Chapter 14Our Galaxy

14.1 The Milky Way Revealed

• Our Goals for Learning

• What does our galaxy look like?

• How do stars orbit in our galaxy?

Stars and planets are part of galactic ecosystem, within which the stars and planets are born, and recycled.

What does our galaxy look like?

The Milky Way galaxy appears in our sky as a faint band of light – to old Greeks it resembled a flowing ribbon of milk (galactos).

Dusty gas clouds obscure our view because they absorb visible light

This is the interstellar medium that makes new star systems

The dusty smoglike interstellar medium hides most of our Galaxy from us.

All-Sky View

We see our galaxy edge-on.

Primary features: disk, bulge, halo, globular clusters (more than a million of stars densely packed in about 100 ly across).

How it would look like if would observe our galaxy from above?

Milky Way is a spiral Galaxy: if we could view it from above the disk, we would see its spiral arms. We know this thank to the edge-on observations and observations of other Galaxies.

Milky way has also 4 smaller galaxy companions:

Large and small magellanic cloud are two small galaxies which orbit Milky way. They are visible only from the southern hemisphere.

Sagittarius Dwarf and Canis Major Dwarf are each in a process of colliding with the Milky Way disk, and will be ultimately ripped apart by our own Galaxies strong gravitational field.

How do stars orbit in our galaxy?

Stars in the disk all orbit in the same direction with a little up-and-down motion. Like on a merry go round.

For stars at sun’s distance it takes about 200 million years to finish one orbit.

It takes about tens of millions of years for each up and down bob.

Thought Question

Why do orbits of bulge stars bob up and down?

A. They’re stuck to interstellar mediumB. Gravity of disk stars pulls toward diskC. Halo stars knock them back into disk

Thought Question

Why do orbits of bulge stars bob up and down?

A. They’re stuck to interstellar mediumB. Gravity of disk stars pulls toward diskC. Halo stars knock them back into disk

There is one important difference between a merry go round and the rotation within a galaxy: the stars in a galaxy all travel with the same speeds, no matter how far from the center they are (on a merry go round horses near the edge travel much faster).

This is connected to the issue of Dark Matter.

Orbits of stars in the bulge and halo have random orientations

Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit:

1.0 x 1011 Msun

This is essentially using Newton’s version of Kepler’s third law. What is this law? Why it gives us the mass of the Galaxy only within the Sun’s orbit?

What have we learned?• What does our galaxy look

like?• The Milky Way Galaxy

consists of a thin disk about 100,000 light-years in diameter with a central bulge and a spherical region called the halo that surrounds the entire disk. The disk contains the gas and dust of the interstellar medium, while the halo contains very little gas.

What have we learned?• How do stars orbit in our

galaxy?• Stars in the disk all orbit

the galactic center in about the same plane and in the same direction. Halo and bulge stars also orbit the center of the galaxy, but their orbits are randomly inclined to the disk of the galaxy.

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?

Generations of stars continually recycle the same galactic material through their cores, gradually raising the overall abundance of elements made by fusion.

How does our galaxy recycle gas into stars?

Star-gas-star cycle

Recycles gas from old stars into new star systems

1) Stars are born in molecular

clouds…2) They shine and produce heavier elements in nuclear fusion…

3) They return much of their content into interstellar medium through stellar winds and when they explode…

High-mass stars have strong stellar winds that blow bubbles of hot gas. It glows where gas pikes up as the bubble sweeps through the interstellar medium.

How do high mass stars end their lives?

Lower mass stars return gas to interstellar space through stellar winds and planetary nebulae. Stellar winds are less strong in low mass stars. Example: by the time sun dies and forms planetary nebula, it will loose about half its mass in stellar winds.

The bubbles created by supernova have one more effect on interstellar medium, they produce shock waves – waves of pressure that move faster than speed of sound. They create a wall of fast moving gas on its leading edge (blue region on the picture, 20 million degree gas).

Supernova remnants are aftermaths of these shock waves.

X-rays from hot gas in supernova remnants.

Supernova remnant cools and begins to emit visible light as it expands

New elements made by supernova mix into interstellar medium.

What kind of spectra is this?

It is not easy to stop the gas from exploding stars: after supernova explosion, ionized gas flies out at speeds of several thousands kilometers – much faster than the escape velocity from the galaxy. What is stopping it from leaving our Galaxy?

Multiple supernovae create huge hot bubbles that can blow out of disk.

