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 |
| 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.
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- Light Spectrum of our Sun. Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
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.
References
Information taken from Universe, 9th edition, Freedman et al, 2011, ISBN-10: 1-4292-3153-X
