February 28, 2006 Astronomy 2010 1

Chapter 24: The Chapter 24: The Milky Way GalaxyMilky Way Galaxy

M51 Whirlpool Galaxy31 million LY distant30,000 LY across

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How Do We Determine the Shape How Do We Determine the Shape of the Galaxy?of the Galaxy?

We are in the galaxy and can’t easily We are in the galaxy and can’t easily step back to see what it looks like.step back to see what it looks like.

William Herschel: measure the William Herschel: measure the distance and direction to stars and distance and direction to stars and make a map.make a map.• Dust obscures view of much of the galaxy.Dust obscures view of much of the galaxy.

Harlow Shapley: measure the distance Harlow Shapley: measure the distance and direction to globular clusters and direction to globular clusters scattered above and below the galaxyscattered above and below the galaxy• away from the dustaway from the dust

Modern: use radio and infrared lightModern: use radio and infrared light

Mosaic of 51 wide-angle photographs. Made over a three year period from locations in California (USA), South Africa, and Germany, the individual pictures were digitized and stitched together to create an apparently seamless 360 by 180 degree view.

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NGC 4103, 55 million LY awayNGC 4103, 55 million LY away edge on spiral galaxyedge on spiral galaxy dark dust band 500 LY thickdark dust band 500 LY thick

side top

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center of galaxycenter of galaxy

obscured by dustobscured by dust

Our Milky Way GalaxyOur Milky Way Galaxy

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The Milky Way GalaxyThe Milky Way Galaxy stars grouped in stars grouped in galaxiesgalaxies our galaxy: our galaxy: the Milky Waythe Milky Way

• roughly disk shaped, 100,000 LY diameterroughly disk shaped, 100,000 LY diameter• 1000 LY thick 1000 LY thick

central spherical central spherical nuclear bulgenuclear bulge, , 4 major 4 major spiral armsspiral arms plus smaller “spurs” plus smaller “spurs”

• arms: Cygnus, Perseus, Sagittarius-Carina arms: Cygnus, Perseus, Sagittarius-Carina • fourth unnamed arm – hard to see (on other side of fourth unnamed arm – hard to see (on other side of bulge)bulge)

• 80,000 LY long 80,000 LY long Sun in short spur – the Orion arm – between Sun in short spur – the Orion arm – between Perseus and Carina Perseus and Carina • 15,000 LY long15,000 LY long• also contains Orion Nebulaalso contains Orion Nebula• Sun in middle of disk, 70 LY from central planeSun in middle of disk, 70 LY from central plane

Spiral ArmsSpiral Arms hot blue stars delineate spiral structure – like Xmas lights on a tree

arms: regions where gas and dust more densely concentrated

need dust to replenish short lived blue stars

cool orange and red stars found in and between spiral arms

interstellar dust limits our view in visible light to dashed circle

Why Spiral Why Spiral Arms?Arms?

stars orbit around center of mass of galaxy – like planets

Kepler’s Laws:• closer in –

faster • farther out –

slower differential

rotation of stars explains curved shape of spiral arms

Rotation SpeedsRotation Speeds• Inner PartsInner Parts: Rise from Zero to few 100 km/sec : Rise from Zero to few 100 km/sec • Outer PartsOuter Parts: Nearly constant at a few 100 : Nearly constant at a few 100

km/sec km/sec Sun has VSun has Vrotrot=220 km/sec at R=8.5 kpc =220 km/sec at R=8.5 kpc Orbital Period: 240 Myr Orbital Period: 240 Myr

Rotation Rotation Spiral ArmsSpiral Arms

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Spiral pattern for billions of Spiral pattern for billions of years?years?

Spiral density wavesSpiral density waves

Spiral Density Waves: Traffic JamSpiral Density Waves: Traffic Jam

Spherical StructureSpherical Structure

Nuclear BulgeNuclear Bulge• Many RR Lyrae Many RR Lyrae

starsstars• A little gas & A little gas &

dust dust

Galactic Halo:Galactic Halo: outer sphere with outer sphere with very few stars very few stars • Old metal-poor Old metal-poor

stars stars • Globular clusters Globular clusters • dark matterdark matter• RR Lyrae Stars RR Lyrae Stars

RR Lyrae starsRR Lyrae stars• pulsate like pulsate like

Cepheid VariablesCepheid Variables• distancedistance scalescale

Galactic CenterGalactic Center

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Galaxy Galaxy Mapping with Mapping with Radio BandRadio Band radio is best radio is best for mapping the for mapping the distribution of distribution of hydrogen in the hydrogen in the galaxy galaxy

most of the most of the hydrogen gas is hydrogen gas is not ionized not ionized because O and B because O and B stars are rare stars are rare

radio waves radio waves pass easily pass easily through dust through dust

Population IPopulation I: Disk Stars: Disk Stars Ordered, roughly Ordered, roughly

circular orbits in a circular orbits in a plane. plane.

