Download The Milky Way

The Milky Way
Our home galaxy, full of stars,
gas and mysterious dark matter
We decompose it into a disk and a
halo and a few other parts
Second Exam Results
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Mean: 53.7 + 20 pt curve = 73.9
Standard deviation: 10.4
Maximum: 76 raw, 92 curved;
Minimum: 36 raw, 56 curved.
Distribution:
>90: 4
>80: 5
>70: 9
>60: 5
>50: 2
Almost a
View of our
Milky Way
NGC 4526, a
spiral galaxy like
the MW but about
30 Mpc away: it
has a similar
size, luminosity
and structure
Edge-on “View” and View of MW
Key Parts of the Milky Way
• HALO Contains most globular clusters, and most
Pop II stars; roughly 30 kpc (105 lt-yr) in diameter.
• THICK DISK roughly 5 kpc thick, and 30 kpc in
diameter; contains Pop I stars (but low density).
• THIN DISK ~500 pc thick: contains MOST stars;
includes spiral arms and great majority of luminosity.
• DUST DISK only 50 pc thick; new stars are born in
the molecular clouds found within this very thin disk.
• SPIRAL ARMS are wrapped within the dust/thin disk;
contain almost all hot, luminous (O and B) stars.
Overall Structure of the Milky Way
Inner Parts
• GALACTIC BULGE
roughly 2 kpc in radius around center;
highest concentration of stars, including
many globular clusters.
• GALACTIC CENTER
in the direction of the constellation Sagittarius,
some 8 kpc from the Solar System (SS).
• Multiwavelength Milky Way
MAPPING the MILKY WAY
• Dust, mainly in molecular clouds, shrouds the Disk;
we see few stars beyond 2 kpc from SS in the thin
disk, where the number of stars is much greater
• Originally astronomers thought the Milky Way WAS
the Whole Universe & SS central to it (because of
visible light extinction by dust)
• Location of Globular Clusters in halo implied center
towards Sagittarius and SS actually towards one side
in early 20th century.
• Atomic Hydrogen gas sends 21 cm radio waves that
allow us to map the far side of the galaxy, and the
outer reaches where there are few stars
Our Nearest Big Neighbor, M31,
the Andromeda Galaxy
Andromeda, about 30 kpc across down to nucleus only 15pc
A Limited Conception of the MW
Herschel’s “map” of the Galaxy from star counts;
More in the MW plane, but thought the Sun near the
center and got the size too small: didn’t understand dust
Distribution of Globular Clusters
How do stars orbit in our galaxy?
Stars in the disk all orbit in the same direction
with a little up-and-down motion
Orbits of
stars in the
bulge and
halo have
random
orientations
Thought Question
Why do orbits of bulge stars bob up and down?
A. They’re stuck to interstellar medium
B. Gravity of disk stars pulls toward disk
C. Halo stars knock them back into disk
Thought Question
Why do orbits of bulge stars bob up and down?
A. They’re stuck to interstellar medium
B. Gravity of disk stars pulls toward
disk
C. Halo stars knock them back into disk
Distances from Variable Stars
• Certain stars act as “standard candles” with fixed
LUMINOSITY (M)
• So, MEASURED BRIGHTNESS (m) lets us compute
their distances.
• RR Lyrae stars all have similar absolute magnitudes
(around -0.5 to -1.5). Their periods are all less than
one day. They can be seen in nearby galaxies
outside the MW.
• Can be seen out to 10’s of Mpc. Cepheid variables
are even more luminous, and have longer periods
(1-50 days).