Gas clouds cooling in the halo can rain back down on disk.

Initially we have an ionized gas, …Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons. It has the usual composition: …

Cooling and cloud formation:

Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons. It has the usual composition: … 70% H, 28% He, 2% heavy elements (half of this are dust grains, C and Si minerals which prevent us from seeing through the disk of Galaxy).

Atomic hydrogen emits a spectral line with a wavelength of 21 cm. What part of spectra does it belong?

Cooling and cloud formation:

Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons. It has the usual composition: … 70% H, 28% He, 2% heavy elements.

Atomic hydrogen emits a spectral line with a wavelength of 21 cm. What part of spectra does it belong? Radio observation of the sky show that this 21 cm line comes from all directions: atomic H is distributed throughout the galactic disk. Gravity slowly draws blobs of this gas together, and the gas becomes cooler and denser.

Cooling and cloud formation:

Molecular clouds form next, after gas cools enough to allow to atoms to combine into molecules.Molecular clouds are dense and heavy compared to the rest of interstellar gas and therefore they tend to settle in the central region of Milky Way’s disk (the dark lanes running through the luminous band of light, when we look at the Milky Way)

Cooling and cloud formation:

Molecular clouds in Orion

Composition:• Mostly H2

• About 28% He• About 1% CO• Many other molecules: H2O, NH3, C2H5OH (ethyl alcohol)

Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycle.

Radiation from newly formed stars is eroding these star-forming clouds. It ionizes and heats up molecular cloud – matter evaporates and joins hotter ionized gas encircling molecular clouds. Stars are formed only in dense knots which remain compact.

Summary of Galactic Recycling

• Stars make new elements by fusion• Dying stars expel gas and new elements,

producing hot bubbles (~106 K)• Hot gas cools, allowing atomic hydrogen clouds

to form (~100-10,000 K)• Further cooling permits molecules to form,

making molecular clouds (~30 K)• Gravity forms new stars (and planets) in

molecular clouds

Gas

Coo

ls

Thought Question

Where will the gas be in 1 trillion years?

A. Blown out of galaxyB. Still recycling just like nowC. Locked into brown dwarfs and in stellar corpses (white dwarfs, neutron stars, black holes)

Thought Question

Where will the gas be in 1 trillion years?

A. Blown out of galaxyB. Still recycling just like nowC. Locked into brown dwarfs and in stellar corpses (white dwarfs, neutron stars, black holes)

We observe star-gas-star cycle operating in Milky Way’s disk using many different wavelengths of light

Infrared light reveals stars whose visible light is blocked by gas clouds

Infrared

Visible

X-rays are observed from hot gas above and below the Milky Way’s disk

X-rays

21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk

Radio (21cm)

Radio waves from carbon monoxide (CO) show locations of molecular clouds – concentrated more in the midplane of the disk

Radio (CO)

Long-wavelength infrared emission shows where young stars are heating dust grains – density of dust is very similar to that of the molecular clouds

IR(dust)

Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds – follows locations of atomic and molecular clouds.

Where do stars tend to form in our galaxy?

Ionization nebulae are found around short-lived high-mass stars, signifying active star formation

They look reddish because when electrons fall from 3 to 2nd energy level in H the emit red light! Transition in other elements produces different colors…

Reflection nebulae - starlight reflected from dust grains.

Why do reflection nebulae look bluer than the nearby stars?

Reflection nebulae - starlight reflected from dust grains.

Why do reflection nebulae look bluer than the nearby stars?

For the same reason that our sky is blue! (interstellar dust grains scatter blue light much more readily than the red light)

What kinds of nebulae do you see?

Disk: Ionization nebulae, blue stars star formation

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

Much of star formation in disk happens in spiral arms

Whirlpool Galaxy

Much of star formation in disk happens in spiral arms

Whirlpool Galaxy

Ionization NebulaeBlue StarsGas Clouds

What are the spiral arms?

Do they rotate around the galaxy center? (Hint: are stars closer to the center, faster or slower from stars farther away?)

What are the spiral arms?

Do they move with the stars around the galaxy center? (Hint: are stars closer to the center, faster or slower from stars farther away?)

Spiral arms are places in Galaxy’s disk where stars and gas clouds get more densely packed. To visualize how spiral density waves propagate through the disk, think about traffic behind a slow tractor on a highway…

Spiral arms are waves of star formation

1. Gas clouds get squeezed as they move into spiral arms

2. Squeezing of clouds triggers star formation

3. Young stars flow out of spiral arms

What have we learned?

