Download Lecture 29 - Empyrean Quest Publishers

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-->Distance (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-->dark matter (about 10 x the visible
mass–much of it in halo).
Wraparound view of the MILKY WAY.
Dust obscures our view of the far side of the galaxy.
William Herschel, a musician,
built the world’s 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!
Herschel concluded that the universe was a single infinite slab.
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’.
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.
Shapley, Curtis Debate: 1920
Is the Milky Way an Island Universe?
Shapley—Yes.
Curtis—No.
There was not enough evidence to resolve the question.
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.
Current view of Milky Way
structure.
The halo is composed of brown dwarfs and dark matter.
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.
21 cm Doppler shift tells us how arms are moving.
Galaxy rotates once every 225 million years (sun-240 km/s).
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).
The halo (in blue) contains MACHOS—Massive
Compact Halo Objects. Many of them are brown
Dwarfs.
Density wave theory: a gravity wave flows through galactic
disk, compressing, & causing star formation in its wake.
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.
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.