Light Curves of RR Lyrae and
WW Cgyni (a Cepheid Variable)
The Instability
Strip
• Both RR Lyrae and
Cepheid variable
stars are post-main
sequence stars
(subgiants and
giants) whose
atmospheres
pulsate strongly due
to opacity variations
Standard Candles via PeriodLuminosity Relations
Variable
Stars and
the
Distance
Ladder:
They take us
out to
moderately
distant
galaxies
Motions Near the Sun
• Measure Doppler shifts of
many stars to get
velocities near the Sun
• Motions are faster closer
to the galactic center so,
on the average, stars
ahead of Sun and inside
get ahead (redshifted)
while those behind and
outside fall behind (also
redshifted)
Gas Velocities from 21 cm Lines
Rotation Gives Mass Distribution
• ROTATION CURVES plot the stellar or gas
velocity (v) against distance from center of
galaxy (r). Mostly measured by 21 cm
emission from H I gas
• Rigid body curve: v  r (like CD in a player or a
rigid arm swinging)
• Keplerian curve: v  1/r1/2  most mass
centrally concentrated.
This would be like Mercury orbiting fastest
and Neptune slowest around the Sun.
• Flat curve: v  constant  M rises significantly;
specifically: M  r
Galactic Rotation Curve
DARK MATTER SEEMS TO
REALLY MATTER
• For the MW a FLAT rotation curve implies there is
MISSING MASS or
• DARK MATTER that isn't Stars or Gas seen out to 20
kpc from galactic center.
• Essentially ALL other Spiral galaxies for which
Rotation Curves can be measured ARE ALSO FLAT,
so DM is EVERYWHERE!
• More evidence for DM comes from CLUSTERS OF
GALAXIES; we'll discuss this later.
• Yet more evidence comes from COSMOLOGICAL
measurements of the structure of the universe as a
whole (last couple of lectures!)
Dark Matter Candidates
• Missing Red Dwarfs (not enough; next slide)
• Planets or Brown Dwarfs on the loose (unlikely to be
enough: gravitational lensing)
• Isolated black holes (very unlikely to be enough)
• Massive neutrinos (evidence for then having a tiny mass
makes them a good candidate, but very unlikely that they
dominate the DM)
• Snowballs (very difficult to form them, unpopular choice)
• As yet undiscovered particles; (Axions; Supersymmetric
particles; WIMPs: Weakly Interacting Massive Particles)
MOST popular now BUT no convincing detections yet.
Few Red Dwarfs Seen in Globular
Cluster 47 Tucanae
Gravitational Lensing by Brown Dwarfs
• Temporary increase in star’s brightness due to a dark mass
moving in front
• A rare detection is shown in the right
Key Properties of MW
• We are about r = 8 kpc from the center.
• We orbit the center at v = 220 km/s
• That makes for a galactic year
(circumference divided by velocity) of
• (2 ) x 8,000 x (3.0857 x 1013 km) / 220 km/s
= 7.1 x 1015 s = 2.24 x 108 yr.
• So, roughly 225 million years is ONE
GALACTIC YEAR.
• How old is the solar system in galactic years?
• At nearly 4.6 billion years of age, the SS is
only about 20 galactic years old!
Weighing the Galaxy
Orbital speed depends on mass inside at a particular radius.
This can be used with any galaxy for which motions can be
measured. Mass vs. Distance Applet
Orbital Velocity Law
rv
Mr 
G
2
• The orbital speed (v) and radius (r) of an object on
a circular orbit around the galaxy tells us the mass
(Mr) within that orbit

Mass of the Milky Way
• Mgal r3/P2 from Newton’s laws.
• This is dominated by DARK MATTER, but
total mass can be estimated by the velocity of
stars at different distances.
• Out to solar distance (about 8 kpc) the mass
is about 1 x 1011 M (mostly stars)
• Out to ~15 kpc, (the visible radius) a good
estimate for the mass is nearly 4 x 1011 M
(now mostly DM).
• Out to about 70 kpc (> 90% dark matter):
2 x 1012 M
Spiral Galaxies
M101 is seen face on (similar to MW); NGC 4565 is edge on
Stellar Populations
• Pop I Stars: Have compositions like the sun:
70% H, 28% He, 2% "metals"; these metals
are mostly Carbon, Oxygen and Nitrogen
• Use the CNO cycle to generate Main
Sequence energy if M > 1.5 M
• Are almost all younger than 8 billion years.
• Most are in the thin disk; the rest are in the
thick disk.
