The Milky WayOur Galaxy

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The Milky Way

Almost everything we see in the night sky belongs to the Milky Way.

We see the Milky Way as a faint band of light across the sky.

Based on this (our only) view of the Milky Way, what would you say the Milky Way would look like from a

position far outside the Milky Way?

1. A flat, round disk.

2. A flat, elliptical disk

3. A sphere.

4. A spiral with one arm wound around multiple times

5. A spiral with multiple arms.

The Structure of the Milky Way

Galactic Plane

Galactic Center

The actual structure of our Milky Way is very 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.

In order to get a good outside view of the Milky Way, you are trying to send a spacecraft high above the plane of the Milky

Way, to a height equal to our distance from the Galactic center. If you had a spacecraft that could travel at almost the

speed of light, how long would it take it to get there?

1) 2 months

2) 5 years

3) 250 years

4) 30,000 years

5) 5 million years

Sun

Galactic Center

The Sun is about

8.5 kpc = 8,500 pc

≈ 30,000 light years

from the Galactic center.

Sun

Galactic Center

=> No spacecraft will ever travel a

significant distance through or even out of the Milky Way.

The Voyager Spacecraft (launched in the 1970s) have just reached the

“edge” of the solar system!

Approx. 1,700 light yearsApprox. 1,700 light years

The Extended Solar Neighborhood

The Milky Way Galaxy

Diameter of the Milky Way: Diameter of the Milky Way: Approx. 75,000 light yearsApprox. 75,000 light years

1. Select bright objects that you can see throughout the Milky Way; trace their directions and distances.

2. Observe objects at wavelengths other than visible (not absorbed by gas and dust); note their directions

and distances.

3. Trace the orbital velocities of objects in different directions.

Viable ways to explore our Milky Way:

To trace out the structure of our Milky Way, you want to select the

brightest stars, which are …

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1. A stars.

2. G stars.

3. O stars.

4. M stars.

5. White dwarfs.

Remember: O and B stars are the most

massive, most luminous stars (unfortunately, also the shortest-lived ones).

=> Look for very young clusters or associations

containing O and B stars: O/B Associations!

Which method might astronomers use to measure the distances to

star clusters across the Milky Way?

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1. Light travel time measurements

2. Cepheid Variables

3. Trigonometric Parallax

4. Cosmological Redshift

5. Gravitational Redshift

Exploring the structure of the Milky Way with bright objects

O/B Associations

Distances to O/B Associations determined using Cepheid Variables

O/B Associations trace out 3 spiral arms near the Sun.

Sagittarius arm

Orion-Cygnus arm

Perseus arm

Sun

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, called Spiral Density 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 and B ionization fronts and supernova shock waves

The Nature of Spiral Arms

Spiral arms appear bright in visible light, from newly formed, massive stars,

Spiral arms are also bright in infrared, from dust.

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

The Whirlpool Galaxy

M 51 (Whirlpool Galaxy)

Self-sustaining star forming regions along spiral arm patterns are clearly visible

Why do the star-forming regions in this image of the Whirlpool Galaxy appear red?

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1. This is the red color of Hydrogen Balmer (H) emission from Hydrogen that has been ionized by young, massive, hot stars.

2. Their light is highly red-shifted because especially the star-forming regions are moving away from us at high speed.

3. This is the red color of interstellar dust that is present in the molecular clouds out of which stars are formed.

4. Star forming regions are red from the dominant red light of cool, low-mass stars.

5. They are red from the intensive Hydrogen burning that goes on in the newly formed stars.

Exploring the structure of the Milky Way (II)

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

Globular Cluster M53

How do we know that globular clusters are old?

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1. They contain many O and B stars.

2. Their light is highly red-shifted.

3. They are at very large distances, so they must have formed a long time ago.

4. The turn-off point in their Hertzsprung-Russell diagram is very low on the main sequence.

5. Stellar surfaces appear wrinkled.

Example:HR diagram of the star cluster M 55

High-mass stars evolved onto the

giant branch

Low-mass stars still on the main

sequence

Turn-off point

The Structure of the Milky Way Revealed

75,000 light years

Disk

Nuclear Bulge

HaloSun

Globular Clusters

Open Clusters, O/B Associations

Infrared View of the Milky WayInterstellar dust (absorbing optical light)

emits mostly infrared light.

