Ch. 14. The Milky Way

Ch. 14

Ch. 14 OUTLINEShorter than book

• 14.1 The Milky Way Revealed

• 14.2 Galactic Recycling (closely related to Ch. 13)

• 14.3 The History of the Milky Way

• 14.4 The Mysterious Galactic Center

14.1 The Milky Way Revealed

• Our Goals for Learning (not exactly like book)

• What does our galaxy look like?

• Where do stars form in our galaxy?

What does our galaxy look like?

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

All-Sky View at visible wavelengths plotted in galactic coordinates

What if we could see through most of that dust?

All-Sky View at visible wavelengths

All-Sky View at infrared wavelengths

Remember Extinction and Reddening: interstellar dust will make stars look fainter and redder. Dust will affect more the shorter (bluer) wavelengths and less the longer (redder) wavelengths. By looking at infrared wavelengths we can see through most of the dust.

We see our galaxy edge-on

Primary features: disk, bulge, halo, globular clusters

The Shape of our Galaxy: a flattened disk

If we could view the Milky Way from above the disk, we would see its spiral arms

Andromeda Galaxy: our twin galaxy

Our Galaxy looks like Andromeda

How do we know what our galaxy would look like if viewed from the top? Infrared and Radio observations penetrate dark interstellar clouds

Stellar Populations• Turns out that there are two types of stars in

the Galaxy– Population I: Relatively young. Similar to the

Sun. Tend to be in the galactic disk. Richer in heavy elements

– Population II: Few heavy elements, very old (12-14 billion years), tend to be in the center of the galaxy or in globular clusters

Remember CH 11 Part III

Star Clusters:

1. Two types of clusters: Open and Globular

Two types of star clusters• Open clusters:

young, contain up to several thousand stars and are found in the disk of the galaxy (Population I).

• Globular clusters: old, contain hundreds of thousands of stars, all closely packed together. They are found mainly in the halo of the galaxy (Population II).

14.2 Galactic Recycling

• Our Goals for Learning• How does our galaxy recycle gas into stars?

• Where do stars tend to form in our galaxy?

How does our galaxy recycle gas into stars?

Star-gas-star cycle

Recycles gas from old stars into new star systems

14.2 Galactic Recycling

• Where do stars tend to form in our galaxy?

14.2 Galactic Recycling

• Where do stars tend to form in our galaxy? In the Disk

Question 4What happens after an interstellar cloud of gas

and dust is compressed and collapses:

A. It will heat and contract

B. If it gets hot enough (10 million K) it can produce energy through hydrogen fusion

C. It can produce main sequence stars

Question 4What happens after an interstellar cloud of gas

and dust is compressed and collapses:

A. It will heat and contract

B. If it gets hot enough (10 million K) it can produce energy through hydrogen fusion

C. It can produce main sequence stars

D. All of the above

How does our galaxy recycle gas into stars?

Cycle of Birth and Deaths of Stars• Interstellar cloud of gas and dust is

compressed and collapses to form stars

• After leaving the main sequence red giants eject their outer layers back to the interstellar medium

• Supernovas explode and eject their outer layers back to the interstellar medium

• Supernova explosions and other events can compress an interstellar cloud of gas and dust that collapses to form stars ………..

Remember the Sun’s Evolutionary Process

Remember mass loss in Intermediate Mass Stars

Remember Supernova explosions

Star-gas-star cycle

Recycles gas from old stars into new star systems

Disk: blue stars star formation

Halo: no blue stars no star formation

Disk: emission nebulae, blue stars star formation

Halo: No emission nebulae, no blue stars no star formation

Much of star formation in disk happens in spiral arms

The Whirlpool Galaxy

Emission NebulaeBlue StarsGas Clouds

Spiral arms are waves of star formation

14.3 The History of the Milky Way

• Our Goals for Learning

• What clues to our galaxy’s history do halo stars hold?

• How did our galaxy form?

What clues to our galaxy’s history do halo stars hold?

Halo Stars: 0.02-0.2% heavy elements (O, Fe, …), only old stars

Disk Stars: 2% heavy elements, stars of all ages

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

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, kept forming

How did our galaxy form?

Our galaxy probably formed from a giant gas cloud

Halo stars formed first as gravity caused cloud to contract

Remaining gas settled into spinning disk

Stars continuously form in disk as galaxy grows older

Note: This model is oversimplified

What have we learned?• What clues to our galaxy’s history do halo

stars hold?• The halo generally contains only old, low-mass

stars with a much smaller proportion of heavy elements than stars in the disk. Thus, halo stars must have formed early in the galaxy’s history, before the gas settled into a disk.

What have we learned?• How did our galaxy form?

• The galaxy probably began as a huge blob of gas called a protogalactic cloud. Gravity caused the cloud to shrink in size, and conservation of angular momentum caused the gas to form the spinning disk of our galaxy. Stars in the halo formed before the gas finished collapsing into the disk.

14.4 The Mysterious Galactic Center

• Our Goals for Learning• What lies in the center of our galaxy?

What lies in the center of our galaxy?

Stars at galactic centerStrange radio sources in galactic center

Stars appear to be orbiting something massive but invisible … a black hole!

Orbits of stars indicate a mass of about 4 million MSun

What have we learned?

• What lies in the center of our galaxy?

• Motions of stars near the center of our galaxy suggest that it contains a black hole about 4 million times as massive as the Sun. The black hole appears to be powering a bright source of radio emission.