The Milky Way Galaxy

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Estimated distances to globular clusters using variable stars and P-M relationship

Concluded size to be 100kpc with Sun 15kpc from center

Still wrong…didn’t account for dust absorption which makes things look further away

History of Galactic (& Extragalactic) Astronomy

Slide 8

Shapley realized that the globular clusters are all orbiting the center of our Galaxy and map out the true extent of the Galaxy.

History of Galactic (& Extragalactic) Astronomy

Slide 9

In 1920, the National Academy of Science hosted the Great Debate concerning the nature of the Spiral Nebulae: were they island universes outside of the Milky Way?

Shapley had MW size too big and therefore argued “NO”, they are part of the Milky Way

Others at that time believed the Kapteyn model of a much smaller MW and argued “YES”, they are separate galaxies.

In 1922-1924 Edwin Hubble resolved the controversy using the superior 100-inch telescope at Mount Wilson. He observed Cepheid variables in Andromeda and, using the P-M relation (distance method), determined its distance to be 300kpc -- well outside of the MW (still off by a factor of 2 due to poor Cepheid calibrations)

History of Galactic (& Extragalactic) Astronomy

Slide 10

Also in the early 1900’s, the first kinematic studies of the MW revealed the velocities of those globular clusters were ~250 km/s, much higher than the mass of the smaller Kapteyn galaxy model would require. So the galaxy must contain more stars (and mass) than Kapteyn originally thought in order to keep the star clusters from flying off.

First detailed kinematic model (Lindblad 1927) revealed

A spherical component with random motions (~250 km/s)  HALO

A flattened component with rotational motion measured at 200 to 300 km/s near the Sun – DISK

A third component, also spherical, exists in the center of the galaxy – BULGE

Stars here also move on mostly random orbits

Morphology of our Galaxy

Slide 11

The three components of our galaxy (disk, halo and bulge) also differ in the mix of the types of stars they contain

Population I: Hot, blue stars and young open clusters accompanied by gas and dust are primarily found in the disk of the Milky Way

Population II: red stars and older globular clusters are found in the halo of the Milky Way

Morphology of our Galaxy

Slide 12

Plotting stars on HR diagrams showed that the populations differed in age and metallicity (enrichment of elements heavier than Helium):