The Sun is some 333,400 times more massive than Earth (mass = 1.99 x 10^30 kg), and contains 99.86% of the mass of the entire solar system. It is held together by gravitational attraction, producing immense pressure and temperature at its core (more than a billion times that of the atmosphere on Earth, and a density about 160 times that of water).
At the core the temperature is 16 million degrees K, which is sufficient to sustain thermonuclear fusion reactions. The released energy prevents the collapse of the Sun and keeps it in gaseous form. The total energy radiated is 383 billion trillion kilowatts/second, which is equivalent to that generated by 100 billion megatons of TNT exploding each second.
In addition to the energy-producing solar core, the interior has two distinct regions: a radiative and a convective zone. From the edge of the core outward, first through the radiative and then through the convective zone, the temperature decreases from 8 million to 7,000 K, and density decreases from 20 gm/cm^3 to 4 x 10^-7 gm/m^3. It takes about 10 million years for photons to escape from the dense core and reach the surface.
Because the Sun is gaseous, it rotates faster at the equator (26.8 days) than at the poles (as long as 35 days). The Sun's "surface," known as the photosphere, is just the visible 500 km-thick layer from which most of the Sun's radiation and light finally escapes, and is the place where sunspots are found. Above the photosphere lies the chromosphere ("sphere of color") that may be seen briefly during total solar eclipses as a reddish rim, caused by hot hydrogen atoms, around the Sun. Temperature steadily increases with altitude up to 50,000 K, while density drops to 100,000 times less than in the photosphere. Above the chromosphere lies the corora ("crown"), extending outward from the Sun in the form of the "solar wind" to the edge of the solar system. The corora is extremely hot - millions of degrees K. The process that heats the corona is very mysterious and poorly understood, since the laws of thermodynamics state that heat energy flows from a hotter to a cooler place. Mysterious phenomena, such as this, are studied by researchers in NASA'S Space Physics Division.
| Spectral Type of Star | G2 V |
| Age | 4.5 billion years |
| Mean Distance to Earth | 150 million kilometers |
| Rotation Period (at equator) | 26.8 days |
| Radius | 695,000 kilometers |
| Mass | 1.99 x 10^30 kilograms |
| Composition | Hydrogen 71%, Helium 26.5%, Other 2.5% |
| Effective Surface Temperature | 5,770 K |
| Energy Output (Luminosity) | 3.83 x 10^33 ergs/sec |
| Solar Constant | 0.1368 watts/cm^2 |
| Inclination of Solar Equator to Ecliptic | 7.25° |
1610: Galileo observes sunspots with his telescope
1650-1715: Maunder Sunspot Minimum discovered
1854: First connection made between solar activity and geomagnetic activity
1868: Helium lines first observed in solar spectrum
1908: First measurement of sunspot magnetic fields taken
1942: First radio emission from Sun observed
1946: First observation of solar ultraviolet using a sounding rocket
1946: 1,000,000 K temperature of corona discovered via corona spectra lines
1949: First observation of solar x-rays using a sounding rocket
1954: Galactic cosmic rays found to change in intensity with the 11-year sunspot cycle
1956: Largest observed solar flare occurred
1959: First direct observations of solar wind made by Mariner 2
1963: First observations of solar gamma rays made by Orbiting Solar Observatory 1 (OS01 )
1967: First measurement of solar neutrino flux taken
1973-4: Skylab observed Sun, discovered coronal holes
1982: First observations of neutrons from a solar flare by Solar Maximum Mission (SMM)
1994-5: Ulysses flies over polar regions of Sun