The sun provides the light and warmth that allows life on Earth to exist. Gravity holds the sun together since it's mainly composed of ionized gases. The star at the center of the solar system may be 150 million kilometers (93 million miles) from Earth, but it's bright due to its enormous size. The heat that produces the brightness is powered by nuclear fusion that takes place in the sun's core under massive gravitational pressure.
The source of the sun's energy and brightness is nuclear fusion: when smaller nuclei join to form one larger nucleus. In this case, four hydrogens and four electrons are used to form helium. This reaction takes place in the sun's core at 15 million degrees Celsius (27 million degrees Fahrenheit) and under enormous pressure. Not of all the reacting mass becomes part of the new helium atom, however. Some of it is converted to energy that goes to the sun's surface. This is in agreement with Albert Einstein's "E = mc^2" equation, which states mass can be converted to energy and vice versa.
From the Core to the Surface
Once the energy is produced from nuclear fusion, it moves from the core to the surface. The radiation bounces around the convective zone surrounding the core, so the process takes about 170,000 years. Hot plasma inside the convective zone propels heat upward. After the energy escapes the convection zone, it goes through the surface, which is 500 kilometers (310 miles) thick, and moves outward in space. Such light reaches Earth's surface in about eight minutes thanks to the speed of light, which is approximately 300,000 kilometers per second, or about 186,000 miles per second.
Solar flares are unusually bright explosions in the sun triggered by a sudden release of energy by a "twisted" magnetic field. They are short, extremely hot bursts of radiation generated in just a few minutes. Solar flares are classified by the type of radiation they emit. Some solar flares, such as those emitting X-rays, are powerful enough to disrupt radio equipment and satellites in the upper atmosphere. Smaller solar flares, however, are harmless and have no impact outside the sun's surface.
Sunspots and Brightness
Sunspots are dark spots on the surface of the sun and are generally correlated with the sun's brightness: The more sunspots there are, the brighter the sun is. They are relatively cool areas with very strong magnetic fields and usually last between several days to several weeks. Sunspots also help eject solar plasma into space with their strong magnetic fields, in the same way as solar flares. The solar plasma reacts with molecules in the Earth's atmosphere to generate the aurora phenomena. This is the display of glowing lights in the sky near the North and South Poles.
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