The Milky Way Galaxy

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Slide 1

Our Galaxy The Milky Way

Chapter 19

Slide 2

Overview & History

Our Galaxy is a collection of stellar and interstellar matter – stars, gas, dust, neutron stars, black holes – held together by gravity.

Our view of the Galaxy….

Slide 3

History of Galactic (& Extragalactic) Astronomy

1610 - Galileo discovered the Milky Way is comprised of many stars

1755 - Immanuel Kant theorized that the galaxy has a planar structure, some nebulae might actually be entire other galaxies or island universes

1774 -1781 - Messier catalog compiled including Andromeda galaxy as M31

1781-1802 - William and Caroline Herschel conducted first “all-sky survey” and cataloged 5000 nebulae, resolving some into their individual stars

1845 - William Parsons (Lord Rosse), using a 72-inch telescope, classified the nebulae into featureless ellipticals and whirlpool-like spiral nebulae

Slide 4

Slide 5

Herschel could not account well for the effects of dust.

More dust along the disk causes the distribution of stars to drop-off artificially – objects more than a few kpc from the Sun are obscured by dust.

1785 - Herschel attempted to determine the shape and size of Galaxy

Assumptions:

All stars have same intrinsic brightness

Star are arranged uniformly throughout the MW

He could see to the edge of the MW

History of Galactic (& Extragalactic) Astronomy

Slide 6

Weird Units of Distance in Astronomy:

Astronomical Unit (AU) = 93 million miles or 1.5 x 108 km

Distance between Earth and Sun

Light Year = 9.4 x 1012 km

Distance light travels in one year

Parsec = 3 x 1013 km

(or 3.2 light years)

Distance ???

pc (parsecs)

kpc (kiloparsecs)

Slide 7

Kapteyn (early 1900s) used stellar parallax to estimate the true size of the Galaxy  Kapteyn Universe

10kpc diameter and 2kpc thick with the Sun less than a kpc from the center (rather heliocentric)

Tried to estimate scattering due to ISM gas but determined it to be insignificant (most obscuration is due to dust absorption which has a smaller wavelength dependence)

Shapley (1919) observed that globular clusters are distributed asymmetrically in the sky and that if one assumes they are distributed about the center of the galaxy, this implies the Sun in not near the center of the Galaxy