The Size of the Milky Way
|Early models for
the Milky Way:
- early 1900s: Jacobus
Kapteyn uses quantitative star counts to measure the size
- ~ 10 kpc in size
- Sun near center
- ~1920: Harlow
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
|Globular cluster distribution
(Note: this figure uses the modern day distance estimates;
Shapley thought everything was ~ twice as big)
Who's right? Actually, both were wrong. Their
were compromised by the effects of interstellar dust, the presence of
was unknown at the time.
Of the two, Shapley was much closer to the truth.
When the effects of dust were realized and corrected for, studies of
structure entered the modern era.
- Kapteyn's problem:
dust make stars fainter, Kapteyn couldn't see the already faint stars
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
see them further away. But since dust made them fainter, he thought
were even further away than they actually were.