A Star is the luminous, massive sphere of plasma, which is held together by its own gravity. Stars are cosmic energy sources and they produce light, heat, x-rays, ultraviolet rays and other forms of radiations. They are made up largely of gas and plasma, which is a superheated state of matter. Our universe, roughly contains more than 100 billion galaxies, and each of the galaxies may contain more than 100 billion stars. It is impossible to see all these stars with a naked eye. Yet on a clear and dark night, we can see around 3,000 stars.
Some stars appear brighter than others. The brightness or luminosity of the star depends on the temperature of the star and also how far they are from earth. They also appear, in different colour. Hot stars appear blue or white, whereas the cooler stars appear orange or red.
The size, temperature and mass of the star may vary, ranging from 450x smaller in diameter to over 1000x larger than the Sun. The temperature of the surface may vary from 3,000 degrees Celsius to over 50,000 degrees Celsius. Our sun with a surface temperature of over 5,500 degrees Celsius, appears yellow in colour.
Depending on the size of the star, they are classified in a range from dwarf star to Supergiant stars. Supergiant stars may have a radii thousand times larger than our sun.
Birth of a Star as a Protostar
The primary building block of the stars is hydrogen. This gas in the form of cosmic clouds of dust circles in the space. These dust clouds are called as Nabulae. The Stars are born in the high density region of Nebula and gets condensed into a big globule of dust and gas. This globule further contract under its own gravity. As the star collapses, the density increases resulting in the dramatic increase in pressure and temperature. At this stage, release of gravitational energy produces the luminosity. This collapse may take millions of years for the low mass stars until they arrive in the main sequence. Huge molecular clouds give birth to massive stars. These stars collapse rapidly than the low-mass counterparts, may be in 10,000 years.
If the mass of the star is not enough to create high temperature and pressure in the core, then the star can never commence the hydrogen fusion. A brief period of deuterium or heavy hydrogen fusion may take place, but ultimately the luminosity of the star will be due to the heat release during the gravitational collapse. The star will finally turn into a glowing cold mass called Brown Dwarf.
If the mass of the Protostar is enough to create a temperature of 15 million degrees centigrade, then the Protostar enters into a Main Sequence Star. At this stage, the fusion of hydrogen to form helium starts and stars stops contracting and begins to release energy and shine. Our Sun is the nearest main sequence star to earth.
A Star with approximately one solar mass will remain in the main sequence for about 10 billion years, until all its hydrogen is exhausted in the fusion reaction to form helium.
The core of the helium now begins contraction and further reactions began to occur. Now fusion of helium to form carbon starts. The outer layer cools down, begins expanding and the shining becomes less. This expanding stage of the star is called as the Red Giant.
Betelgeuse is a red giant. It is 14.000 times larger than the sun, and 20 times as massive as the Sun. It is about 600 light-years away from the Earth.
When the helium of the core exhausts, the outer layer drifts away. The remaining 80% of the original star is the final stage of the star.
This dense and fairly luminous stars are also called as degenerate dwarf. The nearest white dwarf known to us is Sirius B, which is 8.6 light years away. The white dwarfs are said to be the final state of evolution of stars, whose mass is not enough to become a neutron star.
In the twilight of its years, once a magnificent star is doomed to a slow death. The fate of the white dwarf is to cool for billions and billions of years until the whole mass turns into a ultra-cold black cinder.
The main constituent of the white dwarf is oxygen and neon, and some carbon. If a white dwarf has a mass more than 1.4 times that of our sun (the Chandrashekar limit), then the star collapses into a neutron star.
The White Dwarf sufficiently cools down to the cosmic microwave background temperature, over billions of years, and will be no longer capable to emit significant light or heat, it is called as Black dwarf. This is the theoretical end of the stellar evolution.
A few stars, with higher mass, eschew this evolutionary path and go with a big bang detonating as Supernovae. This violent explosion leaves behind a small size core that may turn into a neutron star or even, a black hole.
The sun will also go through the same evolution process and in about 5 billion more years, it may turn into a red giant.