Chapter 14: Our Galaxy, the Milky Way

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14.1 The Milky Way Revealed Learning goals • What is the structure of our galaxy? • How do stars orbit in our galaxy?

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In the night sky, the Milky Way appears as a faint band of light.

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Dusty gas clouds obscure our view because they absorb visible light. This is the interstellar medium that makes new star systems. 
 It comprises clouds of hydrogen gas (atomic & molecular) and dust. 4/15/2009

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All-Sky View of the Milky Way

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Size of the Milky Way (side view) •  •  •  • 

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Diameter ~ 100,000 light years Thickness ~ 1,000 light years (flatter than a CD !) Distance from Sun to center ~ 30,000 light years About 100 billion stars in total.

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Stellar components of the Milky Way 1.  2.  3. 

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Disk: rotating, thin collection of stars, gas & dust. Halo: tenuous outer sphere of stars & globular clusters, and very little gas. Bulge: spherical concentration of stars near the center

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Size of the Milky Way (side view) •  •  •  • 

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Diameter ~ 100,000 light years Thickness ~ 1,000 light years (flatter than a CD !) Distance from Sun to center ~ 30,000 light years About 100 billion stars in total.

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Stellar components of the Milky Way 1.  2. 

3. 

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Disk: rotating, thin collection of stars, gas & dust. Halo: tenuous outer sphere of stars & globular clusters, 
 and very little gas. Bulge: spherical concentration of stars near the center

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

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

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Another spiral galaxy seen edge-on

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Another spiral galaxy seen face-on

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Stellar Orbits: Stars in the Galactic Disk •  Disk stars all orbit in the same direction of rotation, 
 with a small amount of vertical (up-and-down) motion. –  Rotation due to angular momentum from the galaxyʼs formation. –  Vertical motion due to gravitational attraction of the disk stars.

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Stellar Orbits: Galactic Halo & Bulge •  Stars in the halo & bulge also orbit the center of the galaxy. •  But their orbits have random orientations, w/o any overall sense of rotation. 4/15/2009

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How do we measure the mass of the Galaxy? •  Sunʼs orbital motion (radius & velocity) tell us the mass inside Sunʼs orbit: 
 ~1.0 x 1011 Msun. •  Cannot measure the mass outside of the Sunʼs orbit in this fashion.

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Orbital velocity law Mr = r v2 / G Take v = 220 km/s: orbital velocity of Sun around center of galaxy r = 28,000 ly: orbital radius  Mr = 1.9 1041 kg  Mr/MS = 1011

Similar calculations of orbits of distant stars  most of galaxyʼs mass is far from center and distributed throughout halo. But since donʼt see emission  dark matter (otherwise stars far away would have v decreasing with distance like planets) 4/15/2009

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Whatʼs the Milky Way got to do with us?

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It holds onto the gas and allows new stars to form 
 from recycled (and enriched) material Habbal Astro 110-01 Chapter 14 Lecture 31

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How does our galaxy form stars? •  Recycles gas from old stars into new stars.
 •  With each cycle, more heavy elements are made by nuclear fusion in stars. •  “Star-gas-star cycle”

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Star-gas-star 
 cycle •  Recycles gas from old stars into new stars. •  With each cycle, more heavy elements are made by nuclear fusion in stars.

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Gas Cools

Summary of Galactic Recycling

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•  Stars make new heavy elements by fusion. •  Dying stars expel gas and new elements, producing hot bubbles of gas (~106 K). These emit X-rays. •  This hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K). This hydrogen emits at 21-cm wavelength emission line. •  Further cooling permits molecules (CO, etc) to form, making molecular clouds (~30 K). CO emits an emission line spectrum at 3 mm. •  Gravity forms new stars (and planets) in molecular clouds. Process starts over. Habbal Astro 110-01 Chapter 14 Lecture 31

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Effect of low-mass stars on the interstellar medium •  Low-mass stars eject gas through their (very small) stellar winds and mass loss during the planetary nebula phase. •  Overall, these have much less effect on the ISM than high mass stars. 4/15/2009

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Effect of high-mass stars on the interstellar medium •  During their lives, high-mass stars have strong stellar winds that blow bubbles of hot gas. •  High mass stars die as supernovae, injecting heavy elements into the interstellar medium.

10 light-years •  Have a very strong effect on the ISM. 4/15/2009

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Supernova remnants: Xrays

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20 light years Habbal Astro 110-01 Chapter 14 Lecture 31

Supernova remnants are filled with hot gas (~106 K), which emit thermal radiation at 
 mostly X-ray wavelengths.

