THE MILKY WAY

Our Galaxy

• Southern Hemisphere View of the Milky Way
• Northern Hemisphere View of the Milky Way
• View from the South Pole
• Near Infrared View of the Milky Way



• Appearance
• band of light
• milky strip
• through telescope: millions of faint stars
• actually consists of hundreds of billions of stars
• Basic Structure
• Flat disk
• thin disk
• 700 pc
• thick disk
• 2600 pc
• spiral arms winding out
• begin in center
• numerous arms
• Bulge
• older stars clustered around center of disk
• Halo
• spherical swarm
• older stars
• completely surrounds disk
• Corona
• dark matter
• Nucleus
• the innermost core
• Basic Properties
• Diameter
• about 80,000 to 100,000 ly for visible disk
• Mass
• greater than a few times 10¹¹ M_sun
• Composition
• 96% stars, 4% dust and gas
• Disk rotates
• near Sun takes about 240 million years
• velocity of rotation is about 220 km/sec
• Contains Stars and Interstellar matter
• Interstellar matter is gas (mostly H and He) plus tiny dust particles
• Entire assemblage held together by gravity
• each star and gas cloud follows own orbit
• Age of the Milky Way
• about 15 billion years (deduced from age of oldest stars)
• Determining the Information
• appearance on the sky as a narrow band of stars
• Band implies disk shape not sphere shape
• Mass
• treat Milky Way as giant binary star
• with Sun as one star
• rest of Milky Way as other star
• Use modified form of Kepler's Third Law
• gives mass interior to Sun's position
• Size and Sun's Location
• Herschel
• count of stars in different regions of sky
• same count in all directions
• Kapteyn
• measured density
• count of stars and distance from us
• density drops off uniformly in all directions
• "We're at the Center!"
• usually wrong conclusion
• Interstellar Extinction
• dust obscures our view
• Trumpler - 1930
• remote clusters appear dimmer than expected
• Globular Clusters
• spherical distribution of 10⁶ stars
• lie outside plane of galaxy
• less dust in the way
• Harlowe Shapley - 1920s
• Observed globular clusters
• Note they outline Milky Way on sky
• contain variable stars that allow us to use period-luminosity relationship
• Measured distances to 93 known clusters
• Plot Clusters in a scale drawing of system
• Size of galaxy larger than previously thought
• Clusters are not centered around the Sun
• centered about point in Sagittarius
• accepted distance to center
• 8 kpc (+-1 kpc)
• Stars in the Milky Way
• all types are present
• mass function
• # of stars of each mass
• determines evolution of galaxy
• most common star
• dim, low mass
• Stellar Populations
• Walter Baade
• Mt. Wilson Observatory
• looked at nearby galaxies
• stars separated by color
• red stars - bulges and halos (Pop II)
• older stars
• blue stars - disks & spiral arms (Pop I)
• younger stars
• Population I
• young
• blue
• lie in disk
• circular orbits
• heavy element "enriched"
• Population II
• old
• red
• halo and bulge
• high eccentricity and tilt
• heavy element poor
• Smooth gradient between populations not sharp distinction
• Multi-wavelength Milky Way
• Interstellar Medium
• Interstellar Gas
• raw material for stellar formation
• mostly hydrogen
• average density of 1 atom/cm³
• higher density regions
• neutral hydrogen
• HI regions
• high enough densities
• may form molecular hydrogen, H₂
• ionized hydrogen
• HII regions
• protons and electrons
• emission nebula
• Interstellar Dust
• blocks light
• limits view of galaxy
• few thousand light years visibility in disk
• (Dark nebulas)
• Zone of Avoidance
• no galaxies visible in this area of the sky
• reddens light
• blue light scattered more
• longer wavelength light can pass through
• Extinction
• 25 magnitudes between Sun and center of the Galaxy
• small dust particles
• micron size
• carbon, silicates and ices
• enriched with heavier elements from supernova explosions
• Radio Astronomy
• Continuum Radio Astronomy
• ignore spectral lines
• bright visible objects vs. bright radio objects
• not always correlated
• Thermal radiation
• black body radiation - cool object for radio spectrum
• Synchrotron Radiation
• electrons spiral along magnetic field lines
• high energy - fast speed
• circular motion - high acceleration
• highly polarized
• Spectral Lines
• Importance
• Doppler shift can be measured
• Radio waves - long wavelength - low energy
• 21 - cm line
