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Amy holds a Master of Science. She has taught science at the high school and college levels.
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What do you see when you look at the night sky? Depending on where you live, you see mostly stars. If you look at the sky without a telescope, you see white stars, maybe some faintly blue or even sometimes some yellow or orange ones. The color depends on the star's surface temperature.
For example, our sun's surface temperature is about 6,000 Kelvin. Although it looks yellow from Earth, the light of the sun would actually look very white if we were in 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 in the red range, and if the sun were hotter, it would look more blue.
The coolest stars in the universe are the red dwarf stars. These are very tiny stars, some of the tiniest, so they don't burn as hot and their surface temperature is only 3,500 Kelvin. The light they give off looks mostly red to us.
Red is also the color you see with red giant stars, huge 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, making this star cooler than other large stars.
On the opposite end of the color spectrum are the blue stars. These stars are giants and hypergiants - much, much bigger than the sun, and also much, much hotter (between 10,000 and 40,000 K). For us on Earth, though, most stars in the sky, except for the brightest ones, appear white or bluish white because they don't emit enough light for our eyes to see color.
Scientists have been studying stars for a long time, and over time they have learned to tell a lot about a star just by determining its temperature and atmospheric pressure. The temperature tells them the surface brightness of a star, and the pressure tells them an approximate size of the star, which tells them whether the star is a giant, a dwarf or something in between. These two measurements taken together can often give information on the star's age and distance from the earth.
Scientists like to organize and classify things; they developed a classification system called the spectral code and have used it since 1943. To those who can read it, the spectral code tells just what kind of object a star really is: its color, size and luminosity compared to other stars, in addition to its peculiarities, history and future.
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Let's learn a bit of the classification system. Scientists classify stars by temperature and the elements they absorb, which are called their spectra. They have divided stars into seven main types.
There are seven main types of stars: O, B, A, F, G, K and M. The O stars are the bright, hot, blue stars and the M stars are the dimmer, cooler, red stars. A common mnemonic for remembering the order of the classifications is: 'Oh Be A Fine Girl, Kiss Me.' But I like this mnemonic better: 'Oh Boy, An F Grade Kills Me.'
According to the modern spectral classification system:
These categories of stars can also be broken down into tenths by giving them a number of 0-9. So an A5 star is five tenths (5/10) between an A star and an F star.
Class O stars are very hot, bright, and look bluish. The class O star is very rare. Only about 0.00003% of main-sequence stars are O stars. These are also some of the brightest, most massive stars in the sky, shining with over a million times the power of our sun.
Class B stars are very bright and blue. These stars are short-lived, so they don't travel far from where they are born. There are few of these class B stars - only about 0.13% of all stars.
Class A stars are white or bluish-white. About 0.625% of the stars in the sky are class A.
Class F stars are white and make up about 3% of stars.
Class G stars are yellowish white. Our sun is a class G star. These stars are more common - about 7.5% of stars.
Class K stars are orangish stars that are slightly cooler than the sun. They make up about 12% of stars.
Class M stars are the most common class, about 76.02% of stars. We can't see any class M stars with our naked eye, though, because none of them are bright enough. Most class M stars are red dwarfs, but there are also some giants and supergiants in this class, along with some hotter brown dwarfs.
Scientists classify stars by their color and temperature into seven categories. The O stars are the brightest and hottest and the M stars are the coolest and dimmest. The easiest way to remember the categories in order is with a mnemonic such as 'Oh Boy, An F Grade Kills Me.' This spectral classification tells scientists a lot about a star, including the approximate size of the star, what type it is, how old it is and how far away it is.
Class O stars look bluish - only about 0.00003% of main sequence stars are O stars. Class B stars are very bright blue. There are few of these class B stars - only about 0.13% of all stars. Class A stars are white or bluish white. About 0.625% of the stars in the sky are class A. Class F stars are white and make up about 3% of stars. Class G stars are yellowish white. These stars are more common - about 7.5% of stars. Class K stars are orangish stars. They make up about 12% of stars. Class M stars are the most common class - about 76.02% of stars.
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