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Recap: Learning from Light •  How does light tell us what things are made of?
 Every kind of atom, ion, and molecule produces a unique set of spectral lines, seen in emission or absorption spectra.

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•  How does light tell us the temperatures of dense objects?
 We can determine temperature from the (continuous) spectrum 
 of thermal radiation.

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Supernova remnants •  The gas of the supernova remnant expands and cools. •  Begins to emit visible light, mostly emission line spectra.

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Supernova remnants •  The gas of the supernova remnant expands and cools. •  Begins to emit visible light as emission line spectra. •  These spectra show heavy elements (O, Ne, N, S) made by the star, which are distributed back into the ISM.

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SN superbubbles

•  Multiple supernovae can create huge bubbles of hot gas, which blow out of the galactic disk. •  Gas clouds cooling in the halo can rain back down onto the disk. 
 •  These collisions may trigger future star formation.

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Atomic hydrogen in the ISM •  As the hot gas cools, electrons combine with protons to form clouds of atomic hydrogen (H). •  Hydrogen produces an emission line at 21cm wavelength (in the radio). Can use this to map the spatial distribution.

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Radio (21 cm)

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Molecular hydrogen in the ISM •  Atomic hydrogen clouds slowly contract & cool further. •  Once they get cold & dense enough, the single H atoms combine to form molecular hydrogen (H2) clouds.

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Optical image

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Molecular clouds Composition: •  •  •  • 

Mostly H2 About 28% He About 1% CO Many other molecules.

Unlike atomic hydrogen (H), molecular hydrogen (H2) 
 is very hard to detect, as it emits very weak radiation. Detect molecular clouds from 3-mm emission line of CO (a trace constituent by mass). 4/15/2009

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Molecular clouds collapse due to gravity to form new stars, thereby completing the star-gas-star cycle.

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Star formation in molecular clouds

•  Young massive stars can erode the birth clouds, preventing further star formation. •  Only a small fraction of gas in molecular clouds forms into stars.

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Gas Cools

Summary of Galactic Recycling

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•  Stars make new heavy elements by fusion. •  Dying stars expel gas and new elements, producing hot bubbles of gas (~106 K). These emit Xrays. •  This hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K). This hydrogen emits at 21-cm wavelength emission line. •  Further cooling permits molecules (CO, etc) to form, making molecular clouds (~30 K). CO emits an emission line spectrum at 3 mm. •  Gravity forms new stars (and planets) in molecular clouds. Process starts over. Habbal Astro 110-01 Chapter 14 Lecture 31

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QUESTION: Where will our 
 Galaxyʼs gas be in 1 trillion years from now?


A. Blown out of galaxy

B. Still recycling just like now

C. Locked into white dwarfs and lowmass stars

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QUESTION: Where will our 
 Galaxyʼs gas be in 1 trillion years from now?


A. Blown out of galaxy

B. Still recycling just like now

C. Locked into white dwarfs and lowmass stars

Galactic recycling is an imperfect process. 
 More and more gas gets locked up into 
 low-mass stars and white dwarfs, which never return their material to the interstellar medium. 4/15/2009

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We observe star-gas-star cycle operating in the Milky Wayʼs disk using many different wavelengths of light.

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Infrared

Visible

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Infrared light reveals stars whose visible light is 
 blocked by clouds of gas & dust. Habbal Astro 110-01 Chapter 14 Lecture 31

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X-rays

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X-rays are observed from hot gas above and 
 below the Milky Wayʼs disk. Habbal Astro 110-01 Chapter 14 Lecture 31

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Radio (21cm)

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

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Radio (3 mm)

3-mm radio waves from carbon monoxide (CO) show 
 locations of molecular clouds.

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Far-IR (dust)

Long-wavelength infrared emission shows where 
 young stars have heated dust grains.

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Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds 4/15/2009 Habbal Astro 110-01 Chapter 14 Lecture 31

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Where do stars tend to form in our galaxy?

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Much of the star formation in disk galaxies happens in the spiral arms. Ionization Nebulae Blue (massive) stars Dusty Gas Clouds

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Whirlpool Galaxy

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Ionization nebulae •  Regions of ionized gas •  Found around short-lived high-mass stars and signify active star formation. •  The blue light of the massive stars is scattered by nearby dust clouds. •  The nebulae tend to appear reddish, b/c of strong emission lines at these wavelengths. 4/15/2009

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Reflection nebulae are dusty gas clouds which scatter the light from stars.

•  Why do reflection nebulae look bluer than the nearby stars? For the same reason our sky is blue, and sunsets are red. •  Blue light is preferentially scattered by gas molecules and small dust particle. 4/15/2009

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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.

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