Transcript The Milky Way Galaxy

The Milky Way Galaxy
Astronomy 315
Professor Lee Carkner
Lecture 17
Upcoming Extra Credit
Sigma Xi Science Café
6:30 pm, Thursday, April 21
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Planetarium Open House
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Via Lactia
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The band forms a complete ring
We seem to be in the center of a disk of stars
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Clouds of gas and dust block our view so it is
hard to see beyond our local region
Our View of the Milky Way
The Milky Way from Outside
Discovering The Galaxy
In the early part of the century Harlow Shapley
found the distance to globular clusters using
Cepheid variables
Globular clusters
Unlike stars in the disk, we can see distant globulars
Cepheid variables
If we can find luminosity and flux we can get distance (F =
L/4pd2)
The Center
Shapley found the distance to the globulars
and plotted their positions
He found:
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We are not at the center of the galaxy and the
disk extends out much further than we can
easily see
Changing Views of the Galaxy
How Do We Learn About The
Milky Way?
Optical observations
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Radio observations
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Infrared observations
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Observing other galaxies
Since we are in the middle of the Milky Way we
can’t get an overview of it
Structure of the Milky Way
Disk
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Nucleus
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Halo
Spherical distribution of old stars and
globular clusters around disk and bulge
Edge-on and Face-on
The Disk
The disk is very thin
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Younger stars and star forming regions near
the center, older stars above and below
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Disk exhibits differential rotation (inner parts
rotating faster than outer)
Differential Galactic Rotation
Mass and Orbits
Finding the properties of a star’s orbit
allows us to find the mass internal to
the orbit

M = a3/P2
M = mass (in solar masses)
P =
a =
You must use the correct units!
Spiral Structure
We know that other galaxies have spiral
structure, but it is harder to see the Milky
Way’s
We find spiral arms by tracing:
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They are not uniformly distributed but are
found in a loose spiral structure
How do spiral arms form?
Local Spiral Arms
Density Waves
Spiral arms are like traffic jams
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This can trigger star formation in the arms
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The clouds eventually move out the other side
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The spiral arm material changes, only the pattern
stays the same
Density Wave
At the Core
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The nucleus is the hardest part of the galaxy
to observe due to all the gas and dust
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One, Sagittarius A* (Sgr A*), may be the
center of the galactic core
What is it?
Properties of the Core
Stars near the core are very close together and
moving very fast
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Sgr A* is emitting enormous amounts of
energy
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A black hole
Massive Black Holes
Sgr A* does not move and may have jets and
an accretion disk
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Can get mass from orbit of star S2:
Period =
Semimajor axis =
Formed from stars and clouds of material
falling into the center
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Our own is very hard to observe
The Halo
The halo are stars orbiting in a large sphere
around the galaxy
The halo is composed of old stars
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Halo stars -- Population II -- metal poor
Disk stars -- Population I -- metal rich
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Halo stars formed formed early from
relatively unprocessed material
Globular Clusters
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Size:
Shape:
Contents:
Globulars are in elliptical orbits around the
galactic center
Unlike open clusters in the disk, globular
clusters are very tightly gravitationally bound
History of the Milky Way
How did the galaxy form?
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Basic theory has Milky way forming from
smaller protogalaxies
Gas stripped out to form disk
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Remains of protogalaxies form halo
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Next Time
Quiz #2
Covers lectures 10-16