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  1. Senior Member
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    #1

    Stellar Classification - Part 1: "Oh, Be A Fine Girl, Kiss Me!"

    Originally, I wanted to write only about stellar classification, but I let myself get carried away.
    The purpose is to teach myself how to write something worth reading. I do not intend to publish it anywhere but here, just for everyone to see.

    Could you please tell me which parts need improvement?

    "Oh, Be A Fine Girl, Kiss Me!" is a mnemonic written by Annie Jump Cannon, which is meant to help astronomy students remember stellar classification. In my opinion, only bad astronomers feel good knowing mnemonics. Nevertheless, the letter arrangement O, B, A, F, G, K, M seems a bit random and difficult to be memorized. Why are the brightest and the most massive stars classified as O? Why isn't it A, B, C, D, E, F, G?

    The story of stellar classification is a combination of refashioning an older system, and trying to keep old definitions true. Originally, stars were classified by the strength of hydrogen emission spectral lines.

    "Every cloud has a silver lining, and every sun has a hydrogen lining." - Me

    Hydrogen lines

    Each element has a unique 'fingerprint' contained within the light coming from it. Light can be broken down into its component colors, and when done so with the light coming from hydrogen, distinct lines are present at precise wavelengths of different hues of light. Each other element, such as Helium, produces a different set of these lines, so it cannot be mistaken for another. The lines are produced by the unique electron configuration of every element.

    Knowing the spectrum of the light coming from a star, we can determine what the star is made of.


    Helium lines

    Class A stars show strong and sharp hydrogen lines; B a bit fainter ones; O have weak and barely, if at all, visible hydrogen lines. In the old system, the classes were in alphabetical order, A through Q. However, they were later rearranged based on their color, rather than in order of their weakening hydrogen lines.

    At that time, much of the tedious grunt work of cataloging and classifying stars had already been done, so it was decided that the letters would only be rearranged, and that the classes themselves would not be renamed to make the new system more compatible with earlier findings. Notice that a few classes from the old system were merged; this was done to reduce the number of classes, and to make the new system simpler.


    Typical stellar spectra

    The decision to redefine stellar classification was made because the color of a star gives more information about it than the hydrogen lining does, which was making the new system more popular, eventually replacing the old one. It's surprising how much can be told about a star just by knowing its color.

    Please tell me when I should post part 2.
    Last edited by Glizdka; 25-Jun-2019 at 00:08. Reason: oops

  2. Tarheel's Avatar
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    #2

    Re: Stellar Classification - Part 1: "Oh, Be A Fine Girl, Kiss Me!"

    Hm.
    Not a professional teacher

  3. Tarheel's Avatar
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    #3

    Re: Stellar Classification - Part 1: "Oh, Be A Fine Girl, Kiss Me!"

    It reads well. When is the book coming out?
    Not a professional teacher

  4. Senior Member
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    #4

    Re: Stellar Classification - Part 1: "Oh, Be A Fine Girl, Kiss Me!"

    Quote Originally Posted by Tarheel View Post
    It reads well. When is the book coming out?
    It won't be a book per se, just a (too) long post (I've already got about 6 pages in Google Doc).

  5. teechar's Avatar
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    #5

    Re: Stellar Classification - Part 1: "Oh, Be A Fine Girl, Kiss Me!"

    Quote Originally Posted by Glizdka View Post
    "Oh, Be A Fine Girl, Kiss Me!" is a mnemonic written by Annie Jump Cannon, which is meant to help astronomy students remember stellar classification. In my opinion, only bad astronomers feel good knowing mnemonics. Nevertheless, the letter arrangement O, B, A, F, G, K, M seems a bit random and difficult to be memorize. d. Why are the brightest and the most massive stars classified as O? Why isn't it A, B, C, D, E, F, orG?

    Light can be broken down into its component colors, and when that is done so with the light coming from hydrogen, distinct lines are present at precise wavelengths of different hues of light. Each other element, such as Helium, produces a different set of these lines, so it cannot be mistaken for another. The lines are produced by the unique electron configuration of every element.

    Class A stars show strong and sharp hydrogen lines, B a bit slightly fainter ones, O have weak and barely, if at all, visible hydrogen lines. In the old system, the classes were in alphabetical order, A through Q. However, they were later rearranged based on their color, rather than in order of their weakening hydrogen lines.

    The decision to redefine stellar classification was made because the color of a star gives more information about it than the hydrogen lining does, which was making and that made the new system more so popular that it eventually replaced ing the old one. It's surprising how much can be told about a star just by knowing its color.
    .

  6. Tarheel's Avatar
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    #6

    Re: Stellar Classification - Part 1: "Oh, Be A Fine Girl, Kiss Me!"

    I would say:

    hard to memorize
    Not a professional teacher

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