The Milky Way - University of Rochester
The Milky Way
The Shape of the Galaxy Stellar Populations and Motions
Stars as a Gas
March 19, 2024
University of Rochester
The Milky Way
Today's lecture: I The shape of the Galaxy I Stellar populations and motions I Stars as a gas: Scale height, velocities, and the mass per unit area of the disk
Reading: Kutner Sec. 16.4, Ryden Sec. 19.1?19.3, Shu Ch. 12
Wide-angle photo and overlay key of the Sagittarius region of the Milky Way (from Bill Keel, U. Alabama).
March 19, 2024 (UR)
Astronomy 142 | Spring 2024
2/1
The number of stars brighter than f0
Suppose stars are uniformly distributed in space with number density n and typical luminosity L. How many are brighter (i.e., have greater flux) than some value f0?
Presuming there is no extinction, there is a
distance r0 corresponding to the flux f0:
s
f0
=
L 4pr20
!
r0 =
L 4pf0
f03/2
log N
=)
N(f
> f0) = =
4p 3
r30n
4pn
L
3/2
3 4pf0
? f0 3/2
log f0
March 19, 2024 (UR)
Astronomy 142 | Spring 2024
3/1
Shape of the Milky Way
Herschel (1785) and Kapteyn (1922) used
Observed counts:
this idea to characterize the shape of the
Milky Way.
I If the MW has edges, N must decrease
faster than f0 3/2 past the edges. I Actual star counts at large fluxes are
f03/2
log N
less than predicted by the N ? f0 3/2 relationship.
I Implication: the MW has a finite size
with identifiable edges.
Red: star count in direction of Galactic disk Blue: star count ? to Galactic disk
log f0
March 19, 2024 (UR)
Astronomy 142 | Spring 2024
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Herschel's Milky Way (1785)
"Section of our sidereal system" (Herschel 1785). The long axis of the figure runs from 20h22m, +35 in Cygnus (left) to 8h20m, 35 in Puppis (right). The short axis points towards 12h24m, +58 in Ursa Major.
Sun
The Great Rift in the summer Milky Way, interpreted as a dearth of stars, is visible on the left. Herschel referred to this as "an opening in the heavens."
March 19, 2024 (UR)
Astronomy 142 | Spring 2024
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