Stellar Populations, 2
• Pop II Stars: Have compositions with much less
heavy elements than the Sun: 72%H, 28% He, 0.2%
metals is typical
• Use the pp-II on the MS if M > 1.5 M
• Are almost all older than 8 billion years.
• Most are in the halo and galactic bulge; however
plenty pass through the thick disk too.
• Pop III Stars: The very earliest born;
• they have essentially NO METALS,
• formed from only H and He made in the BIG BANG;
• Only a few possible detections.
Spiral Arms
• Fundamentally produced by Gravitational
Perturbations to the galactic disk
• Produced either by a CENTRAL BAR or by a
COMPANION GALAXY
• TWO ROUTES to Spiral Arms:
• First, DENSITY WAVES
• Think: traffic jam in space:
• Second, STAR FORMATION CHAIN
REACTION
Density Wave Analogy to Traffic Jam
• Small extra density holds stars/gas up, like a broken down truck
on the side of the road
--Molecular clouds compressed, stars born
--This best explains beautiful smooth
("grand design") spirals
Density Waves Can Make Spiral Arms
NGC 1566
shows
density wave
features with
dust lanes
and nearby
young star
clusters
Spiral arms are
waves of star
formation
1. Gas clouds get
squeezed as they
move into spiral
arms
2. Squeezing of
clouds triggers
star formation
3. Young stars flow
out of spiral arms
So Can Stochastic Star Formation
• Random birth of Massive Stars
• Their SN explosions compress nearby clouds
& make new stars
• Differential rotation of galaxy yields spiral
appearance by streching the stars out
• This best explains "rattier", broken-up spirals
(like the Milky Way, though some Density
Wave contribution is OK.)
Stochastic or Self-Propagating
Star Formation
Spiral Arm Facts
• Typically, spiral arms have dark, DUSTY CLOUDS on
their edges.
• Some of these are compressed enough to form bright
O-B STAR CLUSTERS, which can in turn ionize and
light up parts of the clouds into H II regions.
• Stars older than about 20-30 Myr are usually outside
the arms.
• NOTE: the arms are barely denser in stars than the
rest of the disk but they stand out because they have
nearly all the hot, bright, young stars.
• Spiral Arms Applet
Stellar Clusters
• ALL clusters contain many more stars than
average within diameters of 3-20 pc.
We usually define three types:
• O-B ASSOCIATIONS
• OPEN (or Galactic) CLUSTERS
• GLOBULAR CLUSTERS
O-B ASSOCIATIONS
• usually < 100 stars,
• found in the THIN DISK
• definitely Pop I -- higher metallicity (similar to
the Sun)
• stand out because these massive MS
stars are so powerful
• ages usually < 30 Myr
• definitely BLUE in color because they have
many hot (O and B) MS stars
OPEN (or GALACTIC) CLUSTERS
• 100's to 1000's of stars,
• found in the DISK and BULGE
• definitely Pop I -- higher metallicity (similar to
Sun)
• stand out because of some pretty massive
MS stars and LOTS of stars
• ages range from 5 Myr up to ~3 Gyr
(M = Mega, million, G = Giga, billion)
• colors are BLUE through YELLOW from
dominant MS stars
Open Cluster: Pleiades
Only 120 pc from the Sun, the Seven Sisters have many
fainter companions; only the most massive have left the
MS
GLOBULAR CLUSTERS
• 104 to > 106 stars
• MOSTLY found in the HALO
(plenty in the BULGE too, and a few found
passing through the DISK)
• All Pop II -- much lower heavy element
abundance than the Sun
• stand out because of HUGE number of
stars in them
• ages all > 5 Gyr
• RED in color: low mass (red) MS stars and
higher mass Red Giants provide most of their
light. Blue stars are gone from the MS.
Globular Cluster: Omega Centauri
Higher mass stars have become RGs, MS are low mass
So the globular clusters look RED since they are OLD.
Why do astronomers love to study
star clusters?
• First, because all the stars in a given cluster are
nearly the SAME DISTANCE from us.
