The Turbulent

A few seconds before the Sun was all hid, there discovered itself round the Moon a luminous ring about a digit, or perhaps a tenth part of the Moon's diameter, in breadth. It was of a pale whiteness, or rather pearl-colour, seeming to me a little tinged with the colors of the iris, and to be concentric with the Moon.

Edmond Halley (1715), describing his observation of the corona, which he took to be of lunar rather than solar origin

The Sun is of huge importance to life on Earth, making it very special. Nevertheless, leaving aside our natural bias one has to say that it is not special at all when compared with other stars.

There are reckoned to be about 400 billion stars in our galaxy, the Milky Way. There are blue-white supergiants, brown dwarfs, pulsars or neutron stars, white dwarfs, red giants, black holes, binary stars we know to be double only from their spectra, X-ray emitting binaries, and too many other distinct categories of stellar creature to mention, let alone describe their properties.

Most stars are rather nondescript, spending most of their lives on what is termed the main sequence, an evolutionary track along which stars with different masses, ages, and chemical compositions are burning hydrogen within their cores. ("Burning" here does not mean simple combustion, which is a chemical reaction with oxygen, but rather nuclear burning, in that hydrogen nuclei join together to produce helium.) As they do so, they generate far more energy than any trivial chemical reaction, just as nuclear bombs liberate rather more energy than dynamite.

Thankfully our Sun is one of these nondescript stars. Our neighborhood nuclear generator behaves in a regular way, not burping out vast quantities of its star stuff and incinerating any nearby planets, nor shrinking to leave its rocky companions to a frigid existence. At least, the Sun will not do so yet. It has been merrily emitting energy generated by those nuclear reactions in its core for about 4.5 billion years. It is expected to do the same for another 5 to 10 billion before swelling up into a red giant, enveloping the planets and asteroids out as far as Jupiter, and then collapsing into a white dwarf, having exhausted its nuclear fuel. As it shrinks it may cast off a nebula of gas and dust, which would eventually be recycled to help produce yet more stars and planets.

Some other stars are massive enough such that their cores attain pressures and temperatures sufficient to burn heavier elements, like carbon and nitrogen, producing elements with ever more particles in their nuclei, and so extending the stellar lifetimes. But our Sun cannot do so. Its lifetime is limited. Let us not weep, though: if the Sun were not just as it is, we would not be here to appreciate it and grieve for its eventual expiration.

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