The Faint Young Sun Paradox
Paragraph 1：When the Sun first formed 4.6 billion years ago, it was about 30 percent less luminous than it is today. The Sun’s luminosity increased slowly at first and then more rapidly as the buildup of helium in its core continued. At present, the Sun is thought to be brightening by about one percent every hundred million years. By the time the Sun ends its lifetime as a normal star, about five billion years from now, it is expected to have brightened by a factor of two to three as compared to today.
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