A star joins the main sequence when it begins to generate energy by consuming hydrogen in nuclear reactions deep in its core. Prior to that time, the star generates energy primarily by gravitational contraction that raises the temperature in the central regions. As the temperature near the star's center increase, individual atomic nuclei eventually move fast enough that they can come close enough (despite the repulsion of their protons) to fuse together. These nuclear fusion reactions generate new heat and radiation pressure that stop the contraction, causing the long-term stability of the star on the main sequence. When the fusion stops gravitational collapse, we say that the star is born.
Hans Bethe. Click here for original source URL.
The Sun provides our closest view of a main sequence star. Recall that the debate over the age of the Earth hinged on understanding the Sun's energy source. Stellar fusion could not be understood without the quantum theory of matter. In classical physics, the electrical force that repels one proton from another is an insurmountable barrier. But in the quantum theory, there is a finite probability that the protons will ignore the barrier and fuse. This small probability is enough to allow the Sun to shine! Who cracked this difficult problem? The calculations were done in the late 1930s by Hans Bethe, a German physicist who had emigrated from Nazi Germany to work at Cornell University. Bethe not only showed that protons could be added like building blocks to make a helium nucleus but also that heavier elements could catalyze the creation of helium from hydrogen. His paper won a prize from a scientific society, and he used the money to help his mother escape from Germany. Later, this same work won him a Nobel Prize.
Hertzsprung-Russell diagram showing color and size of stars. Click here for original source URL.
In the H-R diagram, an imaginary array of stars with different masses which have all just reached the main sequence and just begun to consume hydrogen is called the zero-age main sequence. Calculations show that it would be a line of stars on the H-R diagram spread out according to their mass. When astronomers measure the properties of real stars, the main sequence on the H-R diagram is a band rather than a line. The main sequence broadens because it contains stars of different ages, which have converted different fractions of their hydrogen to helium. Thus they have slightly different structures and slightly different positions on the H-R diagram. As stars of any mass age, they become more luminous and slightly cooler.
After a star reaches the zero-age main sequence, it begins its life as a true star and commences a long sequence of various nuclear reactions. During each such reaction, tiny amounts of matter are converted to energy, according to the equivalence principle established by Einstein early in the 20th century. These nuclear reactions provide the heat and light of the star. The basic cause of stellar evolution must be stressed: nuclear reactions convert light elements into heavier elements, changing the star's composition and its energy generation rate; this in turn causes the star to alter its structure. In a general sense, the sense we have of stars as light bulbs in the sky is misleading. The main business of a star is forging heavier elements by fusion; the light we see is just a byproduct of the nuclear reactions.