Small and Large Magellanic Clouds: Small satellite galaxies of our Milky Way, 160,000 LY and 180,000 from

the center of the Milky Way.

Infrared View of the Milky Way

Radio View of the Milky Way

Radio map at a wavelength of 21 cm, tracing neutral hydrogen

Interstellar dust does not absorb radio waves

=> We can observe any direction throughout the Milky Way at radio waves.

The Milky Way Across the Electromagnetic Spectrum

Radio Waves

InfraredVisible light

X-rays-rays

The Structure of the Milky Way Revealed

Distribution of dust

Sun

RingBar

Distribution of stars and neutral hydrogen

When the Milky Way was formed, the gas contained almost exclusively H and He; the gas is enriched by heavier elements (“metals”) only

through supernovae. For this reason, …

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1. Older stars should be more metal-rich than younger ones.

2. There should be no difference in the metal content of old and young stars.

3. Younger stars should be more metal-rich than older ones.

4. The metal content of a star should depend primarily on its mass.

5. The metal content of a star should depend primarily on its temperature.

Stellar Populations

Population I: Young stars (less than 5 billion years):

metal-rich; located in spiral arms and disk

Population II: Old stars (more than 5 billion years): metal-poor;

located in the halo (globular clusters) and nuclear bulge

Younger stars are more metal-rich than older ones.

Orbits of Stars in the Milky Way

Population I (disk stars)

Population II (halo stars)

Orbital Motions in the Milky Way (II)

Differential Rotation

Sun orbits around Galactic center with 220 km/s

1 orbit takes approx. 240 million years.

What can we infer from the sun’s orbital period around the Galactic center

(knowing its distance from the GC)?

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1. The mass of the sun.

2. The diameter of the Milky Way.

3. The angular momentum of the Milky Way.

4. The shape of the Milky Way’s spiral arm structure.

5. The mass of the Milky Way contained inside the sun’s orbit.

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 MsunM = 11 billion MsunM = 25 billion MsunM = 100 billion MsunM = 400 billion Msun

Method similar to mass determination in binary systems

(variation of Kepler’s 3rd law):

The Mass of the Milky WayTotal 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!

Why do we not have a direct view (in visible light) to the Center of the

Milky Way?

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1. The Galactic Center does not emit visible light.

2. There are too many stars in the way that block our view to the Galactic Center.

3. The Galactic Center is always hidden behind the sun.

4. The Milky Way does not have a center.

5. There is too much dust and gas in the way that blocks our view to the Galactic Center.

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. 4 million solar masses.

Sgr A

The Black Hole in the Center of the Milky Way

By following the orbits of individual stars near the center of the Milky Way, the mass of the central

black hole could be determined to ~ 4 million

solar masses.

The Black Hole in the Galactic Center

Black Hole with 4 million Msun

What keeps the stars in the disk of the Milky Way on their orbits around the

Galactic center?

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1. The angular momentum of the Milky Way.

2. The gravitational attraction of the stars near the Galactic center.

3. The gravitational attraction of the supermassive black hole in the Galactic center.

4. The gravitational attraction of the stars in the halo of the Milky Way.

5. The gravitational attraction of all mass (stars, gas, dark matter) in and around the Galactic center.

The gravitational attraction of all mass (stars, gas, dark matter) inside the respective orbit

keeps the stars in their orbits.

Compare 4 million solar masses (supermassive BH in the Galactic center) to

the 1 trillion masses of the entire Galaxy!

The mass of the supermassive BH in the Galactic center makes up only a tiny fraction

of the total mass of the Milky Way!

X-Ray View of the Galactic Center

Chandra X-ray image of Sgr A*

Supermassive black hole in the Galactic center is unusually faint in X-rays,

compared to those in other galaxies.

Galactic center region contains many black-hole and neutron-star X-ray binaries.

History of the Milky Way

The traditional theory:

Quasi-spherical gas cloud fragments into smaller pieces, forming the first,

metal-poor stars (pop. II);

Rotating cloud collapses into a disk-like structure

Later populations of stars (pop. I) are restricted to the disk of the Galaxy