Stars are born when immense clouds of interstellar dust and gas condense through the mutual attraction of gravity. When the condensed material reaches a critical density, the process of fusion begins, and the material becomes a star. The star can continue to aggregate material from its environment throughout its life. Recent evidence gathered from newly discovered extra-solar systems suggests that large gas giant planets that form around stars can spiral inward, sweeping up more material as they go.
The more massive a star, the faster it uses up its fuel for fusion. The most common element in the universe is hydrogen, and this is the primary fuel for fusion in stars. Hydrogen fusion produces helium. After awhile, the star will begin to exhaust its supply of hydrogen, and will start feeding on the helium. This will produce lithium and heavier elements. In succession, the star will begin to use heavier and heavier elements.
Once the star starts producing iron, it has difficulty making heavier elements. It does produce these elements but in very small quantities. These elements are relatively uncommon in the universe. At this point the star undergoes a variety of changes. Massive stars will explode in a supernova, sometimes producing a blackhole. Tiny stars will slowly cool and die, becoming cold, dark, dwarf stars.
The lifespan of stars varies from thousands of years for massive stars to billions for smaller stars. Our Sun, which is of average mass, is predicted to live for about 10 billion years (it is about halfway through).
Scientists determine how old a star is by looking at its spectra - the data about the chemical composition of the star that is revealed in the frequencies of its light. The more heavy elements present in the star, the older the star. To get a specific age, the mass of the star must also be known. The amount of light coming from the star can be measured (this is the magnitude). The more light there is, the bigger the star. However, less light arrives from stars that are further away, so how far away the star is must be taken into account. This can be measured in a number of ways, including the method of parallax.
By knowing the distance, mass, magnitude, and chemical composition of a star, we can determine approximately how old it is, when it was born, and when it will die.
