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

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 !!! ;-)

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

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