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Galactic Astronomy
Topics
The Milky Way Galaxy;
Milky Way Anatomy, esp. the Galactic Core;
Milky Way Formation;
Galaxy types;
Motivation
Galaxies are like the molecules of our Universe.
They have mysterious masses at their cores,
There are several main kinds.
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The Milky Way Galaxy
Visible…
The glow of stars and star formation regions.
Infrared…
Cirrus dust emissions, star formation regions.
21-cm Radio…
Atomic hydrogen gas.
Continuum radio…
Synchrotron radiation–energetic sources.
X-rays…
Very hot gas.
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The Milky Way Galaxy
Stellar Disk
– About 30 kpc in diameter, 0.3 kpc thick (100:1 ratio);
– Compare to a CD (120:1);
– It contains about 100-400 billion stars.
Gas Disk
– About 3kpc (10× thicker) than the stellar disk.
Overall Stats
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About 13.2 billion years old;
About 600-1000×109 M;
The Sun is about 10 kpc from the galactic center;
The Sun travels at about 220 km/s, taking about
250×106 years to orbit the galaxy.
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Disk and Spheroidal Components
Disk Component
Consists of stars that orbit around the galactic center in an ensemble of
flat orbits.
Spheroidal Component
Consists of stars that buzz around the galactic center in a roughly
spherical cloud. It includes “halo stars”, globular clusters, and even
small satellite galaxies.
Halo
The (very old) stars that orbit the galaxy, at distances up to 30 kpc.
Some halo stars may occur as far as 60 kpc from the galactic core.
Bulge
A greater concentration of stars, in general similar to halo stars. About
3 kpc in radius, bar-shaped (2-3× longer than wide). Slightly higher
metallicity than the halo.
Nucleus
A spherical concentration of stars, including many that are young.
Stellar Populations
Population I stars
Young, with many heavy elements; primarily in the disk.
Population II stars
Mainly in the halo and bulge, they are older than those in the
disk and contain only about 1% of the heavy elements of
Pop I stars.
Population III stars (theoretical)
Pure H & He stars, the first stars born after the Big Bang
when very little metals existed. Stellar models tell us that
Pop III stars would have been massive, shortlived, and none
would have survived to current times.
Galactic Interstellar Medium
Composition
70% H, 28% He, 2% heavier elements. 99% is gas, 1% is
dust.
Distribution
Clumpy, with clouds a few—few hundred LY across.
Density and Mass
Even the densest clouds (104 molecules/cm3) exceeds
current vacuum technology (105-107 molecules/cm3).
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Galactic Interstellar Medium
Effects on Light
– On average, interstellar dust extinguishes light by
40% for each 1 kpc; 1% after 5 kpc;
– Dust reradiates the absorbed energy in the
infrared;
– The most conspicuous type of bright emission
nebula are HII regions.
Detection
– Cold ISM is detected by 21-cm radiation;
– Dense ISM is detected by infrared or microwave
radiation;
– Hot ISM is detected by visible radiation;
– Extremely hot ISM is detected by X-rays.
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The Milky Way’s Spiral Arms
Arms—two or four or more?
– Perseus arm (includes 3-kpc arm);
– Scutum-Centaurus arm;
– Also perhaps Norma & Outer arm;
– Also perhaps Carina & Sagittarius;
– Also perhaps Orion & Cygnus arms (this includes our
Sun).
Tracing spiral arms
– HII Regions (visible radiation);
– Clusters of O and B stars (visible radiation);
– Clouds of interstellar hydrogen (21-cm radiation);
– Molecular clouds (CO radiation).
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The Milky Way’s Spiral Arms
Wind-up problem
Recall that the Galaxy has spun about 20-25 times.
Density Wave Theory
1. The spiral arms sweep through the Galaxy with an orbital
period of approximately 500×106 years (about twice the
Sun’s orbital period).
2. This compresses the interstellar medium a small amount.
3. The compression of the interstellar medium triggers star
formation.
4. The O and B stars, and HII regions light up the arms.
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The Galactic Core
Must be observed at radio, infrared, and x-rays.
Sgr A*
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Streamers of hot gas form a spiral-like pattern;
A bright radio source;
Only about 0.3 au across;
About 4.3×106M;
Near the Sun, there is 1 star/pc3—near Sgr A* there are
105 star/pc3, i.e., they are just a few light-weeks apart;
– They are moving so fast, in response to gravity, that we
can watch them orbit some hidden, ultramassive, compact
object;
– A black hole 0.3 au across would be 7.5×106M.
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Our Galaxy’s Story
1) 11 billion years ago, protogalactic clouds collapsed into
Pop III stars and globular clusters, which began the
pollution process.
2) The Pop III stars and clouds merged into a protogalactic
cloud larger than the Galaxy’s current halo.
3) The bulge stars and halo globular clusters continued to
orbit within their respective regions, while the gas
settled into a rotating disc which developed spiral arms.
4) Each generation of stars—Pop III, Pop II, and Pop I—
increased in metallicity.
5) X-ray gas in the galactic neighborhood is produced by
supernovae and blowouts.
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Galactic Bestiary: Spirals
Spirals (S) and Spiral Barred (SB)
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Blue-white disks (Pop I stars);
Yellowish bulges (Pop II stars);
Bars are often present;
Spiral arms are usually (but not always) present.
Subdivisions
– a=tightly wound arms; big bulge;
– b=intermediate in all respects;
– c=loosely wound arms; tiny, starlike bulge.
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Galactic Bestiary: Ellipticals
Giant and Dwarf Ellipticals
– They look like the spheroidal stellar part of
spiral galaxies;
– Dwarf ellipticals are109 M;
– Giant ellipticals are several×1012 M.
Subdivisions
– E0 ellipticals are round;
– E1-E7 are elongated;
– E7 are the most elongated.
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Galactic Bestiary: Irregulars
They are usually white and dusty, indicating the
existence of young, massive stars.
Irregular galaxies were more common in the early
Universe.
Many irregular galaxies appear to be the result of galactic
collisions and mergers.
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