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Our home star is a yellow dwarf, a medium-size variety that's fairly common in our galaxy. On Earth, the sun can take on warmer hues, especially at sunrise or sunset, because our planet's atmosphere scatters blue and green light the most. At about , miles 1. If it was a hollow ball, more than a million Earths could fit inside it. But the sun isn't hollow: It's filled with scorching gases and soups of electrically charged particles called plasma.
The sun's surface temperature is about 10, degrees Fahrenheit 5, degrees Celsius , and it's 27 million degrees Fahrenheit Deep in the sun's core, nuclear fusion converts hydrogen to helium, which generates energy. Particles of light called photons carry this energy through a spherical shell called the radiative zone to the top layer of the solar interior, the convection zone.
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There, hot plasmas rise and fall like the ooze in a lava lamp, which transfers energy to the sun's surface, called the photosphere. It can take , years for a photon to complete its journey out of the sun, but once it exits, it zips through space at more than , miles a second.
Solar photons reach Earth about eight minutes after they're freed from the sun's interior, crossing an average of 93 million miles to get here—a distance defined as one astronomical unit AU. Out beyond the sun's photosphere lies the atmosphere, which consists of the chromosphere and the solar corona.
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The chromosphere looks like a reddish glow fringing the sun, while the corona's huge white tendrils extend millions of miles long. The chromosphere and corona also emit visible light, but on Earth's surface, they can be seen only during a total solar eclipse, when the moon passes between Earth and the sun.
The corona runs far hotter than the photosphere, hitting temperatures of more than a million degrees Fahrenheit. How the corona gets so hot remains a scientific mystery, which is partly why NASA launched its Parker Solar Probe , the fastest spacecraft ever built, and the first ever sent into the corona.
In addition to light, the sun radiates heat and a steady stream of charged particles known as the solar wind. The wind blows about miles kilometers a second throughout the solar system , extending the sun's magnetic field out more than 10 billion miles.
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Beyond that distance, the solar wind gives way to the colder, dense material that drifts in between stars , forming a boundary called the heliopause. So far, just two spacecraft—Voyager 1 and Voyager 2 —have crossed this cosmic threshold, which defines the start of interstellar space.
Every so often, a patch of particles will burst from the sun in a solar flare, which can disrupt satellite communications and knock out power on Earth.
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Flares usually stem from the activity of sunspots, cool regions of the photosphere that form and dissipate as the sun's internal magnetic field shifts. Solar flares and sunspots obey a regular cycle, rising and falling in number every 11 years as the poles of the sun's magnetic field flip back and forth.
Last on. Sun 30 Dec GMT. BBC World Service. More episodes Previous. Boxing Day GMT. Sat 29 Dec GMT. Realtime imagery of the sun.
The Solar System and Outer Space: Sun
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