Our solar system’s star, the Sun, has inspired mythological stories in cultures around the world, including those of the ancient Egyptians, the Aztecs of Mexico, Native American tribes of North America, the Chinese, and many others. A number of ancient cultures built stone structures or modified natural rock formations to observe the Sun and Moon, they charted the seasons, created calendars, and monitored solar and lunar eclipses. These architectural sites show evidence of deliberate alignments to astronomical phenomena: sunrises, moonrises, moonsets, even stars or planets.
The Sun is the closest star to Earth, at a mean distance from our planet of 149.60 million kilometers. This distance is known as an astronomical unit (abbreviated AU), and sets the scale for measuring distances all across the solar system. The Sun is a huge sphere of mostly ionized gas that does supports life on Earth. It powers photosynthesis in green plants, and is ultimately the source of all food and fossil fuel. The connection and interactions between the Sun and Earth drive the seasons, ocean currents, weather, and climate.
The Sun is 332,900 times more massive than Earth and contains 99.86 percent of the mass of the entire solar system. It is held together by gravitational attraction, producing immense pressure and temperature at its core. The Sun has six regions - the core, the radiative zone, and the convective zone in the interior; the visible surface, known as the photosphere; the chromosphere; and the outermost region, the corona.
At the core, the temperature is about 15 million degrees Celsius, which is sufficient to sustain thermonuclear fusion. The energy produced in the core powers the Sun and produces essentially all the heat and light we receive on Earth. Energy from the core bounces around the radiative zone, taking about 170,000 Earth years to get to the convective zone. The temperature drops below two million degrees Celsius in the convective zone, where large bubbles of hot plasma (a soup of ionized atoms) move upwards.
The Sun’s “surface” - the photosphere - is a 500-kilometer-thick region, from which most of the Sun’s radiation escapes outward and is detected as the sunlight we observe here on Earth about eight minutes after it leaves the Sun. Sunspots in the photosphere are areas with strong magnetic fields that are cooler, and thus darker, than the surrounding region. The temperature of the photosphere is about 5,500 degrees Celsius. Above the photosphere lie the tenuous chromosphere and the corona. Visible light from these top regions is usually too weak to be seen against the brighter photosphere, but during total solar eclipses, when the Moon covers the photosphere, the chromosphere can be seen as a red rim around the Sun and the corona forms a beautiful white halo.
Above the photosphere, the temperature increases with altitude, reaching temperatures as high as two million degrees Celsius. The source of coronal heating has been a scientific mystery for more than 50 Earth years. Likely solutions have emerged from observations by the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE) missions, which found patches of magnetic field covering the entire solar surface. Scientists now think that this magnetic “carpet” is probably a source of the corona’s intense heat. The corona cools rapidly, losing heat as radiation and in the form of the solar wind, a stream of charged particles that flows to the edge of the solar system.
Sun: Facts & Figures
Known by the Ancients
Date of Discovery
By Comparison: 109 x that of Earth
By Comparison: 109 x that of Earth
By Comparison: 1,300,000 Earths
By Comparison: 332,900 x Earth’s
By Comparison: 0.255 that of Earth
By Comparison: 11,990 Earths
Equatorial Surface Gravity
By Comparison: 28 x Earth’s surface gravity
By Comparison: 55 x Earth
25.38 Earth days
Minimum/Maximum Surface Temperature
Spectral Type: G2 V Luminosity: 3.83 x 10 33 ergs/sec.
Age: 4.6 Billion E-y
Composition: 92.1% Hydrogen, 7.8% Helium
Synodic Period: 27.2753 E-d
Rotation Period at Equator: 26.8 E-d
Rotation Period at Poles: 36 E-d
Velocity Relative to Near Stars: 19.7 km/s
Mean Distance to Earth: 149.60 million km (1.000 AU)
Solar Constant (Total Solar Irradiance): 1.365 - 1.369 kW/m2
(at the mean distance of the Earth from the Sun, about 1.000 AU)
Why explore, because the Sun is our nearest star. Solar activity affects us on Earth (electronics and communications on the ground and satellites in orbit can be affected by solar storms). Solar activity also affects human and robotic explorers traveling through our solar system. The Sun is the source of most of the light and heat in our solar system.