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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. Mean: 53.7 + 20 pt curve = 73.9 Standard deviation: 10.4 Maximum: 76 raw, 92 curved ; Minimum: 36 raw, 56 curved.

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The Milky Way

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the milky way

The Milky Way

Our home galaxy, full of stars, gas andmysterious dark matter

We decompose it into a disk and a halo and a few other parts

second exam results
Second Exam Results
  • 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
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

key parts of the milky way
Key Parts of the Milky Way
  • HALOContains 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 ARMSare wrapped within the dust/thin disk; contain almost all hot, luminous (O and B) stars.
inner parts
Inner Parts
  • GALACTIC BULGEroughly 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 20thcentury.
  • 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
Our Nearest Big Neighbor, M31, the Andromeda Galaxy

Andromeda, about 30 kpc across down to nucleus only 15pc

a limited conception of the mw
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

thought question
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 question1
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
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).
the instability strip
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
motions near the sun
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)
rotation gives mass distribution
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
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
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.
gravitational lensing by brown dwarfs
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
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
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
Orbital Velocity Law
  • 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
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 1012M
spiral galaxies
Spiral Galaxies

M101 is seen face on (similar to MW); NGC 4565 is edge on

stellar populations
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
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
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:
  • Think: traffic jam in space:
density wave analogy to traffic jam
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
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

  • Gas clouds get squeezed as they move into spiral arms
  • Squeezing of clouds triggers star formation
  • Young stars flow out of spiral arms
so can stochastic star formation
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.)
spiral arm facts
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
Stellar Clusters
  • ALL clusters contain many more stars than average within diameters of 3-20 pc. We usually define three types:
  • OPEN (or Galactic) 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
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
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
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
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.
  • Plots of individual clusters H-R diagram confirm this evolution!
H-R Diagrams of ClustersTurn-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
New Tools: Radio
  • RADIO maps show H I gas, supernova remnants, & synchrotron emission from filaments of strong magnetic fields
infrared shows fast moving stars
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
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.


Milky Way Center Zoom

x rays from the galactic center
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*
a there s a supermassive black hole in the galactic center sgr a
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
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.