• So differences in apparent magnitude translate
directly to absolute magnitude differences;
• Plot color-magnitude diagram for the cluster;
• Compare it with a H-R diagram made from stars of
known distances;
• Slide MS part up or down until cluster MS overlaps
known MS
• Then can get the distance to the cluster (and ALL its
stars): m - M = 5 log (d/10 pc)
• This is a version of what is called SPECTROSCOPIC
PARALLAX.
Equal Distances and Equal Ages
• Second, because all the stars in a given cluster are nearly the
SAME AGE.
• Theoretical H-R diagrams have the higher mass stars reaching
ZAMS first; It takes 107 years before 2-3 M stars reach ZAMS;
• meanwhile highest mass stars have left MS to become SN
• by 108 years many high mass stars have become RGs and SGs,
but lowest mass stars still not on ZAMS.
• By 109 yr all low mass stars on ZAMS but TURN-OFF down in A
stars I.e., all O, B and some A will have evolved off MS by then.
• By 1010 yr, all stars down to about Sun's mass will have left the
MS, and the cluster will have big RG, Horizontal Branch and WD
contributions.
• THE FURTHER DOWN THE TURN-OFF IS, THE OLDER THE
CLUSTER.
• Plots of individual clusters H-R diagram confirm this evolution!
H-R
Diagrams
of
Clusters
Turn-offs are
lower for older
clusters as
highest mass
stars leave
MS first
Halo Stars:
0.02-0.2% heavy elements (O, Fe, …),
only old stars
Halo stars
formed first,
then stopped
Disk Stars:
2% heavy elements,
stars of all ages
Disk stars
formed later,
kept forming
THE GALACTIC CENTER
• Until the past 20 years, it was very mysterious,
mainly because: VISIBLE light CANNOT
PENETRATE all the DUST in the DISK
• UV light is absorbed even more strongly!
• Confused by stars between us and the Center
New Tools: Radio
• RADIO
maps show
H I gas,
supernova
remnants, &
synchrotron
emission
from
filaments of
strong
magnetic
fields
Infrared Shows Fast Moving Stars
• penetrates dust
much better,
• IR from tall
mountains,
planes, satellites
• some emission
from very center
and also quite a
few INDIVIDUAL
STARS (RGs,
mostly)
• over the past
decade the
ORBITS of some
such RGs have
been determined
Stars appear
to be orbiting
something
massive but
invisible …
a black
hole?
Orbits of stars
indicate a
mass of about
4 million MSun
IR Movie of Stellar Orbits
This is from the Max-Planck Institute for Extraterrestrial
Physics in Germany, based on their measurements over
10 years. Similar results have come from a Caltech group.
• http://www.mpe.de/ir/GC/index.php
Milky Way Center Zoom
X-rays from the Galactic Center
• Since the earth's atmosphere blocks them, we need
SATELLITES!
• The lower energy (soft) X-rays are absorbed by gas,
• BUT Higher energy (hard) X-rays can penetrate out
to us;
• Some are from SNRs near the Galactic Center, but a
modest amount from the very center, also seen as
the strong radio source: Sagittarius A*
• Q.: WHAT DO ALL THESE
MEASUREMENTS TELL US?
A.: THERE'S A SUPERMASSIVE BLACK
HOLE in the Galactic Center, Sgr A*
•
•
•
•
•
•
•
gas moving very fast (from radio measurements)
orbits of some nearby RGs very fast; those further
away are slower;
X-rays consistent with weak emission from accretion
disk
MSMBH = 3.6 x 106 M•
Right now, little mass falls into the SMBH in the MW,
BUT in the past it probably grew fast and was VERY
LUMINOUS.
Such an ACTIVE GALACTIC NUCLEUS (AGN) were
more common in the earlier days of the universe -we'll discuss them later!
The MW's center does NOT (currently) house an
AGN.
Best Evidence for a Supermassive BH at
the Core of the Milky Way
1) Radio core of Sgr A* is unresolved at 43 GHz, very close
to RS for a 3.6 million solar mass BH, 2) as “weighed” by
orbits of stars measured over a decade in the infrared.