All orbit in the same All orbit in the same general direction. general direction.

Orbit speeds similar at Orbit speeds similar at a given radius. a given radius.

Population IIPopulation II: Spheroid Stars : Spheroid Stars Disordered, elliptical Disordered, elliptical

orbits at all inclinations. orbits at all inclinations. Mix of regular and Mix of regular and

retrograde orbits retrograde orbits Wide ranges of orbital Wide ranges of orbital

speeds. speeds.

Stellar PopulationsStellar Populations

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Population IPopulation I LocationLocation: Disk and Open Clusters : Disk and Open Clusters AgeAge: Mix of young and old stars : Mix of young and old stars CompositionComposition: Metal rich (roughly : Metal rich (roughly solar) solar)

70% Hydrogen 70% Hydrogen 28% Helium 28% Helium ~2% "metals" ~2% "metals" EnvironmentEnvironment: Often gas rich, : Often gas rich, especially for the young stars.especially for the young stars.

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Population IIPopulation II LocationLocation: Spheroid and Globular : Spheroid and Globular Clusters Clusters

AgesAges: Oldest stars, >10 Gyr : Oldest stars, >10 Gyr CompositionComposition: Metal Poor (0.1-1% : Metal Poor (0.1-1% solar) solar)

75% Hydrogen 75% Hydrogen 24.99% Helium 24.99% Helium ~0.01% metals ~0.01% metals EnvironmentEnvironment: gas poor, no star : gas poor, no star formation formation

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Contrast & CompareContrast & ComparePopulation IPopulation I Disk & Open Disk & Open Clusters Clusters

Young & Old Stars Young & Old Stars Metal-rich Metal-rich Blue M-S stars Blue M-S stars Ordered, circular Ordered, circular orbits in a plane orbits in a plane

Gas-rich Gas-rich environment with environment with recent star recent star formation. formation.

Population IIPopulation II Spheroid & Spheroid & Globular Clusters Globular Clusters

Oldest Stars Oldest Stars Metal-poor Metal-poor No Blue M-S stars No Blue M-S stars Disordered, Disordered, elliptical orbits elliptical orbits in all directions. in all directions.

Little or no Gas & Little or no Gas & Dust, and no star Dust, and no star formation. formation.

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Mass of the Milky WayMass of the Milky WayObserve orbital period, P, of stars or Observe orbital period, P, of stars or interstellar matter vs. distance, D, from interstellar matter vs. distance, D, from centercenter

Kepler’s Third Law Kepler’s Third Law period of orbit determinedperiod of orbit determinedby mass within orbit: by mass within orbit:

DD33 = (M = (Mgalaxygalaxy + M + Msunsun))PP22

earth orbit earth orbit sun’s mass sun’s massfarther from the galactic farther from the galactic center, the more mass center, the more mass within orbitwithin orbitFIND:FIND: more mass than we see -- more mass than we see -- dark matterdark matter halohalo

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Dark MatterDark Matter Two possibilities to explain the Two possibilities to explain the observed rotation:observed rotation:• Law of gravity is wrong for galaxies.Law of gravity is wrong for galaxies.• There is additional matter that doesn’t There is additional matter that doesn’t emit detectable radiation (dark matter).emit detectable radiation (dark matter).

No evidence that gravity behaves No evidence that gravity behaves differently.differently.

The “dark matter” hypothesis is The “dark matter” hypothesis is favored.favored.• Could be brown dwarfs, black holes, or new Could be brown dwarfs, black holes, or new exotic particles.exotic particles.

Measurements indicate about 90% of the Measurements indicate about 90% of the mass in the universe is dark matter!mass in the universe is dark matter!

Galaxy FormationGalaxy Formation