1. Variable Stars & The Milky Way

2. Variable Stars
Eclipsing binaries (stars do not change physically, only their relative position changes)
Nova (two stars “collaborating” to produce “star eruption”)
Cepheids (stars do change physically)
RR Lyrae Stars (stars do change physically)
Mira Stars (stars do change physically)

3. Eclipsing Binaries (Rare!)
The orbital plane of the pair almost edge-on to our line of sight
We observe periodic changes in the starlight as one member of the binary passes in front of the other

4. Cepheids Named after δ Cephei Period-Luminosity Relations
Two types of Cepheids:
Type I: higher luminosity, metal-rich, Pop. 1
Type II: lower lum., metal-poor, Population 2
Used as “standard candles”
“yard-sticks” for distance measurement
Cepheids in Andromeda Galaxies established the “extragalacticity” of this “nebula”

5. Cepheids
Henrietta Leavitt (1908) discovers the period-luminosity relationship for Cepheid variables
Period thus tells us luminosity, which then tells us the distance
Since Cepheids are
brighter than RR Lyrae,
they can be used to
measure out to further
distances
Extends cosmic distance ladder out to as far as we can see Cepheids (5 Mpc or more, that is, out to neighboring galaxies)

6. Properties of Cepheids
Period of pulsation: a few days
Luminosity: suns
Radius: solar radii

7. Properties of RR Lyrae Stars
Period of pulsation: less than a day
Luminosity: 100 suns
Radius: 5 solar radii

8. Distance Measurements with variable stars
Extends the cosmic distance ladder out as far as we can see Cepheids – about 50 million ly
In 1920 Hubble used this technique to measure the distance to Andromeda (about 2 million ly)
Works best for periodic variables
15 Mpc takes us to neighboring galaxies

9. Cepheids and RR Lyrae: Yard-Sticks
Normal stars undergoing a phase of instability
Cepheids are more massive and brighter than RR Lyrae
Note: all RR Lyrae have the same luminosity
Apparent brightness thus tells us the distance to them!
Recall: B  L/d2

10. The Milky Way Appears as a milky band of light across the sky
A small telescope reveals that it is composed of many stars (Galileo again!)
Our knowledge of the Milky Way comes from a combination of observation and comparison to other galaxies

11. How do we know?
Obviously a bogus picture of our milky way!
Question: How can we say anything about our Milky Way, if we cannot see it from outside?

12. Enter: the Genius William Herschel (XVIII century) Simple model:
Assumed all stars have the same absolute brightness
Counts stars as a function of apparent magnitude
Brighter stars closer to us; fainter stars further away
Cut off in brightness corresponds to a cut off at a certain distance.
Conclusion: there are no stars beyond a certain distance

13. Herschel’s Findings
Stars thinned out very fast at right angles to Milky Way
In the plane of the Milky Way the thinning was slower and depended upon the direction in which he looked
Flaws:
Observations made only in visible spectrum
Did not take into account absorption by interstellar gas and dust

14. Discovering other Island Universes
Data: Lots of nebulous spots known in the night sky
Questions: What are they? All the same? Different things?
Need more observations!
Build bigger telescopes
(The Leviathan of Parsonstown shown, 1845
Biggest telescope of the World until 1917)

15. The first nebula discovered to have spiral structure: M51

16. Enter: next genius
Harlow Shapley used variable stars, e.g. RR Lyrae stars, to map the distribution of globular clusters in the galaxy
Found a spherical distribution about 30 kpc (30,000 pc) across
This is the true size of the galaxy
Sun is (naturally!) not at the center – it’s about 26,000 ly out

17. Standing on the shoulders of Giants
Shapley used methods developed by others to measure the distance to globulars
Cepheid variables show luminosity-period correlations discovered by Henrietta Leavitt
Shapley single-handedly increase the size of the universe tenfold!

18. Structure of a Spiral Galaxy
Three main parts of a galaxy:
Bulge (center of galaxy)
Disk (rotating around center)
Halo (orbiting around bulge with randomly inclined orbits)

19. Properties of Bulge, Disk and Halo
Disk Halo Bulge
Highly flattened spherical football-shaped
young and old stars only old stars young and old stars
has Gas and dust none lots in center
Star formation none since 10 billion yrs in inner regions
White colored, reddish yellow-white
blue spiral arms