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)

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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

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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

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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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