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The Milky Way – Our Galaxy
Exam II
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•
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A: 25 or better
B: 20 or better
C: 17 or better
D: 15 or better
F: 13 or below
• Average: 20.7
Solutions to Exam III (updated)
• 1a,2b,3b,4d,5e,6a,7b,8c,9a,10c,11d,12a,
13c,14c,15c,16c,17b,18d,19c,20a,21b,22c,
23c,24e,25b,26d,27b,28c,29e,30a,31d,32b
Toughest Questions
• Hydrostatic Equilibrium
• Temperature of Sun’s surface
• Star of absolute mag 4.6, apparent magnitude 3.2
Galaxies – Island Universes
• A historic tour of the discovery of the
dwindling significance of humans in the
universe:
• From the center of the universe towards the
edge of an average galaxy amongst 100
billion others
How do we know where we are?
• “Obviously” we are living on a flat Earth at
the center of the universe, as a quick look
tells us:
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The stars, Sun, Moon and planets rotate us
There is no apparent curvature of the ground
The Milky Way is a band that surrounds us
There are no signs for any movement of the
Earth (like wind, or forces throwing us off)
Logic to the Rescue
• How do we avoid these wrong conclusions?
– Sound data
– Flawed interpretation/reasoning
Further observations are necessary to decide!
• Do we have to question everything?
– Yes, in principle.
– The signature of genius is to ask the right
question, not necessarily to answer them.
Exploring our own Island Universe:
The Milky Way
• A galaxy is a
huge
collection of
stars, gas,
dust, neutron
stars, and
black holes,
isolated from
others and
held together
by gravity
Our view of the
Milky Way
• Appears as a milky band of
light across the sky
• A small telescope reveals
that it is composed of many
stars (Galileo again!)
• Our knowledge of the Milky
Way comes from a
combination of observation
and comparison to other
galaxies
How do we know?
Obviously a bogus picture of our milky way!
• Question: How can
we say anything
about our Milky
Way, if we cannot
see it from outside?
Enter: the Genius
• William Herschel (XVIII century)
• Simple model:
– Assumed all stars have the same
absolute brightness
– Counts stars as a function of
apparent magnitude
– Brighter stars closer to us; fainter
stars further away
– Cut off in brightness corresponds to
a cut off at a certain distance.
• Conclusion: there are no stars
beyond a certain distance
Herschel’s Findings
• Stars thinned out very fast at right angles to Milky Way
• In the plane of the Milky Way the thinning was slower
and depended upon the direction in which he looked
• Flaws:
– Observations made only in visible spectrum
– Did not take into account absorption by interstellar gas and
dust
Discovering other Island Universes
• Data: Lots of nebulous spots
known in the nightsky
• Questions: What are they? All
the same? Different things?
• Need more observations!
 Build bigger telescopes
Famous Telescopes - Herschel
Herschel detected Uranus (1781)
(Uranus is visible with the unaided eye)
Famous Telescopes – Lord Ross
• 72 inch Reflector
• built during potato famine in Ireland
• Largest Telescope until Mt Wilson
(1917)
The first nebula discovered to have
spiral structure: M51
Lord Rosse (1845): M51
Hubble Space Telescope (2007): M51
M99 is a
spiral, too!
• Q: do we live in a
spiral?
• Q: Are we in the
center of the
spiral?
• Most probable
answer: No!
Enter: next genius
• Harlow Shapley used variable
stars, e.g. RR Lyrae stars, to
map the distribution of
globular clusters in the galaxy
• Found a spherical distribution
about 30 kpc (30,000 pc)
across
– This is the true size of the
galaxy
• Sun is (naturally!) not at the
center – it’s about 26,000 ly
out
Standing on the shoulders of Giants
• Shapley used methods developed by others
to measure the distance to globulars
• Cepheid variables show luminosity-period
correlations discovered by Henrietta Leavitt
• Shapley single-handedly increase the size of
the universe tenfold!
