As an Astronomy teacher, I'm embarrassed to admit that I've been having a bit of trouble understanding/clarifying the nomenclature for spectral classes. I have been aware of the original "OBAFGKM" classes for years, and have always understood they represent the colors of stars based on the temperature at which there surfaces glow, with M being only red-hot (3000 - 4000 K and O being blue-hot (30000 K).
Then, I began encountering more and more classes. First class "N", then class "L" (representing cooler and cooler stars, dwarfs and brown dwarfs). Then, more classes appeared, based on what a star is made of. Class "C" for example, is used by Celestia to apparently represent not temperature, but stars rich in "carbon".
If I am to teach these properly, could someone please help me understand the answers to four questions:
1. Although it is not clear to me, can I presume that a "C" star is the same thing as an "N" or "L" star? That is, does a class "C" star glow only dully red and have a spectral color class of N or L?
2. Is class "L the lowest color class, based on temperature or is there a dark brown glowing star/"brown" dwarf?
3. Does Celestia display any "L" class stars? If so, what would one of them be named?
4. According to references I have consulted, neutron stars and pulsars have surface temperatures of several million degrees K. At that temperature, the star is so hot it is far beyond blue-hot. It is ultraviolet or X-ray hot. If true, then what visual color is a young neutron star? Does it glow deep violet or is it invisible visually? If so, then does that mean you could look at a young neutron star close up and see .... nothing??? If true, then why don't we have a spectral class for violet-colored stars or hotter ... something above the "O" class? Obviously, these stars all glow hotter than blue?
Your wise and sage counsel will all be very welcomed.
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.gif)
Thanks in advance.
Frank