1. The Milky Way: A Historical Perspective

Before 1920's, Milky Way was believed to be the entire Universe

... and only about 10 kpc x 2 kpc in size

... with the Sun at the center

Three kinds of nebulae:

``emission-line'' nebulae

planetary nebulae

spiral nebulae

-- all thought to be Galactic!

The ``Great Debate:'' Shapley vs. Curtis, 1920

Why so wrong?

No method to measure distances beyond parallax and spectroscopic parallax

What we know today:

- ``spiral nebulae'' lie outside the Milky Way Galaxy

They are galaxies like the Milky Way

- there are other types of extra-galactic ``nebulae'' that represent different types of galaxies

- the Milky Way is roughly 30 kpc in size,

has a flattened disk of young and old stars

a round, centrally located bulge, primarily of old stars,

and a halo of exclusively made of old stars.

- the Sun is about 2/3 the way out from the center of the Milky Way in the disk.

Orientation of Solar System: tilted!

North Celestial Pole vs. North Galactic Pole . . .

. . . off by 60 degrees

2. The distance ladder revisited: Out to Mpc Scales

How did we make this leap in our understanding?

RR Lyrae

- Relatively low-mass, evolved stars

- On the Horizontal Branch

- 10² as luminous as the Sun

- The Sun will be a RR Lyrae in about 5 billion years

- Period and luminosity uncorrelated (``horizontal'')

- But narrow range of luminosity (``horizontal'')

- Plentiful (there are many low-mass stars)

Cepheids

- Massive, extremely luminous evolved stars

- Tomorrow's super novae - Type II

- 10⁴ as luminous as the Sun

- pulsation period and luminosity strongly correlated

Used to map out the galaxy and beyond.

Sun's distance from the Galactic center: 8 kpc

Rotation period (at 8 kpc): 225 Myr

Rotation speed: 210 km/s

But, galaxy disks do not have rigid rotation:       [target]

disks rotate differentially .

If all the mass were concentrated in the center of galaxies:

inner part of galaxies would rotate 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

1 /

.. decreasing with radius.

If rigid rotation:

speed = 2 a / P a

In reality:

speed is nearly constant (!) with a.

4. 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) telescopes were inadequate to observe them

(b) there was no available method to infer distances to these 'nebulae'

(c) these 'nebulae' had appearances identical to confirmed Galactic nebulae

(d) there was no philosophical precedence for believing in external galaxies

(e) The size of the Milky Way was unknown