Stellar Classification

[Glizdka]

Hello,

In this recording, I'm reading the too-long-post about stellar classification. Could I please get your feedback on what I should improve?

 

[teechar]

The link doesn't work. It says "This video is unavailable".

 

[Glizdka]

The link doesn't work. It says "This video is unavailable".

Probably because I've just uploaded it. Please wait a few minutes.

Forgive me the awkwardness... I'm not used to being recorded...

 

[teechar]

Can you upload the transcript? That way, we can highlight words/areas that have issues.

 

[Glizdka]

Can you upload the transcript? That way, we can highlight words/areas that have issues.

"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 it's difficult to memorize. 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 absorption 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 that is done 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 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 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.

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, and that made the new system so popular that it eventually replaced the old one. It's surprising how much can be told about a star just by knowing its color.

 

[teechar]

"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 it's difficult to memorize. 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 absorption spectral 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 that is done 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 light coming from a star, we can determine what the star is made of.

Class A stars show strong and sharp hydrogen lines, B 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.

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.

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

I've highlighted problem areas. Let me know which ones you need individual explanation for.

 

[Glizdka]

I've highlighted problem areas. Let me know which ones you need individual explanation for.

For the most part, I'm aware of my mistakes. These are the ones I'm not aware of:

barely
had already
notice
redefine
much

 

[teechar]

barely

It sounds a little like "barley".

had already

You overdo the catenation in that.

notice

It sounds like "nut-iss".

redefine

It sounds like "red-e-fine"; it should be "ree-define"

much

That's not in the script!

By the way, I highlighted "classification" because you pronounce the "L" in it as a dark "L"; it should be a light "L". Do you know the difference between those?

 

[Tarheel]

For the most part, I'm aware of my mistakes. These are the ones I was not aware of:

barely
had already
notice
redefine
much

We're talking about pronunciation, right?

 

[Glizdka]

By the way, I highlighted "classification" because you pronounce the "L" in it as a dark "L"; it should be a light "L". Do you know the difference between those?

I do.

Thank you a lot. That was very well detailed.