Gargantuan collection of stellar and interstellar matter (stars, gas, dust, neutron stars, black holes)

Held together by its own gravity

Bulge: fattened center of galctic disc

Halo: spherical ball of faint old stars around the galctic disk and bulge

Stars whose luminosities change significantly oer relatively short periods of time; some erratically, some regularly

Only small fraction of stars falls into this category

Usually a part of a binary, but can still be a property of the star itself (pulsating variable stars)

- Two types that are significant: RR Lyrae and Cepheid variables

RR Lyrae stars: pulsate in essentially similar ways with only small differences in period

- Observed periods range from .5-1 day

Cepheid stars: different cepheids have very different pulsation periods (1-100 days)

Both can be identified by observing variations in light they emit

Normal stars that experience a brief (few million years) period of instability as a natural part of stellar evolution

- Post-main sequence stars

Cepheid: high luminosity and mass

Lyrae variables: lower mass/luminosity

Apparent brightness ~ luminosity / distance^2

Pulsating variables are helpful because we can determine their luminosity (and therefore distance)

RR Lyrae: around 100x luminosity of Sun

Cepheids: period-luminosity relationship (between pulsation period and luminosity)

Hub of vast collection of matter; around 8 kpc away from Sun

Globular clusters define overall distribution of stars in Galaxy (Harlow Shapley)

Halo: essentially no gast or dust, redder stars, old (Population II)

Bulge: redder stars, 

Disk: bright blue stars, younger open star clusters (Population I)

- Main sequence O- and B-type supergiants are present; account for disks' blue tint

Blueshifted regions moving toward Sun; redshifted regions receding from it

Differential rotation; based on vicinity to center

Only disk has well-defined rotation; bulge and halo do not

Through merger of several smaller systems

Galaxy used to be irregularly shaped; gas distributed throughout volume. Eventually, gas and dust fell to Galactic plane and formed a spinning disk; stars that were already formed created the halo. New stars would inherit rotation of disk and orbit Galactic center.

Going on today; fraction of heavy elements in disk stars should be much greater than what is actually observed

Gas in disk is gradually diluted by gas from halo (5-10 solar masses/year)

Pinwheel-like structures originating close to Galactic bulge and extending outward throughout the disk

30 kpc diameter

Radio observations in all directions of Galactic plane allow us to map out distribution and picture of disk

Arms are part of disk where star formation takes place

Coiled waves of gas compression that move through Galactic disk, squeezing clouds of interstellar gas and triggering process of star formation as they go

Explanation for why spiral arms retain structure despite differential rotation

Predicted to move more slowly than gas and stars within 15 kpc of center

O- and B-type blue giants are most prominent; short lived

Theory for spiral arm formation; formation of stars drives waves

Emission nebula and supernovae send shock waves through surrounding gas and trigger new star formation

Problem: can only produce pieces of spirals

Could be more than 1 process at work

total mass (solar masses) = orbit size (AU)^3 / orbit period (years)^2

Sun's orbital period determined by portion of Galaxy that lies within orbit of Sun

- Computed in above equation

Plot of rotation speed vs. distance from center

Measure orbital motion of stars and gas at greater distances from Galactic center to determine mass of Galaxy. Use radio observations.

Invisible, extensive region that surrounds luminous region of Galaxy

Dwarfs inner halo of stars and globular clusters and extends well beyond 15 kpc radius

Mostly dark matter

Invisible matter whose gravitational effects we can measure and quantify, but the precise nature is not understood

Undectectable at all electromagnetic wavelengths

Only sign of existence is gravitational pull

Use foreground object when observing a star in the distance

Foreground object (such as a brown or white dwarf) is the gravitational lens

Amount of brightening and duration of effect dpeneds on mass, distance, and speed of lensing object

"Saj A star" - center of our galaxy, in heart of Sagittarius A, galactic nucleus