Most of the visible mass of the galaxy is in the form of stars.
What makes them shine? The most popular idea at 19th century was that the source of energy was the slow contraction of the Sun. In this theory, as the Sun slowly contracts, its gravitational energy increases, so the Sun can use this energy in order to maintain its luminosity. The problem with this theory was that, The Sun’s gravitational energy can only keeps its luminosity for 10 million years. This time-scale is in a serious conflict with the age of our solar system.
We now know that the energy source of the Sun is the nuclear burning of hydrogen into helium and there is sufficient hydrogen fuel in the Sun for it to radiate for about 10 billion years.
The region in which these nuclear reactions can take place occupies the central 10% of the central region. The temperature in the central region of the Sun is about 16 000 000k. Energy is transported from the central region to its outer layers by radiation and this process is responsible for maintaining the Sun’s internal thermal energy. The Internal energy of the Sun is essential since this is the source of the internal pressure which is responsible for holding it up against gravity. The sun is in the state of balance when the inward pull of gravity and the outward pressure of hot stellar material cancel each other out. Normal stars are regions in the universe in which the gas density and the temperature are so great that nuclear reactions take place and the energy released provides the pressure to overcome gravity.
