The length of a star's life depends on how fast it uses up its
nuclear fuel. 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 star¿s life, when the supply of available hydrogen is nearly exhausted, it swells up and brightens. Many stars that are visible to the naked eye are in this stage of their life cycles because this bias brings them preferentially to our attention. They are, on average, a few hundred million years old and slowly coming to the end of their lives. A massive star such as the red Betelgeuse in Orion, in contrast, approaches its demise much more quickly. It has been spending its fuel so extravagantly that it cannot be older than about 10 million years. Within a million years, it is expected to go into complete collapse before probably exploding as a supernova.
Stars are still being born at the present time from dense clouds of dust and gas, but they remain deeply embedded in their placental material and cannot be seen in visible light. The enveloping dust is transparent to infrared radiation, however, so scientists using modern detecting devices can easily locate and study them. In so doing, we hope to learn how planetary systems like our own come together.
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 star¿s life, when the supply of available hydrogen is nearly exhausted, it swells up and brightens. Many stars that are visible to the naked eye are in this stage of their life cycles because this bias brings them preferentially to our attention. They are, on average, a few hundred million years old and slowly coming to the end of their lives. A massive star such as the red Betelgeuse in Orion, in contrast, approaches its demise much more quickly. It has been spending its fuel so extravagantly that it cannot be older than about 10 million years. Within a million years, it is expected to go into complete collapse before probably exploding as a supernova.
Stars are still being born at the present time from dense clouds of dust and gas, but they remain deeply embedded in their placental material and cannot be seen in visible light. The enveloping dust is transparent to infrared radiation, however, so scientists using modern detecting devices can easily locate and study them. In so doing, we hope to learn how planetary systems like our own come together.
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