10 Things You Should Know About the Sun

The Sun casts its glow over the ocean. NOAA/Cmdr. John Bortniak

Our Sun is a major source of light and heat in the solar system. It emits a constant stream of particles called the "solar wind," which bathes the planets in radiation. In particular, that connection and interactions between the Sun and the Earth drive the seasons, currents in the ocean, weather, and climate.

The human connect to the Sun is both scientific and historical. It has inspired mythology in almost all cultures, from the ancient Egyptians, Aztecs, Native Americans, to the Chinese. Today, solar physicists delve into its structure and activities to understand more about how it and other stars work. 

Edited and updated by Carolyn Collins Petersen.

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The Sun is the most massive object in the solar system

Handle shaped Prominence on sun, satellite view
Stocktrek/ Digital Vision/ Getty Images

The sun contains most of the mass in the solar system. It has more than 99.8% of all the mass of the planets, moons, rings, asteroids, and comets, combined. If you could stretch a measuring tape around its middle, you'd need one more than 4,379,000 km long. The Sun's volume is 1,142,200,000,000,000,000 km3  and more than 1,300,000 Earths would fit inside it. 

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Our Sun is actually the closest star to Earth

Pictures of the Sun - Coronal Loops
Pictures of the Sun - Coronal Loops. NASA

The Sun is a star. Astronomers consider it to be a yellow dwarf and they refer to it as spectral type G2 V.  Its size is smaller than many stars in the galaxy. Its age of 4.6 billion years makes it a middle-aged star. While some stars are nearly as old as the universe, about 13.7 billion years, the Sun is a 2nd-generation star, meaning it formed well after the first generation of stars were born. Some of its material came from stars that are long gone.

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The Sun is made up of distinctive layers and regions.

Pictures of the Sun - Sunspot Loops
Pictures of the Sun - Sunspot Loops. NASA

The Sun has several layers and is sometimes colorfully described as a flaming onion. Its energy is produced in the very center, called the core. That's where hydrogen is fused to form helium. The fusion process creates light and heat, which then start a long journey to the surface of the Sun. Above the core lie the radiative zone and the outer convective zone. The temperature decreases from 8 million to 7,000 K between those two zones. It takes a few hundred thousand years for photons of light to escape from the dense core and reach the surface.

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The Sun has used up about half of the hydrogen in its core since it was born.

Pictures of the Sun - Handle on the Sun
Pictures of the Sun - Handle on the Sun. SOHO/Extreme Ultraviolet Imaging Telescope (EIT) consortium

Over the next five billion years or so, the Sun will grow steadily brighter as more helium accumulates in its core. As the supply of hydrogen dwindles, the core must keep producing enough pressure to keep the Sun from collapsing in on itself. The only way it can do this is to increase its temperature. Eventually, it will run out of hydrogen fuel. At that point, it will go through a radical change which will most likely result in the complete destruction of planet Earth.

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The Sun's strong gravitational pull holds Earth and the other planets in place

Pictures of the Sun - STEREO Ultraviolet 3D Images
Pictures of the Sun - STEREO Ultraviolet 3D Images. NASA

Gravity is the force that keeps the planets orbiting inside the solar system. The Sun's surface gravity is 274.0 m/s2. By comparison, Earth's gravitational pull is 9.8 m/s2. If you were on a rocket near the surface of the Sun and wanted to escape its gravitational pull, you'd have to accelerate at a speed of 2,223,720 km/h to get away. That's some strong gravity!

The Sun's own gravity balances out the pressure from heat in its core, and that's what keeps the Sun in a spherical shape. 

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The Greeks named the Sun Helios

Pictures of the Sun - STEREO Ultraviolet 3D Images
Pictures of the Sun - STEREO Ultraviolet 3D Images. NASA

The Sun has had many names around the world. It inspired mythology in almost all cultures. Our current name is a variant on the Roman name "Sol", which is also still in use today. Because of the important role the Sun plays in our lives, it has been studied more than any other object in the universe, outside out own planet Earth.

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We've always known the Sun

Pictures of the Sun - Earth and the Sun
Pictures of the Sun - Earth and the Sun. NASA/ESA

Unlike many other objects in our solar system, the Sun has been known to humans since the dawn of time. There is no discovery date or discoverer. It's the only star to exist in or near our solar system and as far as humans are concerned, it's always been there. 

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Ulysses was the first spacecraft to study our Sun's poles

Pictures of the Sun - STEREO Ultraviolet 3D Images
Pictures of the Sun - STEREO Ultraviolet 3D Images. NASA

Launched aboard the space shuttle Discovery and sent towards Jupiter with powerful booster rockets, the Ulysses mission studied Jupiter for 17 days as it used the giant planet's gravity to hurl it into an orbit out of the ecliptic plane. It went into a polar orbit around the Sun.

Other missions study the Sun, including SOHO, the Solar Dynamics Observatory (SDO), and the twin STEREO spacecraft.. 

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The Sun's "surface" and "atmosphere" is very different from the planets.

Pictures of the Sun - The Plane of the Ecliptic
Pictures of the Sun - The Plane of the Ecliptic. NASA

The "surface," known as the photosphere, is just the visible 500-km-thick layer from which most of the Sun's radiation and light finally escape. It's the place where sunspots exist. Above the photosphere lies the chromosphere ("sphere of color") that can be seen briefly during total solar eclipses as a reddish rim. The temperature steadily increases with altitude up to 50,000 K, while density drops to 100,000 times less than in the photosphere.

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Scientists long puzzled over how the corona got so hot.

Pictures of the Sun - Venus Transit
Pictures of the Sun - Venus Transit. NASA

Above the chromosphere lies the corona ("crown"), extending outward from the Sun in the form of the "solar wind" to the edge of the solar system. The corona is extremely hot, millions of degrees kelvin. Since it is physically impossible to transfer thermal energy from the cooler surface of the Sun to the much hotter corona, the source of coronal heating was a scientific mystery for more than 60 years. Recently solar physicists found that nanoflares (constantly erupting tiny flares) may be the reason the corona is so hot.