1. Galactic Astronomy Topics Motivation 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.

2. Method I: 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.

3. Method II: 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.

4. 21 cm radiation Tracing spiral arms
There is a minute energy difference associated with atomic (neutral) hydrogen electron orientations.
The electron flips, with a half-life of 107 years.
A 105 solar mass H cloud has about 1062 H atoms!
Collisions in the ISM re-excite atoms over time.
Tracing spiral arms
Clouds of interstellar hydrogen (21-cm radiation);
Molecular clouds (CO radiation);
HII Regions (visible radiation);
Clusters of O and B stars (visible radiation). 4 4

5. The Milky Way’s Spiral Arms
Wind-up problem Recall that the Galaxy has spun about times.
Density Wave Theory
The spiral arms sweep through the Galaxy with an orbital period of approximately 500×106 years (about twice the Sun’s orbital period).
This compresses the interstellar medium a small amount.
The compression of the interstellar medium triggers star formation.
The O and B stars, and HII regions light up the arms. 5 5

6. The Milky Way’s spiral arms
Two arms
Perseus arm
Scutum-Centaurus arm
More arms?
Carina/Sagittarius arm?
Orion spur (w/our sun)?
Norma & Outer arm?
3 kpc ring, 5 kpc ring? 6 6

7. Bars and bubbles Bars are poorly understood in galaxies.
Some scientists think galaxies develop bars over time;
Others think bars dissipate over time;
Others think bars may appear/disappear periodically.
Bars appear to stream ISM inward, to form stars—many barred galaxies have active cores, as a consequence of this funneling of material towards the core.
Two enormous bubbles detected at gamma rays, x rays, and radio waves;
each is about 7.7 kpc across.
Originating from outflow from star formation in the central 200 pc of the galaxy. 7 7

8. The Galactic Core Must be observed at radio, infrared, and x-rays.
Sgr A*
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. 8 8

9. 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. 9 9

10. Galactic Bestiary: Spirals
Spirals (S) and Spiral Barred (SB)
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. 10 10

11. 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. 11 11

12. 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. 12 12

13. Appendix: 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. 13 13

14. 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 ( molecules/cm3). 14 14

15. The Milky Way Galaxy Stellar Disk Gas Disk Overall Stats
About 30 kpc in diameter, 0.3 kpc thick (100:1 ratio);
Compare to a CD (120:1);
It contains about billion stars.
Gas Disk
About 3kpc (10× thicker) than the stellar disk.
Overall Stats
About 13.2 billion years old;
About ×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. 15 15