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 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.[note 3] 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 they look smaller (which is why they were originally thought to be much different than 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[note 4] based on the entirely factual history and science text, The Holy Bible. Technically (with extra emphasis), both the Sun and Earth, as well as everything else in the Solar System, orbit a third shared point, known as the barycenter. So technically, neither side is completely right.
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.
 The Good and the Bad
Too much Sun exposure can cause cancer, particularly for fair-skinned people. However, the Sun is vital for all life on Earth and moderate exposure to it produces vitamin D, which is good for bone strength.
There is some evidence that sunlight reduces the risk of cardiovascular disease. Lack of sunlight has been linked to higher levels of cholesterol, and heart disease rates seem to increase in winter and further away from the equator.
All this leads to a lot of debate on how much Sun is good for you. You need to be outside a certain amount of time to produce enough Vitamin D, and the darker your skin the longer it will take. The NHS recommends 10-15 minutes daily for light skinned people in the UK but there are many variables with skin colour and Sun strength. Human beings evolved largely out of doors, but they also evolved dying of all kinds of horrible things.
So should you limit your Sun exposure, and if so by how much? Experts differ. Many things people do outdoors are healthy (although some are unhealthy), and obesity and cardiovascular disease are among the biggest public health problems in most first-world societies. Of course, you could always cover up fully every time you went outdoors, but in practice this might discourage people from going out of doors. The consequence is that you can have draconian health messages saying to avoid all sun, or moderate ones (as in Australia) focused on risk mitigation, saying to wear a t-shirt in the hottest part of the day, protect children, etc.
 Faint young Sun paradox
First raised in 1972 by Carl Sagan and George Mullen, 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. 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. 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 (CO2) 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.
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. 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.
 The Ugly
As has been described above, the Sun's luminosity increases during its evolution as its core contracts to maintain energy production with a smaller fuel supply, heating and thus increasing the rate of nuclear reactions. It's expected that this will cause an increased weathering of minerals, lowering atmospheric CO2 concentrations so much that in around 600 million years plants will begin to disappear as they will be unable to use photosynthesis. No plants means no animals, and in roughly 1 billion years from now just bacteria will remain. Meanwhile, that very same increasing luminosity will also mess with the Earth's climate causing a moist greenhouse and the evaporation of the oceans, leaving Earth as a bone-dry wasteland with water just at the poles — this assuming there's no runaway greenhouse that will render Earth a Venus-like planet or worse — and temperatures eventually so hot that the remaining life is expected to become extinct 2.8 billion years from now.
Around five billion years from now, the Sun will exhaust its central hydrogen supply and will go full red giant 2.6 billion years later (yes, stellar evolution takes its time), things becoming really hellish in Earth before the former is absorbed by the red giant Sun. After burning helium on its core for a 100 million years and other 20 million years of instabilities once core helium is exhausted, the Sun will expel its outer layers to space becoming a white dwarf, a cooling stellar corpse that will fade into oblivion trillions of years later as a black dwarf.
 See also
- Moon — Also floats in the sky and provides light, often at nighttime, but is actually quite different to the sun.
- Starlight problem
- Sun sign
- Stellar metamorphosis
- ↑ Well, compared to the balls of rock, gas, or ice that orbit it. As far as stars go it's pretty average.
- ↑ Well actually, it's white. See Wikipedia
- ↑ From FSTDT, the all-time classic quote (although the blatant description of the Sun in the quote screams Poe's Law).
- ↑ 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.
- ↑ IARC Volume 55: Solar and Ultraviolet Radiation
- ↑ 2.0 2.1 Is there such a thing as a safe tan?, BBC
- ↑ Sunlight, cholesterol and coronary heart disease, DS Grimes, E Hindle, T Dyer, Q J Med 1996; 89: 579-589
- ↑ Vitamin D Sunlight, NHS
- ↑ Earth and Mars: Evolution of Atmospheres and Surface Temperatures, Science
- ↑ Yes, it does. Really.
- ↑ Specifically, evidence from Zircons
- ↑ A Dark and Warm Early Earth
- ↑ "Baby Fat" on the Young Sun?
- ↑ 10.0 10.1 10.2 Future of the Earth. See also references therein
- ↑ Our Sun III. Present and Future
- ↑ Distant future of the Sun and Earth revisited