Structure of the Galaxy
An observer far outside our galaxy
would best describe our galaxy and the
Sun's position in it as a …
a) disk of stars with our Solar System 2/3 towards
the edge.
b) disk of stars with a bulge containing our Solar
System.
c) sphere of stars centered on our Solar System.
d) sphere of stars with our Solar System near the
edge.
Intra-galactic Dynamics
• Three main parts
of a galaxy:
– Bulge (center of
galaxy)
– Disk (rotating
around center)
– Halo (orbiting
around bulge
with randomly
inclined orbits)
Properties of Bulge, Disk and Halo
Disk
Highly flattened
young and old stars
has Gas and dust
Star formation
White colored,
blue spiral arms
Halo
spherical
Bulge
football-shaped
only old stars
young and old stars
none
lots in center
none since 10 billion yrs
reddish
in inner regions
yellow-white
An up-to-date “Reconstruction”
Activity: Milky Way Scales
• Form groups of 3-5
• Work on the questions on the handout
• Hold on the the sheets until we talked about
your findings
• Turn them in with your names on (one sheet
per group)
Other Galaxies: Hubble supersedes
Shapley
• Edwin Hubble identified single stars in the
Andromeda nebula (“turning” it into a
galaxy)
• Measured the distance to Andromeda to be
1 million Ly (modern value: 2.2 mill. Ly)
• Conclusion: it is 20 times more distant than
the milky way’s radius  Extragalacticity!
 Shapley’s theory falsified!
Q: How many galaxies are there?
• Hubble Deep Field
Project
– 100 hour exposures
over 10 days
– Covered an area of
the sky about 1/100
the size of the full
moon
• Probably about 100
billion galaxies
visible to us!
• About
1,500
galaxies in
this patch
alone
• Angular
size ~ 2
minutes of
arc
Other Galaxies
• there are ~ 100 billion galaxies in the observable
Universe
• measure distances to other galaxies using the periodluminosity relationship for Cepheid variables
• Type I supernovae also used to measure distances
– Predictable luminosity – a standard candle
• Other galaxies are quite distant
– Andromeda (M31), a nearby (spiral) galaxy, is 2 million
light-years away and comparable in size to Milky Way
• “Island universes” in their own right
Q: How does our galaxy look like
from the outside?
• Probably like others, so observe them!
Hubble Classification Scheme
• Edwin Hubble (~1924) grouped galaxies
into four basic types:
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Spiral
Barred spiral
Elliptical
Irregular
• There are sub-categories as well
Spirals
(S)
• All have disks, bulges, and halos
• Type Sa: large bulge, tightly wrapped, almost circular
spiral arms
• Type Sb: smaller bulge, more open spiral arms
• Type Sc: smallest bulge, loose, poorly defined spiral arms
Barred Spirals (SB)
• Possess an elongated “bar” of stars and
interstellar mater passing through the center
Elliptical (E)
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•
•
•
No spiral arms or clear internal structure
Essentially all halo
Vary in size from “giant” to “dwarf”
Further classified according to how circular
they are (E0–E7)
S0/SB0
• Intermediate between E7 and Sa
• Ellipticals with a bulge and thin disk, but no
spiral arms
Test: What type is this galaxy?
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•
•
•
Spiral
Barred Spiral
Irregular
Elliptical
And this one?
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•
•
•
Spiral
Barred Spiral
Irregular
Elliptical
Type?
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•
•
•
Spiral
Barred Spiral
Irregular
Elliptical
Here’s a weird one!
• Spiral
• Barred Spiral
Elliptical
Irregular
Solutions
• Sb (Andromeda Galaxy M31)
• E2 (Elliptic Galaxy)
• SBb (Barred spiral galaxy)
•
Ir II (Irregular galaxy M82)
Q: How do we know we live in a
Spiral Galaxy?
