The Sun is the central body in our solar system. It simultaneously provides the energy critically needed for life to persist on Earth, as well as gravitationally bind the planets in their orbits. This main sequence star comprises nearly 99.86% of the total mass in the solar system.
Upon the surface of the Sun there are many distinguishing features, many of which were originally discovered by Galileo. Among these are Sun spots, solar prominences and solar flares. The frequency at which these phenomena appear are an indication of where the Sun is in its solar cycle. Solar flares have also been the subject of many theories of how the Earth may finally meet its end.
The Sun - By the Numbers
- Mass (kg): 1.989 × 1030
- Volume (km3: 1.412 × 1018
- Mean equatorial radius (km): 6.96 × 105
- Mean density (kg/m3): 1.408 × 103
- Core temperature (K): 15.7 × 106
- Mean surface temperature (K): 5,778
- Mean surface gravity (m/s2): 274.0 (~28 times Earth's gravity)
- Mean distance from Earth (km): 1.496 × 108
- Mean distance from Galactic center (km): 2.5 × 1017
- Visual magnitude: 26.74
- Absolute magnitude: +4.83
- Luminosity (1024 J/s): 384.6
- Spectral type: G2V
Layers (structure) of the Sun
The Core: The central part of the Sun, called the core, extends from the center of the star out to about 25% of its radius (~1.6% of its total volume). Inside the core the temperature and pressure are sufficient to cause hydrogen to fuse into helium through the proton-proton chain (p-p chain). This process supplies nearly all of the energy output of the Sun, as it is carried to the surface through light released during the conversion of mass during nucleation. The energy output of the core is roughly equivalent to 100 billion nuclear bombs going off every second.
The Radiative Zone: Outside of the core, stretching to a distance of about 70% of the Sun's radius, the hot plasma state of the material is enough to thermally radiate the energy from the core. During this process the temperature drops from 7,000,000 K to about 2,000,000 K.
The Convection Zone: Once the hot gas has cooled enough, just outside the radiative zone, the heat transfer mechanism changes in favor of convection. Essentially the gas lacks the density and temperature necessary to continue radiation and columns form to carry the heat energy to the surface of the Sun. The gas then cools as the energy is transferred to the surface (and ultimately to the atmosphere and beyond). The cool gas then sinks back to the boundary of the radiative and convection zones and the process begins again.
The Photosphere (visible surface): Normally when viewing the Sun (using only proper equipment of course) we see only the photosphere, the visible surface. (Under eclipse conditions, or using special filters, you can also see the corona (atmosphere) as well.) Once photons are brought to the surface of the Sun by the convection zone they are released into space and proceed into the solar system. The surface of the Sun has a temperature of roughly 6,000 kelvin, which according to Wein's law is why the Sun appears yellow on Earth. While still very hot it pales in comparison to the 15.7 million kelvin nuclear furnace of the core, and is actually about the same temperature as the Earth's core.
The Corona (atmosphere): During a solar eclipse a glowing aura can be seen around the Sun. This is the Sun's atmosphere, known as the corona. The dynamics of the hot gas that surround the Sun remain somewhat a mystery, as temperatures can reach into the millions of degrees, far hotter than the solar surface. Future experiments hope to discover the mechanism for these high temperatures. The corona is the name given to the collective layers of the atmosphere, but it is also specifically the outermost layer. The lower cool layer (about 4,100 K) receives its photons directly from the photosphere, on which are stacked the progressively hotter layers of the chromosphere and corona until the dark void of space is reached.