Transcript C12: Our Milky Way Galaxy

The Milky
Way
(Chapter 12)
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
1,000,000,000,000 (1012)
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
Chandra
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
phys.org
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
NASA Image
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
NASA Image
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
NASA
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?