Student Learning Objectives • Sketch the Milky Way galaxy with all of its components • Describe how the properties of our galaxy are measured Image Credit: NASA
What are the characteristics of our galaxy? The Milky Way galaxy is a relatively large Barred-Spiral galaxy. Diameter Mass Age
25,000 parsecs (81,500 LY) 1011 Msun 15 Billion years
Number of Stars
Nearly everything we see in the night sky, with our eyes, is within our galaxy.
Practice 1) Who observed the Milky Way in the 1600’s and determined that the Milky Way was made up of individual stars?
2) Where is the Sun located in the Milky Way?
Components of our galaxy 1. Sagittarius A 3.7 Million Msun black hole.
At center of our galaxy
How was it found? Emits bursts of radiation Erratic star orbits near black hole
Sag A would fit inside Earth’s orbit
Sag A vs Earth
Sag A Diameter & Earth’s Orbit
2. Nucleus Very Compact Stars, gas, and dust
Heated ISM Young & Old Stars Young SN Remnants Black Hole
3. Bar Structure Condensed region of stars stretching from nucleus
4. Disk Spiral arms of stars & ISM Most stars are mid-lower main-sequence
O & B stars provide most of the luminosity Our Sun Open Clusters ISM
Practice The spiral arms are bright because they contain a. b. c. d.
Mostly mid-lower main-sequence stars O & B stars Stars in giant phase Stellar remnants
5. Halo Diffuse spherical “cloud” of globular clusters and hot gas
Globular Clusters are 11-13 Billion years old Random Orbits
Old Stars Globular Clusters Hot Gas
Image credit: William E. Harris, McMaster U., and Larry McNish.
Enormous cloud of hot gas
Practice 1) How can the ages of stars be determined? 2) Why do the halo stars have different orbits than the disk stars? 3) How can the center of the galaxy be determined?
Stellar populations indicate how our galaxy formed. Population I – Metal rich (2-3% metals) – Nearly circular orbits in disk
Population II – Metal poor (0.1% metals) – Random orbital paths in the halo
Galaxies Form & Evolve 1. Giant spherical cloud 2. High mass stars and globular clusters form 1st 3. Cloud begins to rotate and flatten
4. 1st generation stars produce metals (SN) 5. 2nd generation stars form in disk
Smaller Galaxies are cannibalized & Large Galaxies Merge
Practice Disk or Halo 1) Where would you expect to find Pop I stars? 2) Where would you expect to find Pop II stars? 3) Most main-sequence stars?
How was the mass of our galaxy determined? Estimated with Kepler’s Law Calculated from Rotation Curve
(m1 + m2)P2 = a3
Rotation Curve • Orbital velocities increase • There must be invisible mass causing the acceleration
Dark Matter Visible matter is being accelerated by Dark matter at the edges of our galaxy. Dark matter emits no electromagnetic radiation.
Artist simulation of dark matter halo around the Milky Way
Artist simulation of Milky Way Dark Matter Halo
Dark Matter is being Mapped Observations show dark matter distributed as network of gigantic dense (white) & empty (dark) regions Van Waerbeke, Heymans, & CFHTLens collaboration
Questions 1) What is dark matter? 2) Do other galaxies have dark matter?
Hubble Space Telescope composite image shows ghostly "ring" of dark matter in the galaxy cluster Cl 0024+17 May 2013 NASA, ESA, M.J. Jee and H. Ford
What causes spiral arms? A spiral arm is a region where the density of stars is relatively high. Stars and gas drift into and out of spiral arms.
Orbital Traffic Jam! http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/spiralarms.htm
Density Wave Theory • Gravity draws stars into spiral arms • Inertia carries stars forward and out of spiral arms
• Spiral arms are Gravitational Compressions
Gravity Density Waves The spiral arms don't wind up because they are like a wave disturbance that ripples through the system.
Gas clouds are compressed initiating star formation (make arms bright) Stars pile up (make arms bright) O&B stars shine bright and ionize gas (make arms bright)
Practice 1) Do stars change orbital speed as they travel into and out of spiral arms? 2) Do spiral arms “wind up” resulting in a smooth rather than armed structure? 3) What are the theories for how spiral arms begin?