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Sun
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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 the chromospheric network, plages, prominences, filaments, and spicules. Important features of the corona include coronal holes, coronal loops, coronal mass ejections, helmet streamers, polar plumes, and solar flares.
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) 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). Over the next decade, 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.
| 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 | 5,400 ºC (9,800 ºF) |
| 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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