MadSci Network: Astronomy

Re: Are there different classifications of stars?

Date: Thu Jun 24 04:54:40 2004
Posted By: Matthew Westmore, Medical Physicist
Area of science: Astronomy
ID: 1080764314.As

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.

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 for an example.


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