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.
|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|
|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