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Key Concepts: Lecture 29: Our first steps into the Galaxy
Exploration of the Galaxy: first attempts to measure its structure
(Herschel, Shapley).
The Milky Way
• Faint band of light
circling sky
• First resolved into
stars by Galileo using
his telescope
The Structure of the Galaxy
Our first encounter with Dark Matter
Our Galaxy:
The Milky Way
• Faint band of
light circling sky
• First resolved
into stars by
Galileo using his
telescope
Structure of the Milky Way
• Initially, star counting was used
– Assume all stars are about the
same luminosity
– See how many stars are in each
direction to map the Galaxy
– First done by the (German-born)
British astronomer William
Herschel (1738-1822) and his son.
Herschel also discovered
infrared light
Structure of the Milky Way
• What Herschel found
Shapley Maps the Galaxy with Globular Clusters
Harlow Shapley (1885-1972)
• RR Lyrae Stars
– We are in a flattened disk of stars
– It extends about 6000 light years
– We appeared to be at or near the center!!
SUN
Harlow Shapley (1885-1972)
• Dropped out of school in 5th grade
• Studied at home and became a reporter at 16
• Went to study Journalism in 1907 at the
University of Missouri
• Journalism school would not open for 1 year
• Graduate fellowship to Princeton to work
with Henry Norris Russell (Mr. H-R diagram)
• George Ellery Hale gave him a position at
Mount Wilson Observatory in Pasadena
• Director of Harvard College observatory for
30 years
– Horizontal branch stars
– Brightness varies because they
pulsate
– All have the same average
luminosity
– Found in globular clusters
Shapley Maps the Galaxy
• Shapley mapped the
distribution of globular star
clusters
• Used RR Lyrae Stars to
estimate distances
– RR Lyrae stars all have the
same Luminosity
– Measure Flux (i.e. apparent
brightness)
– Used Inverse Square Law to
get Distance, d
flux = luminosity /(4π d2)
47 Tucana
M5
Shapley Maps the Galaxy
• Distribution of globular clusters
is not symmetric about our
position in the Galaxy
• 30% of G.C.s are found in only
2% of the sky toward
Sagittarius
Shapley’s Results
• Sun is not in center of Galaxy!
– Center of G.C.s is 26,000 LY =
8000 parsecs from Earth
– Center in direction of Sagittarius
– (see Links section of class
webpage for more on Shapley’s
debate with Curtis about the size
of the Galaxy, our location in it,
and the nature of the “nebulae”.)
What Herschel Got Wrong
• The star counts do indeed end
at 6000light
years~=2000parsecs
• They are cutoff by thick clouds
of dust
• Dust tends to:
– Absorb and scatter blue light
– Has little effect on light with
wavelengths longer than the
size of the dust grains
• The actual extent of the Milky
Way is much larger than can be
seen in visible light
The Structure of the Milky Way
The Structure of the Milky Way
• A thin highly flattened disk
– 100,000 LY in diameter
– 1000 LY thick - About the
relative thickness of a CD
– The Sun is in this disk
– Contains younger stars &
gas (called Population I
stars: Pop I)
• Nuclear Bulge - Central
region
– 10,000 LY across
– Contains old stars (Pop II)
• A spherical Halo component
– Contains globular clusters
– Contains old stars (Pop II)
Weighing the Milky Way
• Most stars & gas in the MW disk are on
roughly circular orbits
– This allows us to measure the mass of the Galaxy using
Newton’s version of Kepler’s Third Law
• e.g. for Sun’s orbit: MGalaxy + Msun = a3/P2
– a = 26,000 LY = 1.6 x 109 A.U.
– P = 2.25 x 108 years
MGalaxy + Msun ≈ MGalaxy = 2x1011 Msun
• This estimate tells us how much mass is
inside the Sun’s orbit.
Different Views of the Milky Way
Question?
Hydrogen Gas in the Radio
Star Formation Regions traced by
hot dust at 12 microns
Old Stars in the Near-IR From COBE
• If we were to measure the velocities of more
distant parts of the Milky Way and we find
that they are moving faster than the Sun, so
that their orbital periods are all about the
same as the Sun, what does this tell us about
the total mass of the Galaxy? Hint in the
solar system the more distant planets orbit
much more slowly.
MGalaxy + Msun = a3/P2
Can we write this in terms of the speed?
Newton’s version of
Kepler’s 3rd law
MGalaxy + Msun = a3/P2
P = distance / speed
P = 2π a / v ∝
a/v
MGalaxy + Msun ∝ a v2
We see that v is approximately constant in galaxies (does not
depend on a). Therefore the Mass inside radius a increases
linearly with a.
Mass of the Milky Way
• Most of the luminous matter in the Galaxy drops
off at distances > 30,000 LY
– Mass of Galaxy should be ~ 1011 MSun
• However there are a few globular
clusters and some gas clouds that
lie beyond this boundary
• These objects move faster than the Sun !
– Must be more gravity than supplied by luminous matter
– Total Mass of Galaxy ~ 1012 MSun
– DARK MATTER - Only about 10% of mass is visible
What is the Dark Matter?
• Some kind of faint star ?
– White dwarfs
– Brown dwarfs
• Examples of MAssive
Compact Halo Objects
• Exotic subatomic particles?
– Have mass
– Don’t interact with matter
– Formed in early universe
• Weakly Interacting
Massive Particles
MACHOs
WIMPs
The Search for MACHOs
Gravitational Lensing
See textbook