The Milky Way Galaxy. The Milky Way Galaxy. An early attempt to locate the Sun’s position in the Milky Way was done by William Herschel (who also discovered Uranus) Herschels’ Milky Way Map (1785). Sun. ~3 kpc. ~15 kpc. Dutch Astronomer Jacobus Kapteyn Model (1922).
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An early attempt to locate the Sun’s position in the Milky Way was done by William Herschel (who also discovered Uranus) Herschels’ Milky Way Map (1785) Sun
~3 kpc ~15 kpc Dutch Astronomer Jacobus Kapteyn Model (1922) He used the apparent brightness and proper motion of stars to come to the same conclusion as Herschel.
So why were they wrong? • Imagine driving through a heavy fog. • To get your bearing perhaps a large watch tower could provide guidance
We need to use globular clusters (collection of galaxies above/below the plane of the galaxy) The Milky Way Disk acts just like a thick fog. To get our bearings…
Globular Cluster – approx 106 stars in a volume of only a few hundred LY across.
Globular Cluster M55 (located in constellation Sagittarius) • Observing the luminosity of these stars has allowed us to determine the distance to the cluster. • This technique allows us to triangulate the location of our Solar System.
The Milky Way Galaxy 35 8
The Milky Way Galaxy • The top view is from the ISAS spacecraft • 25,60,100 micrometer wavelengths (interstellar dust) • Shows the distribution of interstellar dust.
The Milky Way Galaxy • Near IR taken from COBE (1.2, 2.2, 3.4 micrometers) • Note the bulge at the center of the galaxy
The Milky Way Galaxy An optical view of the sky An infrared view of the sky A radio view of the sky
Edge on view of NGC 4565 This is what the Milky Way would probably look like from a great distance
Consider a Hydrogen Atom Spin – Flip transitions give off a radio photon
Composition of The Milky Way Galaxy The entire sky at 21cm - Colors indicate the density of Hydrogen gas
Spin – Flip transitions in Medicine • Magnetic Resonance Imaging
Note that Hydrogen is also concentrated in the Spiral Arms • Galaxy M83 • 12 million LY from Earth
High Density due to slow truck So, what are the Spiral Arms? • Consider the following situation
It is thought that the spiral arms are high density regions (waves) where stars form
Hubble Tuning Fork Diagram
Ordinary Spirals Sa Sb Sc
NGC 891 Edge-On Sb NGC 628 Face-On Sc
Flocculent & Grand Design Spirals NGC 4414 Type Sc NGC 628 Type Sc
Barred Spirals SBa SBb SBc
Elliptical Galaxies E1 E5