NOTES: The Galactocentric Perspective

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NOTES The Galactocentric Perspective The
Milky Way Herschel 1800--The slab
universe Kapteyn 1900--The red blood cell
universe Harlow Shapley 1917--We are not at the
center of the disk. Globular clusters orbit
galactic center, sun 2/3rds way out. He used
proper motions of Cepheids--gtDistance (11
stars). Shapley-Curtis debate 1920--Nebulae
are within our island universe
(Shapley). Nebulae may be other galaxies
(Curtis). Edwin Hubble 1923--distance to
Andromeda galaxy found from Cepheid Variable.
2.25 Mill. vs 100,000 LY--Milky way size. Spiral
arms 21 cm radio radiation allows us to see
through dust. Its Doppler
shift tells us how arms are moving. Galaxy
rotates once every 225 million years (sun-240
km/s). Differential rotation--spiral arms
should wind up in 50 turns. Galactic rotation
curves don't obey Newton's Laws for the
visible matter--gtdark matter (about 10 x the
visible massmuch of
it in halo).
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Wraparound view of the MILKY WAY. Dust obscures
our view of the far side of the galaxy.
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William Herschel, a musician, built the worlds
largest telescope with his wages as a musician
for King George.
By 1800, he and his wife had done a survey of
the important objects in the entire sky!
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Herschel concluded that the universe was a single
infinite slab.
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With better observations, Kapteyn concluded that
the Universe was shaped like a red blood cell
and finite, with the sun close to the
center. You might call this an island universe.
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Harlow Shapley gave us a Copernican
Revolution, when he proved we are not even near
the center of the universe.
If we were, the globular clusters hovering about
the Milky Way should be evenly distributed. By
finding the center of their orbits, he
calculated the distance to the center of the
galaxy, though dust made him wrong by a factor
of two.
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Shapley, Curtis Debate 1920 Is the Milky Way
an Island Universe?
ShapleyYes. CurtisNo.
There was not enough evidence to resolve the
question.
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Edwin Hubble later proved there were other island
universes in 1923--distance to Andromeda galaxy
found from Cepheid Variable. 2.25 Mill. vs
100,000 LY--Milky way size.
This was another Copernican Revolution! No
center.
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Current view of Milky Way structure.
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The halo is composed of brown dwarfs and dark
matter.
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21 cm radio emission allows us to peer
through The dust and plot the shape of our
galaxy, The Milky Way
Hydrogen spin flip of electrons emits radio waves.
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21 cm Doppler shift tells us how arms are
moving. Galaxy rotates once every 225
million years (sun-240 km/s).
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Galactic rotation curve doesn't obey Newton's
Laws for the visible matter, implying dark matter
exists (about 10 x the visible mass much of it
in halo).
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The halo (in blue) contains MACHOSMassive Compact
Halo Objects. Many of them are brown Dwarfs.
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Density wave theory a gravity wave flows through
galactic disk, compressing, causing star
formation in its wake.
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ZOOMING IN!
Frames A and B show the whole Galaxy and Galactic
bulge. Frame C shows the increasing
concentration of gas and dark nebula outside the
core. Frame D shows the mix of old stars and
gas, and Frame E displays the ring of gas outside
the Galactic center. Inside of a parsec an
accretion disk forms, ending at the edge of a
galactic mass black hole.
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This panoramic view of the center of our Milky
Way Galaxy is at a radio wavelength (color) of
approximately 1 meter. This image was produced by
N. Kassim and collaborators at the Naval Research
Laboratory. It indicates possible black hole
activity.