Sun

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The Sun (if you can see X-rays)

The Sun is that big,[1] hot, yellow, incredibly bright round thing in the sky.

Contents

[edit] What the sun is

You can't miss it, unless it's nighttime, your climate sucks, or you're the person who said that evolution couldn't be true because it violates the laws of thermodynamics and would require "a huge outside source feeding energy into the planet" to make it work.[2] Astronomers say that the sun is a star and that the stars are all suns; astrologers, on the other hand, would probably say something different. The main difference between the Sun and other stars is that other stars are very much further away from the Earth, hence look smaller (which is why they were originally thought to be much different to the Sun; modern technology has seen off that idea, however). Proxima Centauri is the nearest star to the Solar System, being just over 4 light years away, thus making it the second-closest star to Earth.

Modern educated people generally think that the Earth orbits the sun, based on the mythological concepts of "Observable Reality" and "Empirical Inquiry." People infected with certain strains of religious fundamentalism, on the other hand, know that the Sun goes round the Earth[3] based on the entirely factual history and science text, The Holy Bible.

The Sun, as the most hallowed object in the sky, essential to all life, has the seventh (or first, depending on your calendric issues) day of the week named for her: Sunday.

[edit] Too much of a good thing

Too much sun exposure can cause cancer, particularly for fair-skinned people.[4]

[edit] Faint young Sun paradox

First raised in 1972 by Carl Sagan and George Mullen,[5] the faint young Sun paradox is the apparent contradiction between evidence for liquid water on Earth at a time when solar output was apparently too low for this to be possible.

The evidence tells us that the Sun is a ~4.5 billion year-old main sequence star.[6] What this means is that we can extrapolate back to how it was billions of years ago (as well as billions of years into the future), as we have examples of other stars of this age and thus fairly good models describing how they age. But when we take this back a mere 2.5 billion years, we find that the solar output was a mere 70-75% of the present day value. And this is too low for the Earth to have liquid water, and yet we know that it did.[7] Hence, the paradox.

One solution to the paradox is that a large-scale negative-feedback temperature cycle operates on Earth's climate: changes in atmospheric composition compensate for this change in solar output. What this boils down to is that the amount of greenhouse gases in Earth's atmosphere has been decreasing (on average) from the beginning, and that this has balanced out the increase in solar output. A proposed mechanism involves lower temperatures leading to less weathering and erosion, which in turn causes less carbon dioxide to be taken out of the atmosphere. An increase in temperature causes more weathering and more carbon to be locked up in carbonate rocks that form at the bottom of the sea. However, evidence for high concentrations of CO2 in the atmosphere of the early Earth is lacking.[8]

A competing, less popular line of reasoning involves the condition that the Sun is not on the main sequence: that it was originally a little heavier, but blew much of that mass off in its infancy.[9] This could mean that, as the Sun was heavier in the past, it gave off more energy and thus removing our problem. Additionally, if ideas (usually put forward in relation to global warming denialism) that increased solar wind leads to a decrease in cosmic rays — and that cosmic rays cause cooling — are correct, then this too could contribute to a warmer ancient Earth than we think. The primary problem here is a lack of a mechanism for the loss of solar mass to such a large degree.

[edit] See also

[edit] Don't see also

  • The Sun: The paper that gives tabloids a bad name and steals its own name from a certain yellow star you probably just finished reading about. Unlike said yellow star, looking at it won't damage your eyes- it will just severely damage your brain.

[edit] Footnotes

  1. Well, compared to the balls of rock, gas, or ice that orbit it. As far as stars go it's pretty average.
  2. From FSTDT, the all-time classic quote (although the blatant description of the sun in the quote screams Poe's Law).
  3. Technically one can set up the frame of reference such that everything including the universe as a whole revolves around the Earth. However, the other system uses simpler equations.
  4. IARC Volume 55: Solar and Ultraviolet Radiation
  5. Earth and Mars: Evolution of Atmospheres and Surface Temperatures, Science
  6. Yes, it does. Really.
  7. Specifically, evidence from Zircons
  8. A Dark and Warm Early Earth
  9. "Baby Fat" on the Young Sun?
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