![Page 1: The Milky Way PHYS390 Astrophysics Professor Lee Carkner Lecture 19](https://reader033.vdocuments.net/reader033/viewer/2022042821/56649d395503460f94a1261e/html5/thumbnails/1.jpg)
The Milky Way
PHYS390 Astrophysics
Professor Lee Carkner
Lecture 19
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The Milky Way
We can see the band of the Milky Way on a dark night
Nature of galaxy not known until early 20th century
Basic structure Central dense bulge
Old halo with dark matter
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Disk Most visible area of the MW
Diameter Sun is ~8 kpc from center
Two components Thin disk of younger stars Site of current star formation
Thick disk of older stars Fainter and has fewer stars
(few % of thin disk)
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Metallicity We use metal abundance as a proxy for age
Normally use the iron to hydrogen ratio
compared to the sun[Fe/H] = log [(NFe/NH)star / (NFe/NH)sun]
Range: - 0 (exactly like the sun)
Not perfectly reliable Iron comes from Type Ia supernovae and may vary
with region Not completely mixed
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Age of Disk
Thin disk has broad range of metallicities Started forming stars 8 Gya and still going on
today
-0.6 to -0.4 Formed from episode of star formation
between 10 and 11 Gya
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Spiral Arms
Gas, dust, young stars, bright stars, blue stars all concentrated in arms
Hard to map in our galaxy Form via density waves
As clouds orbit the Milky Way, they get stuck in areas of greater density
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The Bulge
The central part of the MW is a thickened bar-shaped bulge Hard for us to see due to extinction
Due to several waves of star formation
Region within which ½ of the light is emitted
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Halo Above and below the disk are
the globular clusters About 150 total Metallicity around -0.8
May be associated with thick disk
Or else would have broken up over the last ~12 Gyr
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Rotation Curve
period of sun ~ 230 million years
Luminous mass looks concentrated at the core
Instead galaxy has flat rotation curve Rotational velocity constant
with increasing distance from center
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Dark Matter
However, orbits of stars exterior to the sun
indicate that there must be a total of about 1012 Msun
Dark matter is about 95% of total galactic mass Cannot be dust, gas or stars
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Mass to Light
Ratio of mass in solar masses to light in solar luminosities
For total Milky Way ~ 60
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Dark Matter Candidates MACHOs
White dwarfs, neutron stars,
black holes, red dwarfs, brown dwarfs
Not enough detected in microlensing surveys
WIMPs Weakly Interacting Massive
Particles
Should be able to detect in very large isolated detector arrays
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Galactic Center
Galactic center is 8 kpc from the sun in the constellation of Sagittarius Can find from distribution of halo globular clusters
Best data from radio, IR and X-ray (not visible)
stars are “isothermal”
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Radio Observations
A complex series of thermal and non-thermal sources
At the center is a very bright, unresolved source, Sgr A* Less than ~2 AU in size
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X-ray Observations
Sgr A* corresponds to a bright X-ray source
Explosions of material must have occurred in the past
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IR Observations
The K band at 2.2 m is used to observe stars close to Sgr A*
Can use Kepler’s third law to find mass of Sgr A*
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The Core
Sgr A* has a mass of 3.7X106 Msun in a space less than 2 AU in size
Destroys near-by stars to provide mass for accretion disk and outflows
Black hole is fairly quiescent
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Next Time
Read 25.1-25.4 Homework: 24.2, 24.30, 24.33, 25.2b,
25.8a, 25.8b