1. The Distance Scale Ladder A Nested Chain of Cumulative Uncertainty

2. Determining Astronomical Distances Determine the AU (see handout) Determine the AU (see handout) Wait 150 years Wait 150 years First parallax done in 1839 (1/3 asec) First parallax done in 1839 (1/3 asec) Stars are now known to be light years away Stars are now known to be light years away What New Problem is suddenly created with this discovery?

3. Problems with Parallax Measurements Atmospheric motions limit position measurement precision of a single set of measurements Atmospheric motions limit position measurement precision of a single set of measurements Have to average position measurements over about 20 years to get reliable parallax Have to average position measurements over about 20 years to get reliable parallax Systematic error could be present (and indeed it is) Systematic error could be present (and indeed it is)

4. This feature Not Real D = 15 Parsecs D = 25 Parsecs

5. Hipparrcos Satellite Data 100,000 stars

6. Luminosity Scale of the MS is determined from stellar parallax

7. So Any systematic error in parallax leads to systematic error in the main sequence Any systematic error in parallax leads to systematic error in the main sequence Systematic error in luminosity scale of Systematic error in luminosity scale of main sequence leads to systematic error in measuring distances to clusters main sequence leads to systematic error in measuring distances to clusters

9. Cluster Distances Stars are all at the same distance Stars are all at the same distance Clusters contain Cepheid Variables Clusters contain Cepheid Variables

10. Cepheid Variables Long Period variables are very bright Long Period variables are very bright Individual ones can be seen in nearby galaxies Individual ones can be seen in nearby galaxies Can be used to determine Galaxy distances Can be used to determine Galaxy distances Hubble Space Telescope greatly extends the range of distances out to which Cepheids are observed Hubble Space Telescope greatly extends the range of distances out to which Cepheids are observed

11. Cepheid Calibration of Tully Fisher Relation

12. Tully Fisher Relation Can be extended to very large distance because galaxy rotational velocities are easy to measure Can be extended to very large distance because galaxy rotational velocities are easy to measure Can reach the undisturbed Hubble flow to measure the true expansion parameter. Can reach the undisturbed Hubble flow to measure the true expansion parameter. All systematic errors accumulate to this point All systematic errors accumulate to this point Expansion parameter tells us expansion age Expansion parameter tells us expansion age