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![]() The Sun ![]() ![]() ![]() Magnetic Fields ![]() ![]() ![]() Optional: Quantum Physics ![]() ![]() -------------------------- ![]() ![]() ![]() our Galactic Center ![]() of Atoms and Nuclei ![]() ![]() |
The Sun is the source of most of the energy on Earth--the power source for plants, the cause of flows of atmosphere and of water, the source of the warmth which makes life possible. None would exist without it. At the Earth's orbit, neglecting absorption by the atmosphere, each square meter of area facing the Sun receives about 1380 joules per second (nearly 2 horsepower). That quantity is known as the solar constant and sensors aboard NASA's satellites over the 1979-99 interval suggest it varied by only about 0.2%.
See graphs in the section on sunspots . But what powers the Sun itself? How much longer will it shine, before its fuel runs out? For how long has it given out its energy? The first to consider these questions seriously was the great German physicist Hermann von Helmholtz, who noted in 1854 that the Sun's own gravity could supply an appreciable amount of energy. If the Sun were gradually shrinking--if all its matter was gradually falling towards its center--enough energy could be released to keep it radiating for a fairly long time. He calculated that this source could provide the Sun's energy for times of the order of up to 20 million years. Then radioactivity was discovered, the decay of heavy elements into lighter ones through the emission of fast particles, containing a great deal of energy. As it turned out, it was this energy, from radioactive elements in rocks, that provided the internal heat of the Earth. Radioactivity also allowed new estimates of the age of the Earth, since the amount of accumulated decay products in ores indicated how long the process had been going on. This suggested the Earth was much older than Helmholtz'es estimate, perhaps billions of years old. Could perhaps the new source of internal energy also supply the Sun's needs for such a long time? |
My candle burns at both ends; It will not last the night; But ah, my foes, and oh, my friends It gives a lovely light! |
This concludes our discussion of the Sun. "From Stargazers to Starships" continues with sections dealing with spaceflight and spacecraft, starting with The Principle of the Rocket
For section "The Black Hole at the Center of our Galaxy" mentioned above, click here.
Timeline Glossary Back to the Master List
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 9-23-2004
re-formatted 26 March 2006