Physics 162 - Lecture Summary #1
The Spiral Nebulae
Read § 22.1 (907-911), § 22.2 if you are not familiar with the
structure of our Galaxy.
Read § 23.1 on the Hubble Classification scheme
Take a look at our basic astronomy site's description of our
Milky Way Galaxy.
Peruse the discussion of the historic
Shapley
Curtis Debate regarding the nature of the Spiral Nebulae.
Historical Timeline
1750 Thomas Wright proposes that the Milky Way is a stellar disk system.
1755 Immanuel Kant speculates that there may exist "Island Universes" like
our Milky Way.
1785 William Herschel studies star counts along several hundred lines of
sight in the galaxy.
Herschel's model of the Milky Way obtained from "star
gauges" along many lines of sight in the Galaxy. The sun is the brighter star
to the left of center. Herschel, Kapteyn and Shapley were unaware
of the presence of dust in the Galaxy which causes extinction and reddening of
starlight.
1845
William Parsons, Earl of Rosse, using a 72-inch home constructed telescope
discovers the "Spiral Nebulae" (Messier 51), speculates that they may be Kant's
Island Universes.
1912 Vesto
Slipher at Lowell Observatory observes brighter spiral nebulae
spectroscopically. Spectra show emission lines from hot gas, absorption lines
from stars. Radial velocities are nearly all positive with values up to
several hundred km/s - later to be determined to be due to the expansion of
the Universe.
1901-1922
J. C. Kapteyn made extensive star counts from photographic plates to
determine the structure of the Milky Way. His model has become known as
Kapteyn's Universe.
Kapteyn's Universe; the sun is slightly off center in
Kapteyn's model.
1917
G. W. Ritchey observes
novae in spiral
nebulae. Unable to reconcile the "nova" S Andromedae observed in 1885
in the Andromeda nebula with other novae in spiral nebulae - perhaps it is a
particularly powerful nova - a "supernova"?
1923 Edwin
Hubble discovers
Cepheid Variable stars in
Messier 31 - the
Great Nebula in Andromeda, estimating its distance as nearly 0.3Mpc (modern
value is about 0.7Mpc), well outside our Galaxy.
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