Download Lecture 29 Our Galaxy: "Milky Way"

Lecture 29
The Milky Way Galaxy
Disk, Bulge, Halo
Rotation Curve
Galactic Center
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Our Galaxy: "Milky Way"
Milky, diffuse band of light around sky known to ancients.
• Galileo shows to be combined light from large number of faint stars.
Knowledge of structure is based on indirect distance methods for distant
objects:
• For stars close enough for heliocentric parallax (< 200 pc): Diskshaped, stars extend beyond observable region
• For stars with observable spectral types: disk shaped, numbers appear
to fall off with distance from sun. Now known to be only apparent due
to interstellar extinction by dust of about 1 mag/kpc in disk (1 kpc
(kiloparsec) = 1000 pc).
• Most reliable: distances based on stellar "standard candles" like RR
Lyrae stars (post Helium flash stars: pulsation period 1/2 day,
luminosity 50 Lsun). Seen in globular clusters. Shapley (1915) finds
largest numbers of globular clusters centered in region towards
Sagittarius. Suggests is true center of Galaxy
• Modern distance to the center of the Galaxy: 8 kpc.
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Milky Way Regions
• Large number of studies of distances to stars and
interstellar material in Galaxy: four superposed regions. In
all regions the density of stars drops off with distance from
center (no exact boundaries):
From inside out:
• Nucleus: Compact cluster of stars - few 100 parsecs in
diameter. Interstellar gas/dust. Unknown energy source at
very center.
• Central Bulge: Spherical distribution of stars - 5 kpc in
diameter.
• Disk: Stars and gas in disk-shaped volume - 25 kpc in
diameter. Solar System located out toward edge in disk.
• Halo: Spherical distribution of stars and globular clusters.
Goes out beyond disk.
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Milky Way Facts
• Total number of stars: over 1011 (100 billion!)
• Plane ("Milky Way") is highly tilted (62 deg) to
Earth's equatorial plane. "North Galactic Pole" is
just between Leo and Ursa Major.
• Galactic positions are sometimes given in
"galactic latitude and longitude", with longitude 0
in the direction of the center.
• Center (longitude 0) between Sagittarius and
Scorpius
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Galactic Rotation
Galactic material must be in some kind of orbit around the
center, or gravity would pull everything into the center!
• Rotation of Galaxy is real tough to measure (can't actually
see rotation in our lifetime), but get a rough idea as
follows, mainly from doppler radial velocities:
• 1) Assume stars in the flattened part of galaxy (the disk)
are orbiting together, and stars in the spherical part (the
bulge) are just boiling about (orbits are in all directions).
Then can measure rotation of disk stars with respect to the
bulge stars:
– Solar orbital motion - 250 km/sec in direction of Cygnus
(perpendicular to the direction of the center in Sagittarius)
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Motions in the Disk
2) Measure motion relative to the Sun of nearby stars/gas
from doppler effect (radial motion) and "proper motion"
(small angular motions accumulated over years):
• Nearby objects are orbiting at very close to same rate as
Sun (no more than 20 km/sec relative motion)
• Objects in disk that are closer to the Galactic Center than
we are are moving slightly faster, objects farther out are
moving slower. Orbital velocity plotted vs distance from
center is the “rotation curve”
• Interpreted as (nearly) circular motion, with stars closer in
having shorter orbital periods than those farther out
("differential rotation").
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Galactic Orbits and the Mass of the Galaxy
• How long will we have to wait to verify circular orbit
assumption?
• Solar Period = 2 π 8000 pc / 250 km/sec
– = 48000×3×1013 km / 250 km/sec
– = 6x1015 sec = 200,000,000 years!
• If true, the sun is only 25 "galactic years" old.
• How much mass inside sun's orbit is required to bend it
around into a circle? Use Kepler's Third Law!
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Orbital Period = 200,000,000 years
Orbital Size = 8000 pc = 1,600,000,000 AU
Central Mass / Msun = A3/P2 = 1011 !
This is about right if you estimate the number of stars in there and
multiply by an estimated mass/star.
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The Dark Matter Halo
• However, there is a serious, difficult to explain
problem if you try to do a similar estimate using
the orbits of stars more distant from the Galactic
Center:
• Find stars at great distance have orbital speeds just
as large or larger than Sun's => estimate large
mass for Galaxy out there, which cannot be
accounted for by known stars.
• Current explanation: Galactic halo includes mass
that is not stars, and not observable in light –
“Dark matter” (later for this)
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The Galactic Center
The nature of the center of the Galaxy is poorly known.
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Most evidence is from infrared and radio (some strong γ-rays, too).
(interstellar dust blocks visible):
About 1 kpc from center, expanding gas 100 km/sec => some
explosive event about 10 million years ago
About 100 pc across: radio source Sagittarius A (strongest radio source
in sky). Synchrotron radiation (light from accelerated subatomic
particles). Radio images show effects of strong magnetic field.
About 10 pc across: pinwheel-like arrangement of gas
< 1 pc across: "Sagittarius A*" almost point-like radio/infrared/X-ray
source. Orbits of nearby stars => 3 x 106 Msun.
Nucleus: Supermassive Black Hole? Most believe that this much mass
must mean a black hole:
– IR, X-rays from Sag A* varies with t(var) ~ 1 minute => size < 1 AU
– 106 Msun is far above the limit for a neutron star.
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The Milky Way
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Milky Way Structure
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Stellar Orbits in Milky Way
Disk Orbits:
Circular in
Plane
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Bulge Orbits:
Elongated,
Random
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Milky Way Rotation Curve
Position of Sun
Expected from
number of stars
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Galactic Center – X-rays
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Magnetic Arches
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Galactic Center Stellar Orbits
Black Hole ?
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Galactic Center Black Hole
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