Color of Stars

by Fraser Cain on February 4, 2009

Star classifications. Image credit: Kieff

Star classifications. Image credit: Kieff


Look up into the sky and you’ll see the stars twinkling in different colors. Some are dull and red, while others are white and others look bright blue. So how do you get so many different star colors?

The color of a star depends on its surface temperature. Our Sun’s surface temperature is about 6,000 Kelvin. Although it looks yellow from here on Earth, the light of the Sun would actually look very white from space. This white light coming off of the Sun is because its temperature is 6,000 Kelvin. If the Sun were cooler, it would give off light more on the red end of the spectrum, and if the Sun were hotter, it would look more blue.

And that’s just what we see with other stars. The coolest stars in the Universe are the red dwarf stars. These are stars with just a fraction of the mass of our Sun (as low as 7.5% the mass of the Sun). They don’t burn as hot in their cores, and their surface temperature is about 3,500 Kelvin. The light released from their surface looks mostly red to our eyes (although there are different colors mixed up in there too, red is the majority).

This is also the color you see with red giant stars; solar-mass stars that ran out of hydrogen fuel and bloated up many times their original size. The luminosity of the star is spread out over the much larger surface area of the red giant and so they’re cooler,

On the opposite side of the spectrum are blue stars. These are stars with many times the mass of the Sun and so their surface temperatures are much hotter. Blue stars start out above 10,000 Kelvin but they can reach 40,000 Kelvin with the largest hypergiant stars.

We have written many articles about stars on Universe Today. Here’s an article about the biggest stars in the Universe.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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