OT: Astronomy Lesson Of The Week (was: Re: [LST] Making a 'pedia entry Re: was: (a lot of other things)

This has really drifted away from the main focus of the GZG games, but
in an effort to bring everyone onto the same starfield (at least those
willing to suffer reading through this ;-) (I promise no math - this
time!), I post it to the list.

> As mentioned by others, M-class stars are small and cool (relatively
!)
> 
> Class     G		     M
> Temp	   5.8x10e9	    3.3x10e9
> 
> The M class is about 0.3x Sun's radius and 0.2 x Sun's mass.	You
would need
> about 100 M-class to shine as brightly as a G class.

Actually, as someone else pointed out (sorry, I forget who), the "class"
of a star is actually the "spectral type", and refers to the temperature
of the star. It has no bearing on the size. For that you need a
luminosity
component.

Now, again, as has been mentioned, the little diddy "Oh Be A Fine Girl,
Kiss Me [Right Now Sweetie/Susie/S-whatever]" lines up the spectral
elements in descending temperature ranges. Thus:

O - 30,000+ K
B - 21,000 K
A - 10,000 K
F -  8,000 K
G -  6,000 K
K - < 5,000 K
M - < 4,000 K

The temperatures are close approximations.

I'm not going to cover the R, N, S, or even W as they are somewhat more
esoteric. We're going to stick with the "standard" set for this. But I'm
going to do my damndest to NOT go into a full-blown stellar evolution
thing here.  :-)

The *size* of a star comes into play with the *luminosity* of said star.
Just because a star is "cool" does not mean it cannot also be bright.
Ditto for small hot stars. Just because they are small doesn't mean they
can't be cookin'.

Whatever their sizes, these large, bright stars are called Giants or
Supergiants. The small ones are called Dwarfs (in some cases
'sub-dwarfs').

There is a very specific temperature-luminosity relation that you
can look up on the web. Do a search for "hertzsprung-russel diagram"
(or just "hertzsprung-russel"; many of the pages are not in english,
but you don't need english to understand the table! :-)  You will see
essentially the following:

 -10  |..    .	       .	 .	  .
      | ...	  .	   .	    ..	 .  . supergiants 
      |   ...  .     .		.     ..  .. .
  -5  |      ....		     ....  .....
      |        ......	      .    ... giants ..
      | 	  .......      ...  .. .  ....
   0  | 	     ......  .	.  .
      | 	       .......
      | 		 .......
  +5  | 		    .......
      |     .	.	      .......
      |     .	  .		 ......
  +10 |    white dwarfs 	    .....
      |  .     .			....
      |      .	    .			   ....
  +15 | 				       ..
      |-------------------------------------------
	O     B     A	  F	G     K     M

Yeah, it's a poor ASCII diagram. Sorry.  :-/  The main
trend from upper left to lower right is called The Main
Sequence, and these stars are generally referred to
as "dwarfs" (well, at least from A to M, anyway ;-)

But what I'm trying to illustrate here is that *spectral class*
has no bearing on *size* of a star. Only the luminosity of the
star (the Y-axis, which is, by the way, denoted in absolute
magnitude, where the "absolute magnitude" is how bright a star
would appear to the naked eye at a distance of 10 parsecs, where
the magnitude scale is a logrithmic scale, each step being 2.5x
brighter/fainter than the previous; the lower the number, the
brighter the object in question). (btw, a parsec is 3.26 lightyears)

Now, for some example stars (in no particular order):

Name (constellation)   apparent   absolute		    distance
		       magnitude  magnitude   "size"	  (in parsecs)
------------------------------------------------------------------------
Betelgeuse (Orion)	  0.8	   -5.5     M2 supergiant   150
Sirius (Canis Major)	 -1.5	    1.4     A1 dwarf	      2.7
Alpha Centauri		 -0.3	    4.4     G2 dwarf	      1.3
Rigel (orion)		  0.1	   -6.8     B8 supergiant   250
Vega (Lyra)		  0.0	    0.5     A0 dwarf	      8.1
Pollux (Gemini) 	  1.2	    0.8     K0 giant	     12
Castor (Gemini) 	  1.6	    0.9     A1 dwarf	     14
Deneb (Cygnus)		  1.3	   -6.9     A2 supergiant   450
Epsilon Indi		  4.73	    7.0     K5 dwarf	      3.44
Barnard's Star		  9.54	   13.2     M5 dwarf	      1.83
Epsilon Eridani 	  3.73	    6.1     K2 dwarf	      3.28
Wolf 359		 13.66	   16.8     M6 dwarf	      2.32
Wolf 424		 12.7	   14.4     M5.5 dwarf system 4.37
Our Sun 		-26	   +4.2     G2 dwarf	 go outside and
							   look!  :-P

Anyway, what I'm trying to show you is that size does matter - for
brightness. It isn't tied to the temperature of a star. I don't know
how well I succeeded in getting this across (this is probably a slight
jumble since I'm trying to work and prep for a meeting at the same time
:-/ )

> NB All figures are approximate !!! ;-)
> 
> I do not recognise the star references you have quoted, or at least,
neither
> of my astronomy programs have them in their databases in that form, so
I
> can't help you with those.

The designations are given from various catalogs across the world
charting
certain types of stars or objects of interest. So, from the given list
for
example:

> AY Indi
> Wolf 1495
> AC 12’2306-155
> Ross 845
> Ross 848
> CD 55’9073
> CD 60’7821 L
> CP 40’7021 LTT

CD is from the Cordoba Durchmusterung catalog
CP is from Astrographic Catalog
Ross is from an astronomer named Ross (imagine that ;-) who
  studied high proper motion stars
Wolf is from a german astronomer who charted out another class of
  stars
AY is one of a number of designations for a variable star (this is
  a whole "lesson" in and of itself, but I won't go into this now)

Stars are often cross-cataloged. Example:

Wolf 1495 above is also known as GJ 3809, GEN# +9.80064027, G 65-9,
G 64-27, LTT 14032, and UBV M 42607.

Most of those would have zero meaning to most of you.  ;-)  But 
essentially they refer to different catalogs and/or studies.

"Lesson" over. You can wake up now.

Mk