The Size of the Milky Way
Early models for
the Milky Way:
- early 1900s: Jacobus
Kapteyn uses quantitative star counts to measure the size
of
the Galaxy.
- ellipsoidal
- ~ 10 kpc in size
- Sun near center
- ~1920: Harlow
Shapley
uses RR Lyrae variable stars to get distances to globular clusters.
- Galaxy is ~ 100 kpc in size
- Sun is ~ 15 kpc away from the center of
the
GC distribution.
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Kapteyn's model
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Globular cluster distribution
(Note: this figure uses the modern day distance estimates;
Shapley thought everything was ~ twice as big)
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Who's right? Actually, both were wrong. Their
observations
were compromised by the effects of interstellar dust, the presence of
which
was unknown at the time.
- Kapteyn's problem:
because
dust make stars fainter, Kapteyn couldn't see the already faint stars
at
large distances, so he thought the Galaxy was "running out of stars" at
about 10 kpc away.
- Shapley's problem: since
he was observing clusters of stars (instead of individual stars) he
could
see them further away. But since dust made them fainter, he thought
they
were even further away than they actually were.
Of the two, Shapley was much closer to the truth.
When the effects of dust were realized and corrected for, studies of
galactic
structure entered the modern era.