PeiWen
17-03-2004, 01:38 PM
Q: How long do stars usually live?
A: 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.
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Q: What is a blue moon?
A: A "blue moon" once meant something impossible or at least highly unlikely, much like the expression "when donkeys fly!" This was apparently the usage as early as the 16th century.
Then in 1883, the explosion of Krakatau in Indonesia threw enough dust into the atmosphere to turn worldwide sunsets green and the moon blue. Forest fires, prolonged drought and volcanic eruptions can still do this. So a blue moon became synonymous with something rare—hence the phrase "once in a blue moon."
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A: 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.
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Q: What is a blue moon?
A: A "blue moon" once meant something impossible or at least highly unlikely, much like the expression "when donkeys fly!" This was apparently the usage as early as the 16th century.
Then in 1883, the explosion of Krakatau in Indonesia threw enough dust into the atmosphere to turn worldwide sunsets green and the moon blue. Forest fires, prolonged drought and volcanic eruptions can still do this. So a blue moon became synonymous with something rare—hence the phrase "once in a blue moon."
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