1. How Far Away Are The Stars?

2. Distances in the Solar System Kepler’s Third Law relates period and distance Defines a relative distance scale One accurate distance determines everything

3. The Streetlight Analogy: What can the prisoner learn about the outside world?

4. Parallax Nearby Lights Appear to Change Position as the Observer Moves Can Triangulate to get distance Can determine true brightness of lights

5. Parallax and the Distances of Stars Stars appear identical all over Earth They do show slight parallax shift from opposite sides of Earth’s orbit

6. Parallax: pre-1997 Parallax is tiny - was once used as argument against motion of the Earth One second of arc = size of a quarter at 5 km (3 mi.) Parallax angle of nearest star (4.3 l.y.) is 0.75” Accuracy limited by Earth’s atmosphere Fairly accurate to 30-40 l.y., rough to 100

7. Hipparcos Named for ancient Greek astronomer who catalogued the stars High Precision Parallax Collecting System Launched by European Space Agency, 1989 Data Collection 1989-1993 Data Analysis 1993-1997

8. The Hipparcos Data 118,218 stars measured: parallax and motion 22,396 accurate to 10% - a 20-fold improvement Stars out to 200-300 l.y. are known to within 10% 30,000 more accurate to 20% All pre-Hipparcos distance data is obsolete

9. GAIA: the Next Generation To be placed in Earth-Sun L2 point Measure a billion stars out to 100,000 l.y.- 1% of entire galaxy Transmit 1 Mb every 8 seconds for five years Accuracy of five micro-seconds (width of a human hair at 2,500 miles) Data could be available by 2020

10. Beyond Parallax More Distant Lights Show Little Parallax We know how much light nearby lights emit Can use this to estimate distance of faraway lights

11. In nearby towns, lights of known type and brightness can be observed Use brightness to estimate distance

12. We know how much light a town emits per block Can estimate the distance of towns even when individual lights cannot be seen

13. Once we have a good idea how big and bright a typical city is, we can estimate the distance to faraway cities

14. Nearby clusters of cities allow us to gather statistics Statistics can estimate distances to faraway clusters of cities At these distances, some small cities can no longer even be seen.

15. Disasters Sometimes a big fire will outshine the rest of the city Distant fires can be used as distance estimators Sometimes a fire is visible even if the city is too faint to see

16. The Cosmic Distance Scale Makes use of different indicators for different distances Each increase in distance builds on previous distances Faraway distances are only as accurate as nearer distances

17. Distances in our Galaxy Parallax (to 300+ l.y. with Hipparcos) Spectroscopic Parallax (Brightness of stars of known types and absolute brightness) Moving Cluster Method – Radial motions of stars from Doppler Effect – Transverse motions measured directly – Assume velocity distribution uniform

18. Spectroscopic Parallax

19. Variable Stars Henrietta Leavitt, 1917 Measured Magellanic Cloud stars - a lot in a small space Unexpected discovery- some variables have uniform properties Magellanic Cloud stars all about same distance away (170,000 l.y.)

20. Variable Stars as Yardsticks RR Lyrae Stars – Have distinctive light variation curve – All about 100 times as luminous as Sun Cepheid Variables – The brighter, the longer the period – Think of a bell ringing

21. Extragalactic Yardsticks (“Standard Candles”) Cepheids (Governed specifications for Hubble Space Telescope) Supernovae Brightest Galaxy in Cluster Hubble Parameter (25 km/sec/m.l.y. - implies age of universe = 12 billion years)