• How does our galaxy recycle gas into stars?• Stars are born from the gravitational collapse of gas clumps in

molecular clouds. Near the ends of their lives, stars more massive than our Sun create elements heavier than hydrogen and helium and expel them into space via supernovae and stellar winds. The supernovae and winds create hot bubbles in the interstellar medium, but the gas within these bubbles gradually slows and cools as they expand. Eventually, this gas cools enough to collect into clouds of atomic hydrogen. Further cooling allows atoms of hydrogen and other elements to collect into molecules, producing molecular clouds.These molecular clouds then form stars, completing the star–gas–star cycle.

What have we learned?

• Where do stars tend to form in our galaxy?

• Active star-forming regions, marked by the presence of hot, massive stars and ionization nebulae, are found preferentially in spiral arms. The spiral arms represent regions where a spiral density wave has caused gas clouds to crash into each other, thereby compressing them and making star formation more likely.

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

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

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

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

Halo stars formed first, then stopped

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

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

Halo stars formed first, then stopped

Disk stars formed later, kept forming

How does the halo of our Galaxy resemble the distant future that of the galactic disk?

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

Stars continuously form in disk as galaxy grows older

Warning: This model is oversimplified

The problem with this model:

If the Milky Way had formed from a single protogalactic cloud, it would have steadily accumulated heavy elements during its inward collapse as stars formed and exploded within it. In that case, the outermost stars should be the most deficient in heavy elements. Instead, we measure that the percentage of heavy elements does not depend on the distance to the center.

Detailed studies: several smaller gas clouds, already bearing some stars may have merged to form protogalactic cloud. These stars remained in the halo while the gas settled into the Milky Way’s disk.

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?

We can not see the galactic center in visible light (why?), but radio, infrared and X-ray telescopes are able to observe the center of our galaxy.

Galactic center in infrared light

Galactic center in infrared light Galactic center in radio

Swirling clouds of gas and a cluster of several million stars.

In a very center we find a strange source of radio emission called Sagittarius A*

Galactic center in radio

Several hundred stars crowd the region within about 1 ly of Sgr A*.

Strange radio source 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 all packed into a region of space just a little larger than our solar system.

Stars are orbiting within few light hours from the center.

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 3 to 4 million times as massive as the Sun. The black hole appears to be powering a bright source of radio emission known as Sgr A*.

If we could see our own galaxy from 2 million light-years away, it would appear _________.

  A. to be a flattened disk with a central bulge and spiral arms

  B. to fill the sky with widely spaced stars

  C. like a single, dim star

  D. as a faintly glowing band of light stretching all the way around the sky

  A. I

  B. II

  C. III

  D. IV

This diagram represents an edge-on view of our Milky Way Galaxy. Of the four labeled stars, which one is located in what we call the halo of the galaxy?  

How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?

  A. It should be higher in mass.

  B. It should orbit the galactic center at a much higher rate of speed.

  C. It should have a higher fraction of elements heavier than hydrogen and helium.

  D. It should be much brighter.

  How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?

  A. Thanks to the recycling of the star-gas-star cycle, the interstellar medium should look about the same in 50 billion years as it does today.

  B. The total amount of gas will be much less than it is today.

  C. The total amount of gas will be about the same, but it will contain a much higher percentage of elements heavier than hydrogen and helium.

  D. The total amount of gas will be much greater, since many stars will undergo supernovae between now and then.

Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?

  A. The Hubble Space Telescope

  B. The SOFIA airborne infrared observatory

  C. The Chandra X-ray Observatory

  D. The Keck I telescope on the summit of Mauna Kea

  A. The movie would alternate back and forth between being very bright when there is a lot of gas and very dark when there is very little gas.

  B. Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles.

  C. The entire disk of the Milky Way would pulsate in and out as it contracts to form stars and then blows out in supernovae and then contracts to form stars again and so on.

  D. Gas that changes only in very slow and steady ways, so that the movie would in fact be quite boring.

If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?

Why are spiral arms bright?

  A. Because they are the only places where we find stars within the disk of the galaxy

  B. Because they contain more hot young stars than other parts of the disk

  C. Because they contain more molecular clouds than other parts of the disk

  D. Because they contain far more stars than other parts of the galactic disk