The Physical Size Of The

Turning our attention to the Sun, this is a gaseous body so we may expect it to expand and contract. For example, changes in the rate of energy generation (through nuclear fusion in its core) will cause its diameter to vary. The Sun is almost 5 billion years old, and over that time span we know that its energy output has not been constant. After the next 5 billion years, when the hydrogen fuel within it starts to be exhausted, astrophysicists expect the Sun to expand to become a red giant star, with a radius perhaps as large as the orbit ofJupiter, more than a thousand times its present size. After that, with little internal energy production to support it, the Sun will shrink again and attain a dimension rather less than at present, becoming a white dwarf.

Astronomers see these processes occurring in other stars and witness outbursts and oscillations in stellar sizes on all sorts of time scales. Some alter quickly, within days or weeks, but most stars have shown no significant alteration over the decades in which measurements of their brightness have been possible. Although the solar output is reasonably constant in the short-term (which is just as well, otherwise we might get fried), we should be prepared at least to entertain the notion that over centuries or millennia the Sun might grow or shrink. Such variations would of course affect the occurrences of eclipses, and their characteristics. In Chapter 7 we turn our attention to this matter.

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