Sun

An image of the Sun taken by the SOHO spacecraft, showing a large solar flare to the bottom left

The star around which the Earth revolves, and the gravitational center of the Solar System; it lies about 270,000 times closer to us than the next nearest star. The Sun is a solitary, yellow dwarf star of spectral type G2 (see G star) that has been on the main sequence for about 4.6 billion years.

The Sun consists largely of hydrogen (71% by mass) and helium (27%), with much smaller amounts of heavier elements. It puts out 400 trillion trillion watts of energy, produced by the fusion of hydrogen to helium by the carbon-nitrogen cycle in its core. The Sun is 109 times wider than the Earth. It spins on its axis with a period that varies from 25 days at the equator to 33.5 days near the poles.

The visible surface, or photosphere, is surrounded by the chromosphere and, beyond this, the corona.

Important features of the chromosphere include:

Important features of the corona include:

Sun-like stars and solar stability

Since the Sun is unique in having a known (to humans!) habitable planet, it is natural that scientists first turn to stars similar to the Sun (see FGK stars and target stars) in their search for extraterrestrial life. Superficially, there are many such stars, including a handful that are less than 20 light-years away (see Sunlike stars). During this decade and beyond, attention will be focused on trying to detect Earthlike planets orbiting within the habitable zones of such solar look-alikes. However, it may be that in at least one respect the Sun is abnormal. A consensus is emerging that our star is exceptionally stable. Although like all stars it sends out flares from time to time, these tend to be on a very modest scale by stellar norms. What is still unknown is why the Sun is so well-behaved, and whether we just happen to be enjoying a tranquil phase in its career.

Facts about the Sun
distance	149,597,900 km (92,975,699 mi., 8.3 light-minutes)
spectral type	G2V
diameter	1,392,000 km (865,000 mi.)
surface temperature	5,400ºC (9,800ºF)
central temperature	14 million ºC (25 million ºF)
mass (Earth = 1)	332,946
density (water = 1)	1.409
surface gravity (Earth =1 )	27.90
escape velocity	617.5 km/s (383.8 mi./s)
rotation period	25.38 days
apparent magnitude	-26.8
absolute magnitude	4.83

Related category

• SOLAR TOPICS

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Encyclopedia of History

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