1. Galaxies

2. Key Features of Galaxies
Galactic disk – an immense, circular, flattened region containing most of our Galaxy's luminous stars and interstellar matter
Galactic bulge – Thick distribution of warm gas and stars around the center of a galaxy
Galactic halo – region of a galaxy extending far above and below the galactic disk where globular clusters and other old stars reside

5. Spirals
bulge and disk with a spiral structure; the tighter the arms the later the letter in the classification scheme; barred spirals have a bar of interstellar material across the center Stars are forming in the sprial arms (bright O and B type stars)

6. Elliptical
smooth and regular; no disk or bulge; fuzzy football Contain little or no cool gas and dust; no evidence of young stars, made of mostly old-reddish, low mass stars

7. Lenticular
look smooth and regular; has a disk and a bulge; spiral galaxies with NO arms

8. Irregular
no well-defined shape, symmetry or structure

9. Spiral Structure Spiral density waves – coiled waves of gas
Differential rotation makes it impossible for any large-scale structure “tied” to the disk material to survive
Spiral density waves –
coiled waves of gas
compression that move
through the galactic disk

11. Measure the Size of Our Galaxy
Variable Stars – stars whose luminosity changes with time
Examples: Eclipsing binary stars
Pulsating variable stars – which have a consistent pattern
RR Lyrae – luminosity changes in a characteristic way and all have about the same average luminosity
Cepheid variables – stars whose luminosity varies in a characteristic way, rapid rise in brightness followed by a decline.
Determine the luminosity, compare with its’ apparent brightness yields distance

12. Why pulsating? Opacity – how much light is let through
Opacity increase – the internal pressure increases because the radiation is trapped
Opacity decrease – the internal pressure decreases and the star shrinks
Evolved past the main-sequence

13. Instability Strip

14. Period-Luminosity relationship long periods, high luminosity

15. Tully-Fisher Observation of how fast a galaxy rotates
Looking at a particular emission line on the emission spectrum (in this case neutral hydrogen)
The line becomes smeared because of the rotation
Doppler Effect
Red shift as going away
Blue shift as coming towards

16. Visual Example

17. Measure Even Further Out
Standard Candles – easily recognizable astronomical objects whose luminosities are confidently known
Have a well defined luminosity
Be bright enough to be seen at a large distances

18. To the outer reaches
Hubble’s Law – the rate at which a galaxy recedes is directly proportional to its distance from us
Cosmological redshift – as its moving away, the Doppler effect comes into play and it is redshifted
Recessional velocity = Hubble’s constant x distance
H0 = 70km/s/Mpc