D-31: Star Types

An earlier version of the following was originally published on the Stellaris forums.


Good morning everyone. A quick one for today. Fortunately, “quick” does not mean less words. It means less research and calculation for me before I write it.

Let’s talk about stars. It occurs to me that many people may not know their star classifications. This is a tragedy and needs to be rectified.

Main-sequence stars (also called “dwarf stars”) are classified into types depending upon their temperature and size. These are traditionally remembered using the mnemonic “Oh be a fine girl/guy, kiss me.” Personally I prefer “only badass astronomers fight green killer martians”, but mnemonics are a matter of personal taste.

O-class stars are very hot, as hot as giant stars. They’re traditionally called blue and indeed that’s the colour they are. They’re also extremely rare. In the past it was thought that these were very young stars and that they eventually aged out of this class, but now we know that they spend the entirety of their lives like this. O-class stars will almost never have planets: in fact, an O-class star’s formation will destroy partially-formed planets within a light year of it.

B-class stars are called blue-white but to the human eye they’re still pretty blue. They’re much more common than O-type stars but are still fairly rare, about one-seventh of one percent. B-class stars are thought unlikely to have planets but if they did, their habitable zone would be a long way out.

A-class stars are more common yet, about 0.6% of known stars. They’re known as white but look blue-white to the naked eye. A-class stars are much smaller than the bigger O- or B-class stars, down to a minimum of about one and a half solar masses. We think that most of them won’t have planets but those that do will also be fairly scary places to live.

F-class stars are where it starts getting good. We call them yellow-white despite the fact that they would look almost pure white to the naked eye. We know of a number of F-class stars which have planets. At their smallest, these can get down to the same size as the Sun but will be a little brighter.

G-class stars are more common yet, between 7% and 8% of known stars. The Sun is a G-class star. Many G-class stars have planets, and their habitable zones can be large and generous. We call them yellow for traditional reasons but, like the Sun, they’re actually more yellow-white.

K-class stars are smaller than the sun. About an eighth of all stars are K-class. We often find them in binary systems and many have planets. They’re called orange and that’s a pretty good description.

Finally, three-quarters of all stars are M-class (which is why people sometimes split some of them off as R, N and S classes.) These are called red but look more like deep orange. These are small, cool stars. Planets are common, as are binary systems. It’s very easy for us to spot planets around these stars, which is why many known exoplanets orbit M-class stars. Their habitable zone is close to the star itself and may feature moons and gas giants.

Within each class we divide them into ten sub-classes which are denoted by numbers. For example, the Sun is G2-class and 10 Lacertae is O9. Higher numbers are colder.

Larger, bluer stars have shorter lives. A G-class star like the Sun may live 10 billion years; a hot white A-class star will be unlikely to see even one billion; and the smallest red M-class stars may eventually see their trillionth birthday. This is part of the reason why big blue stars are far rarer than small red stars.

For the rest of the series, if you get confused by me saying “A-class star”, you can come back here.


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