|MadSci Network: Astronomy|
Hi Madelyn, An important part of the work astronomers do is to classify objects. They love it! If there isn't a classification scheme for a group of objects then we'll invent one pretty quick. Classification tells us a lot about the similarities and differences between stars, galaxies, planets etc, and this can then be used to understand what physics is going in them. So, yes there is a classification system for stars and it is based on their "spectra". If you take sun light and split it into its constituent colours with a prism, that's its spectrum. Astronomers do this for all the stars in the sky and group them according to how the spectrum looks. If you look up at the night sky amongst all the white stars you should be able to make out some bluer and redder ones; Betelgeuse in Orion is a good one to look for. In reality all the stars are different colours and very few are white. Its just that the eye is much more sensitive in monochrome so faint points of light look white. The Sun itself is a pretty average star and it isn't white. If you look at the spectra of all these stars, you see that not only are they all made up of different colours but that superimposed on this "colourwash" are thin bright and dark lines. These are where light is emitted or absorbed by specific elements (hydrogen, helium, heavier metals etc) in the outer atmosphere of the star. The elements involved, and whether they emit or absorb light is due to the temperature of the star and its composition. The current spectral classification scheme was developed at Harvard Observatory, by Henry Draper, in the early 20th century and groups stars into standard star types O, B, A, F, G, K, and M. There are many sub- groups that complicate the system but the basics are the same in that the group a star is in is determined by its spectrum. A good description is given by. http://lheawww.gsfc.nasa.gov/users/allen/spectral_classification.html If you plot the classification of a star against its brightness you get an extremely important graph called a Hertzsprung-Russell (HR) Diagram. This is where classification stops being simple "stamp collecting" and becomes a powerful tool. The HR diagram can tell us a great deal about the type of star (normal, white dwarf etc) its age, and its evolution; see http://ast.star.rl.ac.uk/hr.html for an example. Cheers Matt
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