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
- thick disk
- 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
- Nucleus
- Basic Properties
- Diameter
- about 80,000 to 100,000 ly for visible disk
- Mass
- greater than a few times 1011 Msun
- 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!"
- 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
- Stars in the Milky Way
- all types are present
- mass function
- # of stars of each mass
- determines evolution of galaxy
- most common star
- Stellar Populations
- Walter Baade
- Mt. Wilson Observatory
- looked at nearby galaxies
- stars separated by color
- red stars - bulges and halos (Pop II)
- blue stars - disks & spiral arms (Pop I)
- 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
- 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
- Four major arms
- Perseus
- Sagittarius
- Centaurus
- Cygnus
- Minor arms
- Rotation of the Galaxy
- Stability requires rotation
- 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
- 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