Temperature of Stars

by Fraser Cain on February 6, 2009

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Artist's impression of a red giant star. Image credit: ESO

Artist's impression of a red giant star. Image credit: ESO


You might be surprised to know that the color of stars depends on their temperature. The coolest stars will look red, while the hottest stars will appear blue. And what defines the temperature of a star? It all comes down to mass.

The most common stars in the Universe are the relatively tiny red dwarf stars. These stars can have as little as 7.5% the mass of the Sun, and top out at about 50%. Red dwarfs use their stores of hydrogen fuel very slowly; it’s believed that a red dwarf star with about 10% the mass of the Sun may live for 10 trillion years or more. Our own Sun will only live for about 12 billion years. Red dwarf stars have a surface temperature of less than 3,500 Kelvin, and this is why they appear red to our eyes.

Our own Sun is classified as a yellow dwarf star. It has a surface temperature of about 5,800 Kelvin. Because of this temperature, the bulk of the light we see streaming from the Sun is yellow/white. Our Sun has been in the main sequence phase of its life for 4.5 billion years, and it’s expected to last another 7 billion years or so.

The hottest stars are the blue stars. These start at temperatures of about 10,000 Kelvin, and the biggest, hottest blue supergiants can be more than 40,000 Kelvin. In fact, there’s so much energy coming off the surface of a blue star that many could actually be classified as ultraviolet stars, it’s just that our eyes can’t see that high into the spectrum.

We have written many articles about stars here on Universe Today. Here’s an article about how red dwarf stars could have habitable zones, and here’s an article about how red dwarfs can clear out their dusty disks.

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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