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stars, masses
The lifespan of a star depends crucially upon its mass – the greater the mass, the shorter the lifetime. This may seem strange, since a more massive star obviously starts out with more available fuel for nuclear fusion. However, such a star consumes its fuel at a faster rate. The more massive the star, the greater the pressure exerted on its core due to the weight of the overlying layers. Higher pressures result in higher temperatures (just as pumping a bicycle pump causes the compressed air inside to heat up). In turn, higher temperatures cause nuclear fusion reactions to take place at a more rapid rate, which increases the energy output of the core and hence the star's luminosity. The effect is dramatic. Observations have shown that, in the case of stars with a mass more than about 3 solar masses, a star's luminosity varies as the cube of its mass. This means that a star of, say, 10 solar masses must consume its fusion fuel about 1,000 times faster than does a Sunlike star. Its lifetime is, therefore, only about one hundredth as long, or roughly 100 million years compared with the Sun's 10 billion years. For a star of 30 solar masses, the observed mass-luminosity relationship indicates that fusion fuel is "burned" 303, or 27,000, times faster than in the Sun, resulting in a lifetime of little over 10 million years. Low mass stars, on the other hand, are energy misers, destined to spin out their meager fuel reserves over colossal periods of time. For example, Proxima Centauri, a red dwarf that is the nearest star to the Sun, has a predicted lifespan of about 16 trillion years-roughly 1,000 times the present age of the universe.
The total range of star masses is believed to be from about 0.08 to 70 solar masses. Below the lower limit, an object would not be able to attain a sufficiently high core temperature to trigger fusion reactions and so would exist either as a brown dwarf or simply a large planet like Jupiter. The upper limit is less certain and there may be a few stars, such as Eta Carinae, that have masses of more than 100 solar masses. Such super-massive stars, however, must be highly unstable and extremely short-lived.
Related category
• STELLAR ASTROPHYSICS
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Stars can only grow so big (Mar 11, 2005)
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