|MadSci Network: Astronomy|
John, Solar radiation has indeed changed with time, though this change is very slow. Stars evolve along what is known as the Hertzsprung-Russell Diagram, which is a plot of luminosity (the total rate of energy released over all wavelengths) vs temperature. This plot can also be shown as abolute magnitude vs spectral type since luminosity is related to absolute magnitude and temperature is related to spectral type. An example of what an H-R diagram looks like is shown below. Once a star reaches the main sequence, it moves very little until reaching its turn off point and becoming a red giant. It does evolve, though, and increases its luminosity while decreasing its temperature. For instance, the sun was only 30 percent as luminous in the early part of the solar system when Earth was a young planet. In 5 billion years, it will have twice the luminosity it has now. This is shown below. That's the long term picture. On the short term, the solar magnetic activity cycle with its 11 year period causes small changes in the radiation released by the sun over time. This fluctuation may be enough to effect weather systems on Earth, such as the "Maunder Minimum" which was an absence of sunspot activity from 1645 to 1705. This time is called the Little Ice Age because global temperatures on Earth dropped about 1 degree Fahrenheit (0.5 degree Kelvin). For more information about H-R diagrams and stellar evolution, refer to any introductory astronomy text book, such as Zeilik & Gregory "Astronomy & Astrophysics" Erika
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