The Sun

The sun is a star, a hot ball of glowing gases at the heart of our solar system. Its influence extends far beyond the orbits of distant Neptune and Pluto. Without the sun's intense energy and heat, there would be no life on Earth. And though it is special to us, there are billions of stars like our sun scattered across the Milky Way galaxy.

The sun has many names in many cultures. The ancient Greeks called it Helios and the ancient Romans called it "Sol," which was translated into sun in modern English. The International Astronomical Union (IAU), has not decided on a single name for the sun (or the Moon for that matter), but supports the common practice of using the name of the sun in any language.

Our solar system's central star 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 and Canada, the Chinese and many others. A number of ancient cultures built stone structures or modified natural rock formations to mark the motions of the sun and Earth's 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. Many cultures believed that the Earth was immovable and the sun, other planets, and stars revolved around it. Ancient Greek astronomers and philosophers knew this geocentric concept from as early as the 6th century B.C.

The sun is the closest star to Earth, at a mean distance from our planet of 149.60 million km (92.96 million miles). This distance is known as an astronomical unit (abbreviated AU), and sets the scale for measuring distances all across our solar system. The sun, a huge sphere of mostly ionized gas, supports life here on Earth. The connection and interactions between the sun and Earth drive the seasons, ocean currents, weather and climate.

About one million Earths could fit inside the sun. 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 (the photosphere); the chromosphere; and the outermost region - the corona.

At the core, the temperature is about 27 million degrees Fahrenheit (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 is carried outward by radiation, which bounces around the radiative zone, taking about 170,000 years to get from the core to the convective zone. The temperature drops below 3.5 million degrees Fahrenheit (2 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-km thick (300-mile-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 number of sunspots goes up and down every 11 years as part of the sun's magnetic activity cycle. Also connected to this cycle are bright solar flares and huge coronal mass ejections that blast off the sun.

The temperature of the photosphere is about 10,000 degrees Fahrenheit (5,500 degrees Celsius). Above the photosphere lie the tenuous chromosphere and the corona ("crown"). 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 while the corona forms a beautiful white crown with plasma streaming outward, forming the points of the crown.

The temperature increases with altitude, reaching temperatures as high as 3.5 million degrees Fahrenheit (2 million degrees Celsius). The source of coronal heating has been a scientific mystery for more than 50 years. Likely solutions have emerged from observations by the SOHO and 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.