NOTES: The Galactocentric Perspective The Milky Way: Herschel The slab universe Kapteyn The red blood cell universe Harlow Shapley 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 Nebulae are within our island universe (Shapley). Nebulae may be other galaxies (Curtis). Edwin Hubble distance to Andromeda galaxy found from Cepheid Variable 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 distance to Andromeda galaxy found from Cepheid Variable 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.

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. ZOOMING IN!

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.Naval Research Laboratory