each second, the sun converts 500 million metric tons of hydrogen to helium. In turn, every second 5 million metric tons of excess material is converted into energy. This means that every year, 157,680,000,000,000 metric tons are converted into energy. The material from one second energy is about 1x1027 (one octillion thousand) watts of energy. On Earth, we receive about 2/1,000,000,000 (two billionths) of that energy, or about 2x1018 (two quintillion) watts. This is enough energy to power 100 average light bulbs for about 5 million years -- longer than humans have been standing upright (1).

Stars are born in giant clouds of dust and gases. Sometimes part of the cloud shrinks because of gravity. As it shrinks it becomes hotter when it is hot enough, nuclear reactions can start in the centre and a star is born. Once nuclear fusion is producing heat in the centre of the new star, this heat stops the rest of the star collapsing. The star then stays almost exactly the same for a long time (about 10 billion years for a star like the sun). The balance between gravity trying to make star shrink and heat holding it up is called Thermodynamic Equilibrium.

During its life a star will not change very much. However, different stars are different color, sizes and brightness. The bigger a star, the hotter and brighter it is. Hot stars are blue in color. Smaller stars are less bright, cooler and red. Because they are so hot, the bigger stars actually have shorter lives than small, cool ones.

Life span:

The most massive stars have the shortest lives. Stars that are 25 to 50 times that of the sun live for only a few million years. They die so quickly because they burn massive amounts of nuclear fuel. For example, Betelgeuse (the second brightest star in Orion) is a red supergiant that is about 20 times more massive than the sun. It is about 14,000 times brighter than the sun and burns nuclear fuel at a rate of 14,000 times faster than that of sun (2).

Sun-like Stars

(Up to 1.5 times the mass of the Sun)

Huge Stars

(From 1.5 to 3 times the mass of the Sun)

Giant Stars

(Over 3 times the mass of the Sun)

Evolved star

An evolved star is an old star that is near the end of its existence. Its nuclear fuel is mostly gone. The star losses mass from its surface, producing a stellar wind (gas that is ejected from the surface of a star). Older stars produce more stellar winds than younger one(3).

Death of stars

Stars expand as they grow old. As their core runs out of hydrogen and then helium, the core contacts and the outer layers expand, cool, and become less bright. This is a red giant or a red super giant (depending on the initial mass of the star). It will eventually collapse and explode. A star's life span and eventual fate are determined by the original mass of the star.

Eventually, the hydrogen (the fuel for the nuclear fusion) in the centre of the star will run out. No new heat is made and gravity will take over and the centre of the star will shrink.  This makes the very outside of the star "float up" and cool down, making the star look much bigger and redder, this is called Red giant star.

As the centre collapses, it becomes very hot again, eventually getting hot enough to start a new kind of nuclear fusion with helium as the fuel. Then the red giant shrinks and the star looks normal again. This does not last very long, though, as the helium runs out very quickly and again the star forms a Red Giant.

For a star like the sun, no more nuclear fusion can take place, so the centre of the star will then keep collapsing. Eventually it can become almost as small as the earth, but with the same mass as a whole star. This very dense object is called a White Dwarf. A piece of White Dwarf the size of a mobile phone would weigh as much as an elephant on the earth.

The end of sun like star

The outer parts of the star (that formed the Red Giant) then drift off into space and cool down making a planetary Nebula. Planetary Nebulae have nothing to do with planets. Of course, they just look a bit like them in small telescope. Example of planetary Nebula is M57 with its white Dwarf in the middle.

The end of massive star

For more massive (bigger) stars than the sun, many more types of nuclear fusion can take place. This means several more Red Giant stages. However, eventually even the biggest stars run out of fuel and finally collapse. For the biggest stars, this collapse causes a huge explosion called Supernova. A Supernova can be brighter than an entire Galaxy of 100,000,000,000 stars.

Because the star was so big, the collapse does not even with a White Dwarf, but an even most dense object called a Neutron star is made. The density of a Neutron star is about 1x1018 Kg/m3 (i.e. 1,000,000,000,000,000,000). Sometimes the collapse can not stop at all and a Black hole is made, from which not even light can escape. The debris of the explosion is blown away and forms a glowing cloud called a Supernova remnant. e.g. the crab supernova remnant.

When the nuclear power source at the center or core of a star is exhausted, the core collapses. In less than a second, a neutron star (or a black hole, if the star is extremely massive) is formed. The formation of a neutron star releases an enormous amount of energy in the form of neutrinos and heat, which reverses the implosion. All but the central neutron star is blown away at speeds in excess of 50 million kilometers per hour as a thermonuclear shock wave races through the now expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst that can be as intense as the light of several billion Suns(4).

Reference

1.Life and Death of Stars Page 1 of 3

http://burro.astr.cwru.edu/stu/stars_lifedeath.html

2.Star Death - Zoom Astronomy Page 1 of 3

http://www.enchantedlearning.com/subjects/astronomy/stars/lifecycle/stardeath.sh tml 02/08/2010

3.Chnadra x-ray observatory.

http://chandra.harvard.edu/xray_sources/Supernovas.html

4.Virtual Trips to Black Holes and Neutron Stars from NASA

http://www.enchantedlearning.com/subjects/astronomy/stars/lifecycle/stardeath

 

About the Author

I was born in Kolkata, Qualified Ph.D on 1989 from Calcutta University (Spl. Endocrinology), acquired research experience of more than 22 years with publications of around 29 papers in various national / international journals , acquired teaching experience of more than 15 years, acquired experience of writing biology text book under ISC course which is currently under Cambridge press (Kolkata) for publication. Awarded Sangit Prabhakar and Prayag Sangit Samiti in Indian Classical music.  Awarded certificate of appreciation – Celebrations of the centenary of Ramkrishna Mission & of Swamy Vivekananda's historic return from west in 1897 as well as of 66th foundation day of the pratisthan.  Ramakrishna Mission Seva Pratisthan (Kol-India), expressed its participation of my contribution to the success of the seminar in 1997. 

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