How long will it be before the Sun's expansion makes life on Earth impossible?

Most people cite the fact that in about 5-6 billion years the Sun will become a red giant star, and swell to the orbit of Venus or even the Earth in size. Actually, even now, the Sun grows brighter and brighter as its is evolving 'off' the main sequence. In another 500 million years of this steady increase, it will be about 10 percent more luminous. This means that the surface of the Earth will be a LOT hotter as the oceans begin to dump more water vapor into the atmosphere and thereby increasing the terrestrial greenhouse effect. Some forecasts suggest that in as little as another few hundred million years, the Earth's biosphere may turn very inhospitable. Fortunately, there are 'thermophylic' bacteria that live in nearly boiling water, so again in the far future, the Earth will end its years as a host for life by being a breeding ground for bacteria. We had better not be here when that happens.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the
NASA IMAGE/POETRY Education and Public Outreach program.