the milky way Flashcards
Thomas Wright’s Milky Way (1750)
The Milky Way is “an optical effect due to our immersion in
what locally approximates to a flat layer of stars.”
First to suggest that faint nebulae were galaxies similar to
the Milky Way, but much further away.
Harlow Shapley (1921)
Noticed Globular Clusters are:
1. Uniformly above & below the MW.
2. Concentrated toward Sagittarius
Observations:
Used RR Lyrae variables to
map the cluster
distribution in space.
Why was Herschel’s strategy for mapping our Galaxy
flawed?
He relied on visual wavelengths, which are obscured by
dust.
Schematic of the Milky Way
Disk is 100,000 light years (30 kpc) in diameter
* Sun ~8 kpc from the center (Sagittarius)
* Galactic Center and disk obscured by dust
Imagine that you could travel at the speed of light.
Starting from Earth, how long would it take you to travel
to the center of the Milky Way Galaxy?
26,000 years
The image below represents the Milky Way Galaxy.
What is the approximate diameter of the white dot?
10,000 light years
Thin disk
400 light years thick
* youngest stars (1-10 Byr)
* gas & dust
* Sun is 70 ly above the mid-plane
Thick disk
2000 light years thick
* older stars (11 Byr)
* 5% the mass of the thin disk
Bulge
Inner region of the Galaxy (within 10,000 ly of the center)
* contains the central bar
* old stars
* little gas and dust
* visible through the disk at infrared wavelengths
halo
Spherical distribution of very old (13 Byr),
faint stars, centered on bulge
* sparsely populated
* extends well beyond (150,000 ly) the
visible light from the disk
* globular clusters
* dark matter
spiral arms
Pattern of hot stars, star clusters,
gas & dust in the disk
tracers:
* O&B Stars
* emission nebulae (HII regions)
* gas and dust Clouds
Spiral Arms in the Milky Way
Two major, several smaller arms
* Sun sits in a minor arm called
the ‘Orion Spur’ (10,000 ly long)
Rotation Speeds
Inner Parts
Rise from Zero to few hundred km/sec
rotation speed outer parts
Nearly constant at a few hundred km/sec
Density Waves
Pass through disk like water waves pass over the
ocean.
* Stars move through the spiral arms.
* Gas clouds try to move through, but some
form stars (collision)
For an orbit
the speed depends only
on the mass inside the orbit:
What is the extra mass if not stars?
dark matter
What observations suggest the mass of the Galaxy goes
much farther out than its visible disk
the rotation curve of the outermost portions of the disk
