Measuring the Stars
(Part II)
A Quantitative Way to Characterize Stars:
Color (Temperature) vs. Magnitude (Luminosity)
A “Hertzsprung-Russell” (“H-R”) Diagram (Stellar Luminosity vs. Temperature)
(Done here for a “random”
selection of stars…
…No apparent pattern…)
H-R Diagram for stars lying within ~5 parsecs of our Sun
(Our Sun is actually
hotter & brighter
than most stars in a typical
galactic neighborhood…)
H-R Diagram for the 100 brightest(-looking) stars in our Night Sky
(…but our Sun is
not very bright
compared to the
typical stars
one sees in the sky!)
(…in other words,
compared to
the “celebrities”
of the star kingdom…)
The Main Regions of the H-R Diagram (“Cartoon View”)
A condensing Protostar “hops” onto the Main Sequence…
…becoming an official Star as its core Nuclear Fusion initiates!
 With its permanent Main Sequence “status” (position) depending upon its Mass!
(Generally) Life-Long Main Sequence Position, as a function of Stellar Mass:
(vs. Mass of
our Sun)
Typical Stellar Abundances – by Mass of the Star
(…but why are Low-Mass stars so much more common…?)
Of “All” Milky Way Stars (roughly):
~9% White Dwarfs (“dead”)
~1% Red (Super-)Giants (“dying”)
~90% Main Sequence (“living”)
Some Reasons for the great abundance of Low-Mass stars…
1) Obviously requires less material to make (many!) Low-Mass Stars, than High-Mass ones;
2) Even though High-Mass stars have more “fuel” (Hydrogen)…
 They “burn through it” (fusion) much sooner!
Stellar Luminosity
increases dramatically
as Star’s Mass
grows Larger…!
“Main Sequence Turn-Off” as Stars Die…
o “Sun-like” (G-Type) Stars,
 ~  × Sun :
Live ~10 Billion Years ;
o “Blue Giant” (O-Type) Stars,
 ~  × Sun :
Live (“only”) ~25 Million Years ;
( Not even as long ago
as the Dinosaurs!)
o “Red Dwarf” (M-Type) Stars,
 ~ .  × Sun :
Live ~1 *Trillion* Years!
(…older than the Universe ,
so far…!)
Spectacularly Bright, Massive Stars Die “Young”!

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