Saturday 5 October 2013

Life cycle of stars


Stars are mainly formed from the dust and gases present in a galaxy. The dust and gases are collectively known as nebula. From a cloud of dust and gases many stars are formed. Gravitational force is mainly responsible for the formation of stars. The dust and gases in the cloud begin to shrink and as they shrink their temperature rises. Due to the rise in temperature a nuclear fusion reaction takes pace.
In this reaction four hydrogen atoms combine to form one helium atom. The mass of one helium atom is a little bit less than that of these four hydrogen atoms. The difference in the mass is called as mass defect. This mass defect appears in the form of binding energy which binds the nucleons (protons and neutrons) in the nucleus. According to Einstein’s equation “E =mc2” a tremendous amount of energy is released.
The largest stars contain matter which is about 120 times higher than the matter in our sun. They rapidly generate large amounts of energy by consuming the matter inside them. This energy is useful to prevent the collapse of the star due to gravitation. The temperature of a very bright star would be about 70,000. They will be radiating a large amount of heat and light and will exist up to lakhs of years only.
Our sun is at a temperature of about 10,000 and will be radiating up to thousands of crores of years. Very small stars which have only 8% matter of the sun at 3-4 will exist of lakhs of crores of years.
After hydrogen is exhausted in a star its end approaches. The star will expand rapidly such that its diameter will be hundreds of crores of miles. This stage is called as “red giant”. So, the temperature of the star will be reduced. At this stage it might radiate energy due to some nuclear reactions occurring inside it. Eventually it will run out of fuel. The outer layers expand, cool and will shine less brightly. The core becomes a “white dwarf” that gradually cool down and dims. When it finally stops shining altogether, the dead star will become a black dwarf.
Massive stars have a mass at least three times that of the sun. these stars after passing through the stage of being a red giant explode and the outer layers are blown away by a shock wave. These supernovae shine brighter than an entire galaxy for a short time. The core may or may not survive the explosion. If the core survives and its mass is between about one and a half solar masses, it contracts to become a tiny, dense neutron star. If the core is considerably greater than three solar masses, it contracts to become a black hole