Star classifications. Image credit: Kieff

Star Classification

Article Updated: 24 Dec , 2015


Like most scientists, astronomers like to categorize the Universe around them, and stars are no exception. Here’s how astronomers classify stars into different groups; let’s learn a bit about star classification.

Stars are classified based on the spectral characteristic of the light it’s giving off. As you probably know, the light we see with our eyes is actually a mixture of colors. You can break them up into their different parts just like you can use a prism to break sunlight into all the colors of a rainbow. The rainbow that we see is actually the spectrum produced by the Sun, and it’s different for different stars depending on their temperature. A cooler star will have a spectrum that has more red in it, while a hotter star will be shifted up towards the blue end of the spectrum.

Astronomers classify stars by color using a series of letters: O, B, A, F, G, K and M. You can remember the sequence with the handy mnemonic, “oh be a fine girl and kiss me”. Under this classification, O stars are the hottest, and M stars are the coolest, with the other letters coming in between. O stars are “blue”, A stars are “white”, G stars are “yellow”, and M stars are red.

Since that doesn’t provide enough detail, astronomers put a second number after the letter to distinguish where on G, for example, a star like our Sun should be positioned. Each number is a further 10% towards the next spectral letter. For example, our Sun is classified as a G2 star. This means it’s 20% along the way towards an orange main sequence star.

Astronomers use another roman numeral at the end of the spectral letter to define the size and luminosity of a star. They range from I supergiants to V dwarfs, or main sequence stars. Because our Sun is a main sequence star, it would get the V designation.

So the full classification for the Sun is G2V.

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


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