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[Hertzsprung-Russell] HR-diagram - log plot of temp against brightness

Main sequence: most stars are in NW diagonal band from 2000K (band M),0.00001
	luminosity (sun=1) to 20,000K (band B), 10,000 luminosity.
	sun at 7000 (band F), 1 is near the middle.  Altair 10,000,10 is a
	bit NW, whereas 61 Cygni SE and Barnards star 3000 (M-band),0.01
	further SE
Red dwarfs towards bottom right (low luminosity) will have long life
	~ 10^10 years (e.g. Proxima Centauri)
White dwarf - size of earth but mass of sun; this book-sized vol may have
	mass of 10K tons.  At the bottom (low-luminosity) of HR diagram.
	Sirius B (band B, 0.005)
Red giants : much larger and brighter, above main sequence - mass similar to
	sun, low surf temp (K,M band) but would envelop all inner planets
	(upto earth?) Antares, Aldebaran.
Supergiants: Betelgeuse - 600mn miles, (upto orbit of Mars).  Rigel - 60mn
	miles - abt 100x size of sun - hot blue supergiant.

Milky way - larger than many other galaxies.  2x10^11 stars.  disk 15K ly
	thick, abt 150K ly dia, stars in spiral arms upto 75K ly, then old
	stars and star clusters.   At the center, prob a black hole
	(Sagittarius A*) emitting radio waves when new mass falls into it.

Contents

Part One : A tour of the universe
     - Stars and Galaxies
     - Solar System ("nine" planets)
Part Two:  Observing the sky 65
     - Tools and Techniques
     - Sky and Constellation Charts 95 (bulk of book
Part Three: Exploring Space
     - Breaking away from earth
     - The outer limits