Presentation on theme: "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."— Presentation transcript:
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
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.
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 10 6 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 top view is from the ISAS spacecraft u 25,60,100 micrometer wavelengths (interstellar dust) u Shows the distribution of interstellar dust.
Near IR taken from COBE (1.2, 2.2, 3.4 micrometers) u Note the bulge at the center of the galaxy
An optical view of the sky An infrared view of the sky A radio view of the sky
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
The entire sky at 21cm - Colors indicate the density of Hydrogen gas
Spin – Flip transitions in Medicine Magnetic Resonance Imaging
Radio Astronomers can map the Milky Way using the Doppler Effect
Galaxy M83 12 million LY from Earth Note that Hydrogen is also concentrated in the Spiral Arms
The Milky Way
So, what are the Spiral Arms? Consider the following situation High Density due to slow truck
It is thought that the spiral arms are high density regions (waves) where stars form
As the galaxy rotates, the spiral arms get tighter and tighter
Sa Sb Sc Ordinary Spirals
NGC 628 Face-On Sc NGC 891 Edge-On Sb
Andromeda Spiral Galaxy
Spiral Galaxy M100
NGC 253 is a Starburst Galaxy
NGC 4414 Type Sc Flocculent & Grand Design Spirals NGC 628 Type Sc
SBa SBb SBc Barred Spirals
E1E5 Elliptical Galaxies
Irregular Galaxies Large Magellanic Cloud Small Magellanic Cloud