• spin of electron
• opposite to proton - lower energy
• absorption and emission
• collisions - can flip electron
• Other Lines
• molecular lines
• formation of large molecules?
• using dust grains
• lots of theories and questions
• Spiral Arms
• neutral hydrogen map
• spiral structure
• Four major arms
• Perseus
• Sagittarius
• Centaurus
• Cygnus
• Minor arms
• We live in Orion
• Rotation of the Galaxy
• Stability requires rotation
• Gravitational collapse
• Rotational velocity
• Determined by Doppler shift of 21 cm line
• Relative to Sun
• Need reference frame to determine speed of Sun
• Distant galaxies and globular clusters
• Speed of Sun
• Period - T = distance/speed
• T = 2pi (8000 pc)*(3.09 x 10¹³ km/pc)/220 km/s
• T =7.1 x 10¹⁵ s = 220 million years
• Sun's orbit roughly circular
• Mass inside orbit affects motion
• Stars and gas all orbit in the same direction
• Orbital speed roughly constant
• Rotation curve of galaxy
• Orbital speed as a function of distance
• Rotation of the Galaxy
• Disk like rotation
• Keplerian Rotation
• Galactic Rotation Curve
• Spiral Arms
• Disk-like Structure results from
• Conservation of angular momentum
• Formation of Arms
• ????
• Pattern in place from the start??
• Only if galaxy rotates as a disk
• Our galaxy
• objects have approximately the same speed
• Winding dilemma
• Spiral Structure disappears "quickly" as arms wind up
• Few hundred million years
• Spiral arms cannot be like spokes
• Density Waves
• proposed by Bertil Lindblad - Swedish Astronomer
• Waves
• Crests (regions of higher density) and troughs (regions of lower density)
• Spiral Arms are the crests
• waves move slower than stars or interstellar material
• areas of higher densities propagating around galaxy
• Interstellar dust and gas move into region
• Higher density - star forming regions
• brighter than surroundings
• OB stars seen close to high density regions
• Do not have time to move out
• What maintains the waves?
• Energy source?
• Bar type galaxy
• Asymmetric pull could generate waves
• Gravitational interaction between galaxies
• Two types of Spiral galaxies
• Grand-design spiral
• Thin, graceful, well-defined spiral arms
• Flocculent
• Broad, fuzzy, chaotic, poorly defined arms
• Self-propagating Star Formation (SSF)
• disturbance produced by supernovae
• causes new stars to form
• star forming regions move outward
• inner regions move faster
• results in spiral structure
• explains flocculent spirals
• Density waves explain grand Design
• The Galactic Center
• in constellation Sagittarius
• radio source
• Karl Jansky
• SgrA
• invisible at optical wavelengths
• too much dust
• use radio, infrared, and gamma ray
• Large number of stars
• packed densely
• about 1/1000 separation near Sun
• intense, small radio source
• synchrotron radiation
• nonthermal emission
• high-energy electrons in magnetic field
• 5 kpc
• a necklace of clouds of molecular hydrogen, ionized hydrogen
• 3 kpc
• arc of cold hydrogen outward at 100 km/s
• Core
• giant swarm of stars
• 1000 light years across
• millions of stars/cubic light year
• at center - IRS16
• extremely luminous stars at 20 light years
• 30,000 solar masses
• luminosity of 20 million Suns
• Powerful Energy Source
• SgrA*
• 10 AU in diameter
• million times the mass of the Sun
• no light
• Black hole?
• How Formed?
• Starts small- few solar masses
• Grows by collecting gas
• Eventually gets big enough to "eat" stars
• History and Formation of the Milky Way
• Explain
• shape
• motion
• stellar populations
• Intergalactic gas cloud
• pure H and He
• 100 billion solar masses
• slow spin
• Due to interactions with other proto-galaxies
• Collapses under its gravity
• some stars form in collapsing gas
• collapse is slow-takes several 100 million years
• stars retained motion once they formed
• massive stars
• evolve
• generate heavy elements in their core
• explode as supernova
• add heavy elements to gas
• additional stars form in collapsing gas
• now contain some heavy elements
• these become Pop II stars
• some gas left over
• "Polluted" gas collects in disk
• conservation of angular momentum causes flat shape (like solar system)
• Spiral arms form as "density wave"
• Clouds in arms collapse
• form Pop I stars in disk
• Why no Pop III stars (no heavy elements)?
• early conditions unsuitable for low mass stars
• surface layers may have been contaminated