3. Motions in the Disk

Sun's distance from the Galactic center: 8 kpc

Rotation period (at 8 kpc): 225 Myr

Rotation speed: 210 km/s

But, galaxies' disks have differential rotation:

inner part of galaxy rotates faster than outer parts

Analogy to solar system:

inner planets rotate faster than outer planets around Sun; not just shorter period, but faster speed!

Just Kepler's 3rd law:

Period (P)² = semi-major axis (a)³

(where P and a are measured in suitable units)

In general:

speed = distance / time

Here:

time = Period

distance = 2 a (circumference of orbit)

So

speed = 2 a / P

= a / a^3/2

= a^-1/2

= /

4. Spiral Arms

Problem: Why don't spiral arms wind up and disappear?

Spiral arms are density waves in galactic disks, moving at different speeds than stars and gas.

Density waves travel

faster in outer regions,

slower in inner regions

relative to rotation of underlying disk.

Density waves move through the disk in a spiral pattern.

These "waves" produce star formation in their wake by collapsing gas

Spiral arms appear as regions of recent star formation in the disks of galaxies.

The waves are an 'instability' in the disk.

Imagine throwing a pebble into a pool of still water, producing ripples.

Next imagine the same, but with the water swirling.

You will get something like a spiral pattern

Neighboring satellite galaxies (like pebbles) may help

produce these instabilities.

5. The Smooth Component

With the exception of globular clusters,

old stars lie in a smooth distribution,

either in the disk or halo of our galaxy

(and other galaxies).

Why is the distribution smooth?

- Dust and gas are sticky (they "dissipate")

(dust and gas particles tend to collide and stick together)

- Stars are not sticky ("collisionless")

(stars rarely collide unless very close)

After star formation occurs in a "sticky" environment

i.e. clumped, and unsmooth distribution,

the stars diffuse, and "relax" into a smoother distribution,

i.e. the stars randomize their motions and positions.

6. What lies at the center?

Sagittarius A - strongest radio source in Sagittarius

Optically obscured by dust

Two ways to get a handle on the center of the Milky Way:

(1) look at external galaxies' nuclei

problem: galaxy nuclei come in many varieties even for galaxies which look similar at larger radii

(2) peer directly into center at other wavelengths:

radio

infrared

Moving inward:

super metal-rich stars (1 kpc)

a region of extremely high stellar density (100-500 pc)

a region of extremely hot gas (10-100 pc)

gas motions imply a massive, dense object 10⁶ M

A central black hole of massive proportions?

massive: yes, compared to individual stars

massive: no, compared to other galaxy nuclei

probably 'normal'

(a) age

(b) presence of red giants

(c) metalicity

(d) kinematics

(e) spatial distribution

(a) they hold the stars together in a spiral pattern.

(b) they shepherd the stars into spiral patterns.

(c) they have nothing to do with producing spiral arms.

(d) they induce star formation in spiral patterns.

(e) they transport bulge stars into the disk.