Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Classification of Stella Spectra

  1. Apr 28, 2004 #1

    jimmy p

    User Avatar
    Gold Member

    Just a simple quickie, why are stars classified using the letters O, B, A, F, G, K and M? Is there any significance in the letters?
     
  2. jcsd
  3. Apr 28, 2004 #2
    Indeed, there is a significance. This Link provides an overwiev
     
  4. Apr 28, 2004 #3
    Back in the late 1800s, when Professor Draper of Harvard Observatory began his photographic collection of stellar spectra, the underlying physical significance of spectral line intensities was not understood, nor did he have a way to guess the temperatures of the stellar atmospheres that produced these spectra. But he best understood the visible Hydrogen dark lines (Balmer series) and decided to base a classification on letters of the roman alphabet: A, B, C, D,... A would be (and still is) the darkest hydrogen Balmer lines, B would be next darkest and so on down. This project was continued by Annie Cannon after Draper died. With time it became clear that the Draper system was not a particularly natural one, though thousands of records had been classified using this letter scheme. The matter of determining effective temperatures, excitation versus ionization in line spectra and their statistical distribution, and also the effects of stellar size differences on spectra led to a realization that these letters needed to be rearranged differently. For instance, O represents a hotter stellar surface condition than A, but is weak in Hydrogen lines. That is because almost all of the Hydrogen is at equilibrium in the ionized state at that temperature, so practically no Balmer lines can be detected. Most of the letters turned out to be single rare and peculiar stars, better represented by certain common letters in the system, so they were dropped. The remaining were W,O,B,A,F,G,K,M and a few others, going from hottest surface temperatures on down. The revised scheme is also complicated by existence of bright lines in the spectra of some stars.

    Quart
     
  5. Apr 28, 2004 #4

    jimmy p

    User Avatar
    Gold Member

    So what is the meaning of the letters?
     
  6. Apr 28, 2004 #5

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Several mnemonics, e.g. "Oven Baked Apples From Grandpa's/Grandma's Kitchen. Mmmm."
     
  7. Apr 28, 2004 #6

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    "Oh, Be A Fine Gorilla, Kiss Me."

    - Warren
     
  8. Apr 28, 2004 #7
    Indeed, your list is not complete. It extends to 4 more classes (R,N and S by the right and W by the left)
    So the list must be W O B A F G K M R N S
    You can memorize it like "Wow, Oh be a fine girl, kiss me right now soon"
     
    Last edited: Apr 29, 2004
  9. Apr 29, 2004 #8

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    "girl": I understand this is no longer "PC" in certain countries in the North American continent ... and why not "guy"? or "gorilla"?

    "soon": used to be "sweetheart", but also no longer PC

    In any case, we now have T, L, C (?), and maybe more as classes for stars (not necessary, entirely, spectral classifications).
     
  10. Apr 29, 2004 #9

    Labguy

    User Avatar
    Science Advisor

    A bunch of memory tricks tell you nothing about the meaning of the letters. Go to:

    http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit1/sptypes.html and read the progression of the classification systems all to the bottom of the page, and then click on the top or bottom link that shows comparative spectra of the different classes. The letters just happened to be rearranged as explaned on the link, and could have just as well been 1, 2, 3 etc. had letters not been originally chosen.
     
  11. Apr 29, 2004 #10
    The meaning of the letters is that A is the first letter of the Roman alphabet, B is the second letter, C is the third letter, etc. It is a list.

    The sequence was set up to begin with stars of darkest Hydrogen spectral lines first, next darkest second, and so on. Most of the classes (letters) were later thought unnecessary. Effective temperature became more physically important than the original idea of Hydrogen dark lines. So the remaining classes (letters) got their order changed to go from highest effective emission temperature on down.
     
  12. Dec 15, 2008 #11
    oh yeh!!! no prob u lady gorilla u!!! pphhwwaa!
     
  13. Dec 15, 2008 #12
    legend jimmy p!!! {:-)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Classification of Stella Spectra
  1. Interpreting spectra (Replies: 5)

  2. Emission Spectra (Replies: 9)

  3. Spectra Classification (Replies: 1)

  4. Stellar spectra (Replies: 2)

  5. Stellar Classification (Replies: 2)

Loading...