1 1 The Milky Way 2 The Milky Way Almost everything we see in the night sky belongs to the Milky Way. We see most of the Milky Way as a faint band of light across the sky. From outside, our Milky Way might very much look like our cosmic neighbor, the Andromeda Galaxy. 3 First Studies of the Galaxy First attempt to unveil the structure of the galaxy by William Herschel (1785), based on optical observations. The shape of the Milky Way was believed to resemble a grindstone, with the sun close to the center 4 Determining the Structure of the Milky Way Galactic Plane Galactic Center The structure of our Milky Way is hard to determine because: 1) We are inside. 2) Distance measurements are difficult. 3) Our view towards the center is obscured by gas and dust.
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The Milky Way2
The Milky Way
Almost everything we see in thenight sky belongs to the Milky Way.
We see most of the Milky Way as afaint band of light across the sky.
From outside, our Milky Way mightvery much look like our cosmic
neighbor, the Andromeda Galaxy.
3
First Studies of the Galaxy
First attempt to unveil thestructure of the galaxy by
William Herschel (1785), basedon optical observations.
The shape of the Milky Way wasbelieved to resemble a grindstone,
with the sun close to the center4
Determining the Structureof the Milky Way
Galactic Plane
Galactic Center
The structure of our Milky Way is hard to determine because:1) We are inside.2) Distance measurements are difficult.3) Our view towards the center is
The Mass of the Milky WayIf all mass was concentrated in thecenter, Rotation curve would follow amodified version of Kepler’s 3rd law.
Rotation Curve = orbital velocityas function of radius.
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The Mass of the Milky Way (II)Total mass in the disk
of the Milky Way:
Approx. 200 billionsolar masses
Additional mass in anextended halo:
Total: Approx. 1 trillionsolar masses
Most (>80%) of themass is not emitting
any radiation:
dark matter!
One of Vera Rubin's importantcontributions to astrophysics is thecollection of conclusive data pointing tothe presence of dark matter in galaxies.These data are measurements of theorbital velocities of interstellar matter ingalaxies. She studied the variation ofthese velocities with distance from thecenter of the galaxy.
It is assumed that matter orbits aboutthe center of a galaxy owing to acentripetal force which is thegravitational attraction of other matterin the galaxy. Assuming all other matterin the galaxy is luminous,astrophysicists cannot account for thecentripetal accelerations observed.These can be accounted for, however, ifadditional matter is present.
Consequently, Rubin's measurementswere of fundamental importance asempirical evidence for dark matter.
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The Nature of Spiral Arms
Chance coincidence of small spiral galaxyin front of a large background galaxy
Spiral arms appearbright (newly formed,
massive stars!)against the dark sky
background,
but dark (gas and dustin dense, star-forming
clouds) against thebright background of
the large galaxy
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The Galactic Center (I)
Wide-angle optical view of the GC region
galactic center
Our view (in visible light) towards the Galactic center (GC)is heavily obscured by gas and dust:
Extinction by 30 magnitudes
Only 1 out of 1012 optical photons makes itsway from the GC towards Earth!
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Radio View of the Galactic CenterMany supernova remnants;
shells and filaments
Sgr A
Arc
Sgr A*: The center of our galaxy
The galactic center contains a supermassiveblack hole of approx. 2.6 million solar masses.
Sgr A
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Measuring the Mass of the BlackHole in the Center of the Milky Way
By following the orbits ofindividual stars near the centerof the Milky Way, the mass ofthe central black hole could bedetermined to be ~ 2.6 million
solar masses.
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Astrometry of Galactic Center Stars
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Flare of Sag A*
http://www.mpe.mpg.de/ir/GC/index.php
May 9, 2003
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33Chandra X ray image of Sgr A*
Supermassive black hole in the galacticcenter is unusually faint in X rays,
compared to those in other galaxies.
Galactic center region contains many black-holeand neutron-star X-ray binaries.