Sun

The Sun (if you can see X-rays)

It's not rocket science, it's... Astronomy

The Final Frontier

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The abyss stares back

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Not to be confused with The Sun, the paper that gives tabloids a bad name. Looking directly at it won't hurt your eyes; just severely damage your brain.

The Sun is that big, hot, blinding, yellow, and incredibly bright round thing in the sky.

What the Sun is

Motion of the barycenter of the Solar System relative to the Sun

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. 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.

By Solar System standards, as commented above, the Sun is really big — even Jupiter, the largest planet, is peanuts next to it — being more than one hundred times as large as Earth and more than three hundred thousand times more massive. However, even if there are stars out there that dwarf it in one way or in another, the Daystar is not only more luminous than many stars of our galaxy, but it also has the distinctions of being the only star that we can study in detail and the only one known to have at least one life-supporting planet orbiting it.

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 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. Even more technically, that common barycenter is in motion around the centre of the Milky Way galaxy. And the Milky Way is moving relative to other galaxies, too. So technically, neither side is completely right.

The Sun, as the most hallowed object in the sky, being essential to all life, has the seventh (or first, depending on your calendric issues) day of the week named after it: 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 largely evolved outdoors, 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 outdoors. The consequence is that you can have draconian health messages saying to avoid all sun or moderate ones (such 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. This means 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 involves a large-scale negative-feedback temperature cycle operating 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 its mass off in its infancy. This could mean that, as the Sun was heavier in the past, it gave off more energy - 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 whole faint young Sun paradox thing holds no terrors for Young-SunEarth Creationists, who know due to inerrant Faith that the Sun originated on day 4 of Creation Week after the first emergence of light and day and night and subsequent to the phenomena of evening and morning.

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. This will have a profound impact on Earth, rendering it uninhabitable within around 1-3 billion years. However, by that time, humanity would most likely be able to get off the planet entirely, assuming they didn't do anything stupid beforehand and get themselves killed.

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 the planets of the inner Solar System as our star expands before all but Mars are 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 as a planetary nebula and becoming so a white dwarf, a cooling stellar corpse that will fade into oblivion trillions of years later as a black dwarf. Despite the white dwarf being about the size of Earth, it will be about half the mass the Sun is right now.

That's not to say the Sun can't pose any imminent danger to humans. The Sun can occasionally cause geomagnetic storms (also known as solar storms), massive disturbances in Earth's geomagnetic field. While these wouldn't normally matter to fleshbags like humans, they matter immensely to the electronics and electrical grids that us fleshbags rely on for communication, transportation, medicine, and food production. The last significant geomagnetic storm happened in March 1989, and caused a nine-hour long blackout in Quebec's power grid, as well as radio blackouts across the globe. A more powerful solar storm, along the lines of the Carrington Event in 1859, could wipe out power networks over much larger areas. Fortunately, NASA has since started closely observing solar flares so we don't get blindsided by the next major solar storm.

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
• Sungazing
• Stellar metamorphosis