Our sun, in many ways an average sort of star, has been around for nearly five billion years and has enough fuel to keep going for another five billion years.
Almost all stars shine as a result of the nuclear fusion of hydrogen into helium. This takes place within their hot, dense cores where temperatures are as high as 20 million degrees. The rate of energy generation for a star is very sensitive to both temperature and the gravitational compression from its outer layers.
These parameters are higher for heavier stars, and the rate of energy generation--and in turn the observed luminosity--goes roughly as the cube of the stellar mass. Heavier stars thus burn their fuel much faster than less massive ones do and are disproportionately brighter. Some will exhaust their available hydrogen within a few million years. On the other hand, the least massive stars that we know are so parsimonious in their fuel consumption that they can live to ages older than that of the universe itself--about 15 billion years.
But because they have such low energy output, they are very faint. When we look up at the stars at night, almost all of the ones we can see are intrinsically more massive and brighter than our sun. Most longer-lasting stars that are fainter than the sun are just too dim to view without telescopic aid.
At the end of a stars life, when the supply of available hydrogen is nearly exhausted, it swells up and brightens. Given that the Universe is only The luminosity of the star is the energy released per unit time. For main sequence stars, the energy comes from hydrogen fusion and we have:. The mass converted into energy through burning will be a fraction f of the total mass of the star. Curious about black holes?
As a bonus, you'll also receive our weekly e-newsletter with the latest astronomy news. Average stars with up to 1. As they run out of hydrogen to fuse in their cores, they swell into red giant stars before shedding their outer layers.
The remnant left behind in these planetary nebulae is a white dwarf star. Like neutron stars, white dwarfs no longer fuse hydrogen into helium, instead depending on degeneracy pressure for support — this time, the electrons are degenerate, packed together and forced into higher energy states, rather than the neutrons.
Left to their own devices, white dwarfs will eventually fade into black dwarfs. No black dwarfs have been observed yet because a white dwarf takes longer than the current age of the universe to fade away. And if the white dwarf is part of a binary system, it may avoid that fate altogether. By accreting matter from its companion star, the white dwarf can explode in a Type Ia supernova , leaving no remnant behind. The smallest stars in the universe have exceedingly long lives — in fact, none have faced their end yet.
Red dwarfs, stars with less than 0. Explore single, multiple, and variable stars in the ultimate Sky Atlas
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