Seeing in the dark: Lives of Stars
How are Stars formed?
A typical atom is surrounded by a kind of cloud of electrons. The electrons are electrically charged as the name suggests and they determine the chemical properties of the atom, for example, the glitter of the gold or the transparency of the solid. Deep inside the atom is the nucleus that is composed chiefly of protons and neutrons. Atoms are very small. Most of the mass in an atom is in the nucleus. The electrons are by comparison just bits of moving fluff. Atoms are mainly empty spaces. The matter is composed chiefly of nothing.
There are some 92 chemically distinct kinds of atoms naturally found on earth, they are called the Chemical Elements. Virtually everything we see and know, all the beauty of the natural world is made of these few kinds of atoms arranged in harmonious chemical patterns.
The fact that atoms are composed of only three kinds of elementary particles – protons, neutrons, and electrons is a comparatively recent finding. The neutron was not discovered until 1932 and it like the electron and the proton was discovered at Cambridge University.
A neutron is electrically neutral as its name suggests. A proton has a positive electrical charge and an electron has an equal negative electrical charge. Since every atom is electrically neutral, the number of protons in the nucleus must equal the number of electrons far away in the electron cloud. The protons and neutrons together make up the nucleus of the atom.
The nature of a chemical element depends only on the number of electrons which equals the number of protons which is called the Atomic Number. Protons have positive electrical charges but like charges repel each other so how does the nucleus is held together?? Why not the electrical repulsion of the protons makes the nucleus fly to pieces??
Because there is another force in nature – The Nuclear Force. We can think of it as short-range hooks which start working when protons and neutrons are brought very close together. Since the neutrons exert nuclear force but not electrical forces, they are a kind of glue that holds the atomic nucleus together. The nuclear force can overcome the electrical repulsion of the protons. This can happen only at very high temperatures where particles are moving so fast that there is no time for electrical repulsion to act. Temperatures of tens of millions of degrees, such high temperatures are common in nature…where???…in the inside of the stars!!!!
Why the Stars Shine?
Atoms are made in the insides of the stars. In most of the stars, we see hydrogen nuclei are being jammed together to form helium nuclei. Every time a nucleus of helium is made, a photon of light is generated and this is why the stars shine.
Life of a Star
Stars are born in great clouds of gas and dust like the Orion Nebula, 1500 light-years away, parts of which are collapsing under gravity. Collisions among the atoms heat the cloud until in its interior hydrogen begins to fuse into helium and the stars are born. Stars are born in batches. Later they wander out of the nursery to pursue their destiny in the Milky Way. Adolescent stars like the Pleiades are still surrounded by gas and dust. Eventually, they journey far from home.
It is the destiny of the stars to collapse. Of the thousands of stars, you see when you look up at the night sky, every one of them is living in an interval between two collapses – an initial collapse of a dark interstellar gas cloud to form the star and a final collapse of the luminous star on the way to its ultimate fate.
Gravity makes stars contract unless some other force intervenes. The hot gas in the interior of the stars expands while the gravity tries to make the star contract, thereby maintaining the equilibrium, but when the nuclear fuel is exhausted the interior cools and the pressure is no longer enough to support its outer layers, at that time the collapse resumes.
There are three ways in which stars die. Everything depends on their initial mass.
- A typical star with a mass like sun collapses until its density becomes very high and then the contraction is stopped by the mutual repulsion of the overcrowded electrons in its interior.
- A collapsing star twice as massive as the sun is not stopped by the electron pressure and it goes on falling in on itself until nuclear forces come into play and they hold up the weight of the star.
- A collapsing star three times as massive as the sun is not stopped even by the nuclear forces. There is no force known that can withstand this enormous compression and such a star has an astonishing destiny. It continues to collapse until it vanishes utterly.
Interesting Facts about Stars:
- A star that is supported by the gas pressure is a normal star like the sun.
- A collapsed star that is held up by the electron forces is called a White Dwarf – It’s a sun shrunk to the size of the earth.
- A collapsed star supported by nuclear forces is called a Neutron Star – It’s a sun shrunk to the size of a city.
- A star so massive that in its final collapse it disappears altogether is called a Black Hole – It’s a sun with no size at all.
Before the final gravitational collapse, all stars go through a stage called Red Giant in which the star swells up many times its original size.
Such is the life of an ordinary star – born in a gas cloud, maturing as a yellow sun, decaying as a red giant, and dying as a white dwarf.