In popular culture, the Sun is yellow. But did you know that the color of the Sun is actually white? It’s only when light from the Sun passes through the Earth’s atmosphere that in changes in color, from white, to the yellow we see here on Earth.
All stars have a color. From red dwarfs and red giants, to white and yellow stars to blue giants and supergiants. The color of a star comes from its temperature. As photons escape the interior of a star out into space, they have different amounts of energy. A star can be emitting infrared, red, blue and ultraviolet light all at the same time. They’re even emitting X-rays and gamma rays.
If a star is cool, less than 3,500 Kelvin, its color will be red. This is because there are more red photons being emitted than any other kind of visible light. If a star is very hot, above 10,000 Kelvin, its color will be blue. Once again, because there are more blue photons streaming from a star.
The temperature of the Sun is approximately 6,000 Kelvin. The Sun, and stars like our Sun appear white. This is because we’re seeing all the different color photons coming from the Sun at the same time. When you add all those colors up, you get pure white.
The white color inside this black box is approximately the color of the Sun.
True color of the Sun
So why does the Sun appear yellow here on Earth? The atmosphere of the Earth scatters sunlight, removing the shorter wavelength light – blue and violet. Once you reduce those colors from the spectrum of light coming from the Sun, it appears more yellow. But if you could fly up and see the Sun from space, the color of the Sun would be pure white.
Temperature of the Sun
The surface of the Sun, the part we can see, is called the photosphere. Photons streaming from the surface of the Sun vary in temperature from 4,500 kelvin to more than 6,000 kelvin. The average temperature of the Sun is about 5,800 kelvin. In other temperature measurements, the Sun is 5,500° C, or 9,900° F.
The Suns photosphere is looking particulary boring (NASA/SOHO)
But this is just an average temperature. Individual photons can be cooler and redder, or hotter and bluer. The color of the Sun we see here on Earth is an average of all the photons streaming from the Sun.
But this is just the surface. The Sun is held together by the mutual gravity of its mass. If you could descend down into the Sun, you would feel the temperature and pressures increase all the way to the core. And it’s down in the core where the temperatures reach 15.7 million kelvin. At this pressure and temperature, hydrogen fusion can take place. This is where atoms of hydrogen are fused together into helium, releasing photons of gamma radiation. These photons are released and absorbed by atoms in the Sun as they slowly make their way out into space. It can take 100,000 years for a photon formed in the core to finally reach the photosphere and make the leap into space.
Surface of the Sun
Perhaps the most familiar feature on the surface of the Sun are sunspots. These are relatively cooler regions on the surface of the Sun, where its magnetic field lines pierce the surface of the Sun. Sunspots can be the source of solar flares and coronal mass ejections.
Hinode Views the Sun’s Surface
When we look at the Sun, we notice that the center of the Sun looks much brighter than the edges of the Sun. This is called “limb darkening”, and it happens because we’re seeing light that has passed through the surface of the Sun at an angle, and has been blocked more – and so it’s darker.
Huge coronal mass ejections can also be seen blasting out of the surface of the Sun. These are created when the coiled up magnetic field of the Sun snaps and reconnects. This reconnection releases a tremendous amount of energy, and throws charged plasma into space. When this plasma reaches the Earth, it creates the beautiful auroras best seen near the Earth’s poles.
Astronomers measure the brightness of stars with various tools, but they need a way to compare. That’s where our Sun comes in. As everyone knows, the Sun gives off roughly 3.839 x 1033 ergs per second in energy. Other stars in the Universe might only give off a fraction of solar luminosity, or several multiples of it. Our Sun is a stellar yardstick.
Massive coronal mass ejection on. This image shows the size of the Earth to scale. NASA / SDO / J. Major.
Imagine that the Sun is enclosed in a series of transparent spheres – like layers in an onion. The amount of energy, the solar luminosity, passing through each one of these spheres every second is always the same. However, the surface area of that sphere gets larger and larger. This is why the further away you get from a star, the less light you see.
This is called the inverse square law, and it allows astronomers to calculate the solar luminosity; in fact, it lets them calculate the luminosity of all stars. Scientists have sent up missions into space that measure the total amount of energy falling upon their detectors. From this information, astronomers can calculate how much energy is falling onto the entire Earth, and then how much is coming from the Sun.
And this works for stars as well. The spacecraft detects the luminosity of another star, factors in the distance, and helps calculate the original luminosity of the star.
Although our Sun is roughly stable, it does experience slight variations in solar luminosity. These changes are caused by sunspots that darken regions, and bright structures on the solar disk during the Sun’s 11-year sunspot cycle. Detailed measurements made over the last 30 years have found that they aren’t enough to cause the accelerated global warming we’re detecting here on Earth.