The Milky Way Galaxy

Chapter 12:

The Milky Way Galaxy appears in our sky as a faint band of light.

Dusty gas

clouds obscure

our view

because they

absorb visible

light.

This is the

interstellar

medium that

makes new

star systems.

Determining the Structure

of the Milky Way

Galactic Plane

Galactic Center

The structure of our Milky Way is hard

to determine because:

1) we are inside.

2) distance measurements are difficult.

3) our view towards the center is

obscured by gas and dust.

Strategies to explore the

structure of our Milky Way

I. Trace the direction and distance to

bright objects in the Milky Way.

II. Observe objects at wavelengths other than visible

and catalogue their directions and distances.

III. Trace the orbital velocities of objects in the

Milky Way.

First Studies of the Galaxy

First attempt to unveil

the structure of the

Galaxy by William

Herschel (1785), based

on optical observations

The shape of the Milky was believed to resemble a

grindstone, with the sun close to the center.

Measuring Distances:

The Cepheid Method

Instability Strip

The more luminous a Cepheid Variable, the

longer its pulsation period.

Observing

the period

yields a

measure

of its

luminosity

and thus

its

distance!

The Cepheid Method

Allows one to

measure the

distances to

star clusters

throughout

the Milky

Way

Exploring the Galaxy Using

Clusters of Stars

Two types of clusters of stars:

1) Open clusters = young clusters

of recently formed stars; within the

disk of the Galaxy

Open clusters h

and c Persei

2) Globular clusters = old, centrally

concentrated clusters of stars; mostly

in a halo around the Galaxy Globular Cluster

M 19

Globular Clusters

• Dense clusters of 50,000 – a million stars

• Approx. 200 globular clusters in our Milky Way

• Old (~ 11 billion years), lower-main-sequence stars

Globular Cluster M80

Locating the Center of the Milky Way

Distribution of

globular clusters is

not centered on

the sun,

but on a location

which is heavily

obscured from direct

(visual) observation.

Radio Observations

21-cm radio observations reveal the distribution

of neutral hydrogen throughout the galaxy.

Distances to

hydrogen clouds

determined through

radial-velocity

measurements

(Doppler effect!)

Galactic

Center

Sun

Neutral

hydrogen

concentrated

in spiral arms

The Structure of the

Milky Way Revealed

Distribution of dust

Sun

Ring Bar

Distribution of stars

and neutral hydrogen

Our Milky

Galaxy is a

bar galaxy…

… and it’s

huge!

The Structure of the Milky Way

75,000 light years

Disk

Nuclear Bulge

Halo

Sun

Globular Clusters

Open Clusters,

O/B Associations

We see our galaxy edge-on.

Primary features: disk, bulge, halo, globular clusters

If we could view the Milky Way from above the

disk, we would see its spiral arms.

Infrared View of the Milky Way

Interstellar dust

(absorbing optical light)

emits mostly infrared.

Near infrared image

Infrared emission is

not strongly absorbed

and provides a clear

view throughout the

Milky Way.

Nuclear bulge

Galactic Plane

Far infrared image

Orbital Motions in the Milky Way

Disk stars:

Nearly circular

orbits in the disk

of the Galaxy

Halo stars:

Highly elliptical

orbits; randomly

oriented

Orbital Motions in the Milky Way

Differential Rotation Sun orbits around

Galactic center

with 220 km/s

1 orbit takes approx.

240 million years.

Stars closer to the

Galacic center

orbit faster.

Stars further out

orbit more slowly.

Mass determination

from orbital velocity:

The more mass there is

inside the orbit, the faster

the sun has to orbit

around the Galactic

center.

Combined mass:

M = 4 billion Msun M = 11 billion Msun M = 25 billion Msun M = 100 billion Msun M = 400 billion Msun

The Mass of the Milky Way If all mass was concentrated in the

center, rotation curve would follow a

modified version of Kepler’s 3 law.

Rotation Curve = orbital velocity

as function of radius.

The Mass of the Milky Way

Total mass in the disk

of the Milky Way:

Approx. 200 billion

solar masses

Additional mass in an

extended halo:

Total: Approx. 1 trillion

solar masses

Most of the mass is not

emitting any radiation:

Dark Matter!

Stellar Populations

Population I: Young stars:

metal rich; located in spiral

arms and disk

Population II: Old stars: metal

poor; located in the halo

(globular clusters) and

nuclear bulge

Metal Abundances in the Universe

Logarithmic Scale

All elements

heavier than He

are very rare.

Linear Scale

Metals in Stars Absorption lines almost exclusively from Hydrogen: Population II

Many absorption lines also from heavier elements (metals):

Population I

At the time of formation, the gasses forming the Milky Way consisted

exclusively of Hydrogen and Helium. Heavier Elements (“metals”)

were later only produced in stars.

=> Young stars

contain more

metals than older

stars.

Halo Stars:

0.02–0.2% heavy elements (O, Fe, …),

only old stars

Disk Stars:

2% heavy elements,

stars of all ages

Halo stars

formed first,

then stopped.

Disk stars formed

later, and kept

forming.

The History of the

Milky Way The traditional theory:

Gas cloud fragments into

smaller pieces, forming the

first, metal-poor stars

(pop. II).

The rotating cloud

collapses into a disk-like

structure.

Later populations of stars

(pop. I) are restricted to

the disk of the Galaxy.

Star Formation in Spiral Arms

Shock waves from supernovae, ionization fronts

initiated by O and B stars, and the shock fronts

forming spiral arms trigger star formation.

Spiral arms

are stationary

shock waves,

initiating star

formation.

Star Formation

in Spiral Arms

Spiral arms are basically

stationary shock waves.

Stars and gas clouds orbit

around the Galactic center

and cross spiral arms.

Shocks initiate star formation.

Star formation self-

sustaining through O/B

ionization fronts and

supernova shock waves.

Spiral arms are

waves of star

formation:

1. Gas clouds get

squeezed as

they move into

spiral arms.

2. The squeezing of

clouds triggers

star formation.

3. Young stars flow

out of spiral

arms.

The Nature of Spiral Arms

Chance coincidence of small spiral galaxy

in front of a large background galaxy

Spiral arms appear

bright (newly

formed, massive

stars!) against the

dark sky

background,

but dark (gas and

dust in dense, star-

forming clouds)

against the bright

background of the

large galaxy.

Exploring the structure of the Milky

Way with O/B Associations O/B Associations

Distances to O/B Associations can be

determined using Cepheid Variables.

O/B Associations trace out

3 spiral arms near the Sun.

Sun

Self-Sustained Star

Formation in Spiral Arms

Star forming regions

get elongated due to

differential rotation.

Star formation is self-sustaining, due to

ionization fronts and supernova shocks.

Star forming regions show

up bright in infrared (from

dense gas and dust).

Grand-Design Spiral Galaxies

Grand-Design Spirals have

two dominant spiral arms.

M 100

Flocculent (woolly)

galaxies also have spiral

patterns, but no dominant

pair of spiral arms.

NGC 300

The Galactic Center

Wide-angle optical view of the GC region

Galactic center

Our view (in visible light) towards the Galactic center

(GC) is heavily obscured by gas and dust:

Extinction by 30 magnitudes

Only 1 out of 1012 optical photons makes its

way from the GC towards Earth!

Radio View of the Galactic Center

Many supernova remnants;

shells and filaments

Sgr A

Arc

Sgr A*: The Center of our Galaxy

The Galactic Center contains a supermassive

black hole of approx. 3.7 million solar masses.

Sgr A