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Describe The Sun here.
All right, I'll do it for you. The Sun is big, glowing, hot (approx. 5750 K. on the surface on average), basically a huge globe of plasma composed mostly of hydrogen and helium. Yeah, just like every star you see. Altogether, it's nothing special, just a common G-class star, white (not yellow, though atmospheric scattering of blue light may make it seem so) and rather small, on the outer third of the Galaxy on the Orion arm or in the uncharted backwaters of the unfashionable end of the western spiral arm. It's just a little bit (on the cosmological time scale) older than Earth itself. It is currently around 5 billion years old, and it has enough fuel for another 5 billion. On the end of its life cycle, it will inflate to a red giant, consuming Mercury, Venus and probably Earth. Mars gets off with a sunburn, and Titan might thaw out and develop its own life... at least until the sun collapses into an Earth-sized white dwarf and slowly cools to the ambient temperature of the universe over the next trillions of years.
Still, for us humans it holds a very huge significance and thus it's featured quite frequently in fiction as it is the closest star to us. In fact, the Insignificant Little Blue Planet called Earth is orbiting around it. Yeah, we're orbiting in a roughly circular shape with a radius of 8.3 light minutes and the Sun in the center. And yes, you see it every (cloudless) day (unless you are living on a latitude greater than 66.7° North or South), so you must be kind of familiar with it.
As were people in the ancient times. The Sun has been revered as a deity, or as an object of divine origins by many religions. Helios (or Apollo/Apollon, as his better known name) was the Greek god of the Sun. The ancient Egyptians saw it originally as the Pharaoh's soul; later it was associated with Ra and Horus. And this is only a very small part of the great number of solar deities found in cultures. In such cultures, the Sun is often contrasted with The Moon.
Despite being made of ionized gas, the tremendous pressure in its interior squeezes the core into a very dense ball, bringing the sun's average density up above that of liquid water. As one astronomy professior said, if you put the sun in a bucket of water it'll sink. The only thing keeping the sun from collapsing in on itself still further is the nuclear fusion reaction going on in its core -- so much heat is produced that the radiation literally lifts up the outer layers against their own weight. Nevertheless, when compared to even the largest planets in the solar system, the sun is freakin' huge. It's over a hundred times bigger across than the Earth, and weighs over 300,000 times as much. In fact, all of the planets, asteroids, and comets in the solar system combined would have less than 1% of the sun's mass. If the sun had a solid surface -- which it doesn't -- you'd experience 28 times Earth's surface gravity if you stood on it.
As said above, when its core runs outta gas some 5 billion years from now, it will inflate a lot and consume at least some of the inner planets - likely including Earth, causing a Class 5 or Class X apocalypse. This inflation will take place in the blink of an eye in geological terms, but will be very gradual by human-lifetime standards: it may take anywhere from 10,000 to 100,000 years for the sun to grow from its end-of-main-sequence normal size to its full red-giant glory. After a million or so years in this red giant phase, it will shrink again, then expand a second time as its core fuses helium into carbon, then finally shed its outer layers in a breathtaking display known as a "planetary nebula." What will remain afterward is the tiny, exposed core of the sun, now shrunk to a super-dense white dwarf the size of the Earth, slowly cooling to a black dwarf over the next quadrillion years.
But we might not have that long to wait before things get ugly. According to current models of stellar evolution, in a paltry one billion years the sun will brighten enough that Earth's oceans will evaporate. We won't be around then, though -- either because Science Marches On and we will be on planets far away, or because we will kill ourselves.
Note: some fictional extraterrestrial cultures will worship their central star as our ancient cultures worshiped the Sun. While it is definitely not the same star, for its significance for the given aliens it falls into this category.
- ↑ To be precise, it's unimpressive compared to the stars visible in the sky which are mostly giants and supergiants. But most of the stars that actually exist are hard-to-see red dwarfs, and old Sol clearly outshines most of those.
- ↑ Important nitpick: a red giant is not at all the same thing as a nova. A nova requires a white dwarf star siphoning material off another star it orbits, and only lasts for a few weeks or months; singular stars like the sun will not, and cannot, "go nova." Oh, and don't confuse either of these phenomena with a supernova, which is another beast entirely.
- ↑ "But wait!" I hear you cry. "If the core is no longer providing any radiative pressure to support the sun's upper layers, why will it expand instead of shrinking under its own weight?" I'm glad you asked. When the core fizzles, the layer immediately above the core will collapse down upon it, and in the process this layer will get more and more compressed until it ignites in nuclear fusion itself, forming a hydrogen-burning shell.
- ↑ Delta Pavonis, a star extremely similar and very close--about 20 ly--to the Sun is currently going through this phase. It started the process during the time that modern humans have existed--possibly even during recorded history--but only our descendants to the umpteenth generation will get to see the transformation in full. Astronomers have a mild interest in this star, since being the Sun's "near-identical older brother"--as we put it--its evolution will give hints about what's to come for old Sol.
- ↑ one teaspoon of white dwarf matter would weigh several tons at the surface of the Earth