• After correcting for absorption by dust, it is possible
to plot location of O- and B- (hot young stars) which
tend to be concentrated in the spiral arms
• Radio frequency observations reveal the distribution
of hydrogen (atomic) and molecular clouds
• Evidence for
– galactic bulge
– spiral arms
Rotation of the Galaxy
• Stars near the center
rotate faster; those near
the edges rotate slower
(Kepler)
• The Sun revolves at
about 250 km/sec
around the center
• Takes 200-250 million
years to orbit the
galaxy – a “galactic
year”
How do spiral arms persist?
 Why don’t the “curl up”?
“Spiral Density Waves”
• A spiral
compression wave
(a shock wave)
moves through the
Galaxy
• Triggers star
formation in the
spiral arms
• Explains why we
see many young hot
stars in the spiral
arms
Density (Shock)
Waves
The Mass of the Galaxy
• Can be determined using Kepler’s 3rd Law
– Solar System: the orbital velocities of planets determined by
mass of Sun
– Galaxy: orbital velocities of stars are determined by total
mass of the galaxy contained within that star’s orbit
• Two key results:
– large mass contained in a very small volume at center of our
Galaxy
– Much of the mass of the Galaxy is not observed
• consists neither of stars, nor of gas or dust
• extends far beyond visible part of our galaxy (“dark
halo”)
Galaxy Masses
• Rotation
curves of
spiral galaxies
comparable to
milky way
• Masses vary
greatly
The Missing Mass Problem
• Dark Matter is dark at all wavelengths, not just visible
light
• The Universe as a whole consists of up to 25% of Dark
Matter!  Strange!
• What is it?
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–
–
–
–
Brown dwarfs?
Black dwarfs?
Black holes?
Neutrinos?
Other exotic subatomic particles?
• Actually: Most of the universe (70%) consists of Dark
Energy  Even stranger!
Missing Mass Problem
Actual data
Hypothetical
Keplerian motion
• Keplerian Motion: more distance from center 
less gravitational pull  slower rotational speed
Galaxy Formation
• Not very well understood
– More complicated than stellar formation, and
harder to observe
• Formation of galaxies begins after Big Bang
• Different than star formation because
galaxies may collide and merge
Galaxy Formation
• Galaxies are
probably built up
by mergers
– Contrast to break
up of clouds in star
formation
• Our own Milky
Way is eating up
the neighboring
Sagittarius Dwarf
Galaxy
Galaxy Mergers
• Start with high density of small proto-galaxies
• Galaxies merge and turn into bigger galaxies
Actual photo (HST): lots of
small galaxies
Galaxy Interaction
Galaxy Collision: NGC2207 vs. IC2163
Collision between NGC 4038 and NGC 4039
The Tully-Fisher Relation
• A relation between the rotation speed of a spiral galaxy
and its luminosity
• The more mass a galaxy has the brighter it is  the
faster it rotates  the wider the spectral lines are
• Measuring rotation speed allows us to estimate
luminosity; comparing to observed (apparent)
brightness then tells us the distance
Active Galaxies
Types of Active Galaxies
• Radio galaxies: radiate a long radio
frequencies
• Seyfert Galaxies: between normal and
active, compact core
• Quasars: quasi stellar objects, very far
away, maybe early stage of galaxy
• Note: most active galaxies look ‘normal’ in
visible frequencies
Seyfert Galaxies
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•
•
•
Look like normal spiral galaxies
Energy output mostly in IR and radio frequencies
Emitted from small region: nucleus of galaxy
Nucleus of Seyfert galaxy 10,000 times brighter than of
normal galaxy
NGC 5728
Energy Output
• Active galaxies
emit most of their
energy in radio
frequencies
Quasars
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•
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Quasi-stellar objects
Appear like stars on photographs
Very distant objects
Very high luminosity
Essentially a quasar is a galaxy with
exceptionally bright core
• Might be young galaxies
A typical Quasar
• Very distant
Very faint
Appears starlike
Beyond the Galactic Scale –
Clusters of Galaxies
The Local Group
The Virgo Cluster
Superclusters
Beyond Superclusters
• Strings, filaments,
voids
• Reflect structure
of the universe
close to the Big
Bang
• Largest known
structure: the
Great Wall (70
Mpc  200 Mpc!)