What are the different types of stars?

In my last few posts I have talked mainly about the Sun, our closest star but what other types of star are there? Clearly by looking at the Hertzsprung-Russell diagram there are different types; cooler, dimmer smaller stars to brighter, hotter and massive stars. Astronomers class the stars according to a property called it’s Spectral Class which is related to it’s temperature and chemical make up. Each category of Spectra is given a letter of the alphabet OBAFGKMLT with L and T being added most recently to classify Brown Dwarf stars. For all but the brown dwarfs this can be remembered with the mnemonic Oh Be A Fine Girl/Guy Kiss Me. Can you think of a new mnemonic that includes all the spectral classes?

The relationships between a stars spectral class and it temperature is shown below in the table. The constellation Orion is a good place to look in the sky to see the different colours of the stars Rigel, Spectral Class B, and Betelgeuse, Spectral Class M. On a clear night you can see these stars fairly easily with the naked eye. Our Sun is a Spectral Class G star and is roughly in the middle of the list but is significantly cooler than Rigel.


Classification of stars
Spectral Class Colour Temperature (K) Example Star
O Blue Violet 30000 – 50000 Mintaka in Orion
B Blue White 11000 – 30000 Rigel in Orion
A White 7500 – 11000 Vega in Lyra
F Yellow – White 5900 – 7500 Procyon in Canis Minor
G Yellow 5200-5900 The Sun
K Orange 3900 – 5200 Aldebaran in Taurus
M Red – Orange 2500 – 3900 Betelgeuse in Orion
L Red 1300 – 2500 Brown dwarf Teide 1
T Red below 1300 Brown dwarfs Gliese 229B

Below is an image of the spectrum from our Sun. Click to enlarge it.

There are several points here that I have either not mentioned or glossed over without much detail (The use of Kelvin (K) as  the temperature, what is a spectral class, for example) and I hope to pick these points up in the next few blog posts. In the meantime see if you can find where these spectral classes may sit on the H-R diagram from my previous post.


Information taken from Universe, 9th edition, Freedman et al, 2011, ISBN-10: 1-4292-3153-X

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