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The Milky Way and Beyond. Galaxies and the Larger Scale Structure of the Cosmos. The Milky Way. Our Home Galaxy We live on the “fringes” 75% of the distance out from center Our solar systems makes one orbit of Galactic center every 250 million years!
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The Milky Way and Beyond Galaxies and the Larger Scale Structure of the Cosmos
The Milky Way • Our Home Galaxy • We live on the “fringes” • 75% of the distance out from center • Our solar systems makes one orbit of Galactic center every 250 million years! • Makes Galaxy difficult to describe due to our perception
The Milky Way • Home to some 100 billion stars • Believed to have a Spiral Structure • This is inferred from various observations • Interstellar dust thwarted early observers • Advent of Radio and IR telescopes improved model
Shapley’s work • His work disputed earlier work by Kapteyn • Shapley said that the Milky Way was larger than initially believed • Kapteyn didn’t take into account the dust present causing dimming • Shapley took the dust into account but didn’t think about dimming
Shapley’s work • Also, he didn’t realize there were TWO classes of variable stars • RR Lyraes • Cepheids • The two classes have different brightness and different periods for variablity • Using the Period-Luminosity relationship (Leavitt) he estimated distance
What Shape is the Milky Way? • Dust initially confused observers • Thought we were in the center of the Galaxy • Stars seemed to be equally distributed • Shapley demonstrated that the 100+ Globular Clusters didn’t orbit us • They orbit a point 30,000 ly distant
The Milky Way Galaxy Side View Nuclear Bulge Disk Sun Halo Globular Clusters
Side View Structure • Disk- 100,000 ly across, 2000 ly thick • Contains Spiral Arms • Nuclear Bulge- 20,000 ly across • Contains Nucleus of Galaxy • Halo- 300,000 ly across • Contains Orbiting Globular Clusters and Dark Matter • Each part has a different population of Stars
Stellar Populations • Mass function- # of stars of each mass • Observations tell us that a variety of masses are made • Observations tell us that star formation is ongoing • Baade grouped the stars according to location and color
Stellar Populations • Blue disk stars = Population I • Red bulge and halo stars = Population II • Further study yields • Bulge- Aging Population I Stars and Pop II • Disk- Young Population I Stars • Halo- Old Population II stars
Stellar Populations • Population I • lots of metals • Young and blue • Circular orbits • Population II • metal poor • Old and red • Elliptical and tilted orbits
Stellar Populations • Not all stars (i.e. Sun) fit easily into either category • Subdivisions include extreme and intermediate Populations, and the “old disk” category • Open clusters contain Pop I • Globular clusters contain Pop II
Galactic Motions • Globular Clusters orbit around the nucleus randomly • Bulge stars are “semi-random”
Galactic Motions • Sun and other disk material orbits nucleus of Galaxy in an orderly way • Experiences Differential rotation • Observed in other Spiral Galaxies • “Rotation” occurs due to a Density Wave • It is not a rigid motion of an “arm” • Wind-up problem
Density Waves • Material in the wave is not fixed • Material can move through the wave • Not a material wave but a disruption wave • Like a traffic jam behind a slow moving vehicle • Wave passes through ISM and triggers star formation
Spiral Arm Structure • Number of Arms isn’t well know. • All numbers between 2-10 have been suggested • Use Spiral Arms Tracers to map the arms • Molecular Clouds (Radio) • H II regions (Optical) • Cepheid Variables (Optical) • OB Stars (Optical)
Variable Stars • Cepheid and RR Lyrae Variables • Variable Stars • Luminosity Varies in predictable ways • RR Lyrae vary over 0.5-1 day • Cepheids vary 1-100 days • Both on Instability Strip of HR diagram
Period-Luminosity Relation • Relationship of Period of Pulse and Luminosity of Star • Linear for Cepheids • Constant for RR Lyraes • Cepheid distances can then be determined • Used for large distances because they are brighter
RR Lyrae • Found in Globular Clusters • Shapley used observations to establish distances to GC
Other Tracers • Molecular Clouds emit in Radio • Use Doppler shift to map arm structure • H II regions and OB stars • Luminosity is known • Distance obtained from Inverse Square Law • Group objects by distance, spiral structure seen
Nucleus • Very Obscured • Very crowded • Sagittarius A- powerful radio source, x-ray jets • Million M Black Hole? • Radio reveals two H arms shooting out
Nucleus • Jansky first looked into the heart of the Galaxy with Radio waves • Evidence of star formation ongoing with giant molecular clouds and HII regions • Cool hydrogen and a ring of molecule rich gas exist even closer to the center • As we approach the center, we use many “eyes” to see
The Heart of the Galaxy • Swarm of stars circle the center of the Galaxy • Millions packed into a cubic light year • At the very center is a ring of dust and gas • This surrounds a very small (10 AU) but very powerful source • This is the suspected black hole
In the Halo • If mass were condensed in the center of the Galaxy, rotation would obey Kepler’s 3rd law • More distant objects would orbit more slowly and we can calculate speeds • This relationship doesn’t hold true
Rotation Curve • Plotting speeds of objects based on distance from Galactic center • Appears that most of the mass is contained in the halo
Formation of the Galaxy • Similar to Star Formation • Everything is on a much larger scale • Halo objects form first • Globular Clusters • Halo Stars • Disk and Nucleus collapse next • Collapse generates star formation
A Universe of Galaxies • Normal Galaxies come in 3 types • Spirals • Ellipticals • Irregulars • Each galaxy has a different morphology • Also different stellar populations • Classified on Hubble Tuning Fork Diagram
