Editing Hadean Eon (section)

== Properties of the Sun ==

The Sun (symbol: ☉) is by far the brightest object in <u>Earth's</u> sky, with an '''apparent magnitude''' of −26.74. This is about 13 billion times brighter than the next brightest star, <u>Sirius</u>, which has an apparent magnitude of −1.46. A '''G-type''' (dwarf) '''main-sequence''' (hydrogen-to-helium fusion-powered) star, the Sun's original chemical composition was inherited from the local interstellar cloud out of which it and its neighbors formed.

The radius of the Sun (R☉) is 109 times that of <u>Earth</u>, but its distance from <u>Earth</u> is 215 R☉, so it '''subtends''' an angle of only 1/2° in the sky, roughly the same as that of the Moon. The mass of the Sun, M☉, is 743 times the total mass of all the planets in the Solar System and 330,000 times that of <u>Earth</u>.

The Sun is a very stable source of energy; its radiative output, called the '''solar constant''', is 1.366 kilowatts per square metre at <u>Earth</u> and varies by no more than 0.1 percent; but superimposed on this stability is an 11-year cycle of magnetic activity manifested by regions of transient strong magnetic fields.

Originally the Sun would have been about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements (now 74.9% hydrogen, 23.8% helium, 1.3% heavier elements). The hydrogen and most of the helium were produced by Big Bang '''nucleosynthesis''' in the first moments of the universe, and the heavier elements were produced by previous generations of stars and spread into the interstellar medium during the final stages of their stellar lives; their remnants became dust in our cloud.

Most of the collapsing cloud (99.86% of the total mass) gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core, which formed the star we call the Sun. It has an '''absolute magnitude''' of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way galaxy (most of which are '''red dwarfs'''). The Sun is a Population I, or '''heavy-element-rich''', star. Its formation may have even been triggered by shockwaves from one or more nearby supernovas; this is suggested by a high abundance of heavy elements such as gold and uranium relative to the abundances of these elements in so-called Population II, '''heavy-element-poor''', stars.

Apparent magnitude measures how bright a star or celestial object ''appears from Earth''. It is a '''scale''', traditionally '''inversely logarithmic''', where lower (or more negative) values denote brighter objects. This scale is rooted in an ancient system devised by the Greek astronomer '''[[Hipparchus]]'''. Absolute magnitude is a measure of the ''intrinsic brightness'' of a celestial object. It is defined as the apparent magnitude the object would have if it were placed at a standard distance of 10 parsecs (about 32.6 light-years) from Earth. This allows astronomers to compare the true luminosities of celestial objects without the distortion caused by distance. The Sun's absolute magnitude is +4.83; although it appears very bright in the sky due to its proximity, in terms of the vast array of stars, the Sun is of average brightness. Sirius has an absolute magnitude of +1.4, indicating it is intrinsically brighter than the Sun if both were observed from the same distance; though it appears far less so, because of the distortion of distance. This fundamentally illustrates the difference between apparent and absolute magnitude.