an early attempt to locate the sun’s position in the milky way was done by william herschel (who...
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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
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Dutch Astronomer Jacobus Kapteyn Model (1922)
~15 kpc
~3 kpc
He used the apparent brightness and proper motion of stars to come to the same conclusion as Herschel.
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So why were they wrong?
• Imagine driving through a heavy fog.• To get your bearing perhaps a large watch tower could provide guidance
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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…
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Globular Cluster – approx 106 stars in a volume of only a few hundred LY across.
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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.
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The top view is from the ISAS spacecraft 25,60,100 micrometer wavelengths (interstellar dust) Shows the distribution of interstellar dust.
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Near IR taken from COBE (1.2, 2.2, 3.4 micrometers) Note the bulge at the center of the galaxy
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An optical view of the sky
An infrared view of the sky
A radio view of the sky
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This is what the Milky Way would probably look like from a great distance
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Consider a Hydrogen Atom
Spin – Flip transitions give off a radio photon
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The entire sky at 21cm - Colors indicate the density of Hydrogen gas
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Spin – Flip transitions in Medicine
Magnetic Resonance Imaging
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Radio Astronomers can map the Milky Way using the Doppler Effect
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Galaxy M83 12 million LY
from Earth
Note that Hydrogen is also concentrated in the Spiral Arms
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The Milky Way
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So, what are the Spiral Arms? Consider the following situation
High Density due to slow truck
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It is thought that the spiral arms are high density regions (waves) where stars form
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As the galaxy rotates, the spiral arms get tighter and tighter
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Sa
Sb
Sc
Ordinary Spirals
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NGC 628Face-On Sc
NGC 891Edge-On Sb
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Andromeda Spiral Galaxy
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Spiral Galaxy M100
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NGC 253 is a Starburst Galaxy
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NGC 4414Type Sc
Flocculent & Grand Design Spirals
NGC 628Type Sc
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SBa
SBb
SBc
Barred Spirals
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E1 E5
Elliptical Galaxies
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Elliptical Galaxies
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Irregular Galaxies
Large Magellanic Cloud Small Magellanic Cloud
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Irregular Galaxy
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Galactic Clusters
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