19. Life Cycle of Stars

A light year is a unit of distance, not of time. It is the distance travelled by light in one year, which is 9.5 × 10¹² km.

Stars of mass less than 8 M_Sun end as white dwarfs, those between 8 and 25 M_Sun as neutron stars, and those above 25 M_Sun as black holes.

When the pressure of the electrons balances gravity, the star becomes a white dwarf. A neutron star is formed when the pressure of the neutrons balances the gravitational force.

Nothing at all can come out of a black hole, not even light. Its gravitational force is so great that light falling on it is absorbed and never reflected, which is why the star cannot be seen.

The supergiant stage comes only in the life of stars of mass more than 8 M_Sun. The mass of the Sun is 1 M_Sun, so it will pass through the red giant stage, not the supergiant stage.

The mass of the Sun is far less than 8 M_Sun, so after the red giant stage its outer gas envelope will be thrown off and the inner part will contract to about the size of the earth — a white dwarf.

• Huge clouds of gas and dust, a few light years across, lie in the empty spaces between the stars of a galaxy. These are called interstellar clouds.
• Because of some disturbance, such a cloud begins to contract. As it contracts its density increases, and its gravitational potential energy is changed into heat energy, so the temperature also rises. A dense sphere of hot gas is formed.
• When the temperature and the density at the centre of this sphere become high enough, nuclear fusion begins there — in the Sun, hydrogen nuclei fuse to form helium nuclei.
• Because of the energy generated by this fusion the gas sphere becomes self luminous, and we say a star is born.
• One gigantic cloud can give birth to many stars at the same time, which is why we see clusters of thousands of stars formed from a single cloud.

A star is stable only so long as the outward pressure of its hot gas balances the inward gravitational force. To keep the temperature, and hence the pressure, steady, energy must be generated all the time at its centre by the burning of fuel.

But the fuel at the centre is limited and goes on being used up. When it is finished, the energy generation stops, the temperature falls, the gas pressure falls, and gravity — now the stronger of the two — makes the star contract. The contraction raises the temperature again, so a new fuel starts burning; when hydrogen is finished, helium begins to fuse, and so on. Because of this the star expands at some times and contracts at others and passes through different stages. This change in the properties of a star with time, caused by the using up of its fuel, is its evolution. How many fuels are used depends on the mass of the star, and the greater the mass, the faster the evolution.

In these stars no pressure is able to balance the enormous gravitational force, so they go on contracting for ever. As the size becomes smaller the density and the gravitational force increase tremendously. Everything near them is pulled in, and nothing can come out of them, not even light. Light falling on them is not reflected but is absorbed inside. So nothing can be seen at that place — only a small dark patch, a ‘hole’, that looks completely black. That is why this end stage is called a black hole.

Stars whose initial mass is between 8 and 25 times the mass of the Sun. They pass through the red giant and supergiant stages, in which their size may grow a thousand times, and then a tremendous supernova explosion takes place — so bright that the star can be seen even during the day. The central portion left behind contracts to a sphere of only about 10 km, made entirely of neutrons. The pressure of these neutrons does not depend on temperature and balances gravity for ever, so this neutron star is the end stage.

“I am the Sun, the star nearest to you. I look much bigger than all the other stars only because I am the closest one — in truth I am quite an ordinary star, for there are billions of stars with greater or smaller mass, size and temperature than mine.

I am a gigantic sphere of hot gas. Hydrogen makes up 72 % of my mass and helium 26 %; the remaining 2 % is of elements heavier than helium. At my centre, hydrogen nuclei fuse into helium nuclei and give out the energy that makes me shine and keeps your earth warm and alive.

I stay steady because two forces inside me are balanced — my gravitation pulling all my gas inward, and the pressure of my hot gas pushing outward. Because of this balance my properties have not changed in the last 4.5 billion years, and your scientists know this from their study of the earth itself. Light from me takes about 8 minutes to reach you.

After about another 4.5 billion years my hydrogen will be used up. I shall then swell into a red giant, so large that I shall swallow Mercury and Venus, and perhaps your earth too. At the end I shall throw off my outer gas envelope, and my inner part will shrink to the size of the earth — a white dwarf. Then I shall only go on cooling quietly for ever.”

• A white dwarf is the end stage of a star whose initial mass is less than 8 times the mass of the Sun.
• Such a star first swells into a red giant, and at the end of its evolution it explodes gently and throws off its outer gas envelope. The inner part contracts and its size becomes about the same as the size of the earth.
• Its mass is far greater than that of the earth while its size is that of the earth, so its density is enormous — one spoonful of its material would weigh a few tons.
• At this density the pressure of its electrons no longer depends on the temperature of the gas, so it can balance the gravitational force for ever, even though no energy is being generated any more.
• The star is white in colour and very small in size, and that is why it is called a white dwarf.
• After this its temperature goes on decreasing, but its size and mass remain unchanged for ever. The Sun too will end its life in this way.