Sun+4B

The sun is by far the largets object in the solar system.It contains more than 99.8% of the total mass of the solar system.The sun is personified in many mythologies: the greeks called it helios and the Romans called it Sol.The sun is, at present,about 70% hydrogen and 28% helium by mass evething else (metals) amont to less than 2%.This changes slowly over time as the sun converts hydrogen to helium in its core.Conditions at the sun's core (approximately the inner 25% of tis radius) are exreme.The temperature is 15.6 million kelvin and the pressure is 250 billion atmosperes.At the center of the core the Sun's density is more than 150 times that at water. The surface of the sun, called the photospere,is at a temperature of about 5800 k.Sunspots are cool regions,only 3800 k (they look dark only by comparion with the surroundings regions).Sunspots can be very large, as much as 50,00 km in diameter.Sunspots are caused by complicated and not very well understood interactions with the sun's magmetic field.A small region known as the chromosphere lies above the photospere.In addition to the heat and light,the sun also emits a low density stream of charged particles (mostly electrons and protons) kown as the solar wind which propagates throughout the solar system around at about 450 km/sec.The solar wind and the much higher energy particles ejected by solar flarers can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis.The sun's orbit output is not entirely constant.Nore is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the maunder minimun.It coincides with an abnormally col period in northern europe sometimes kown as the Little Ice Age.Since the formation of the solar system the sun's out put has increased by about 40%.The sunis about 4.5 billion years old.Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate peacefully for another 5 million year ago or so (although its luminosity will approximately double in time).But eventually it will run out of hydrogen fuel.It will then be forced into radical changes which, though commonplace by stellarstandards, will result in the total destruction of the earth (and probably the creation of a planetary nebula).