White Stars

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Stars can look many colors, from the dim red dwarfs to the bright blue supergiants. But what about white stars, can you have a star that looks white? Actually, our own Sun is one of the best examples of a white star. But wait, isn’t the Sun yellow? Actually, the atmosphere of the Earth changes the color of the light from the Sun so that it looks more yellow. But if you could actually go out into space and look at the Sun, it would look like a pure white star. (Here’s a link to an article that explains, why is the Sun yellow?

The color of a star depends on its temperature. The coolest stars are the red dwarfs/red giants, with surface temperatures of 3,500 Kelvin or less. As the surface temperature gets hotter, the color of the star turns orange, and then yellow-orange, and then yellow, yellow-white, and then around 5,800 Kelvin it appears white.

But a star like the Sun isn’t actually giving off pure white light, it’s giving off photons across the entire spectrum of the rainbow; some from the red, orange, yellow, green, blue and indigo regions of the spectrum. When we see the collection of all the photons with our eyes, we average it out and call it white light.

Stars hotter than the Sun also look white. It isn’t until you reach a temperature of around 11,000 Kelvin before a star starts to look blue from our perspective.

Most white stars are going to be hotter and more massive than our Sun. This means they’re more luminous and use their hydrogen fuel up more quickly.

Of course, another kind of white star are the white dwarfs. These were once stars like our Sun, but they used up all the hydrogen fuel in their core. After a brief time as a red giant, they blasted out their outer layers and then collapsed inward to become a white dwarf. These extreme objects pack about 60% the mass of the original star down into a size similar to the Earth. Just a single spoonful of white dwarf material weighs more than a tonne. White dwarfs are white because they’re so hot. But they’re not producing any new energy any more, so they’ll slowly cool down to the background temperature of the Universe.

We have written many articles about stars here on Universe Today. Here’s an article about how you can find the white star Sirius with binoculars, and here’s an article about a new class of white dwarf stars discovered.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

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?

Reference:
http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html