The Distance Scale LadderThe Distance Scale Ladder

Determining Astronomical Determining Astronomical DistancesDistances

Determine the AU (see handout)Determine the AU (see handout)Wait 150 yearsWait 150 yearsFirst parallax done in 1839 (1/3 asec)First parallax done in 1839 (1/3 asec)Stars are now known to be light years Stars are now known to be light years

awayaway

What New Problem is suddenly created with this discovery?

Problems with Parallax Problems with Parallax MeasurementsMeasurements

Atmospheric motions limit position Atmospheric motions limit position measurement precision of a single set of measurement precision of a single set of measurementsmeasurements

Have to average position measurements Have to average position measurements over about 20 years to get reliable parallaxover about 20 years to get reliable parallax

Systematic error could be present (and Systematic error could be present (and indeed it is)indeed it is)

This feature Not Real

D = 15 ParsecsD = 25 Parsecs

Hipparrcos Satellite Data

100,000 stars

Luminosity Scale of the MS is Luminosity Scale of the MS is determined from stellar parallaxdetermined from stellar parallax

SoSo

Any systematic error in parallax leads to Any systematic error in parallax leads to systematic error in the main sequencesystematic error in the main sequence

Systematic error in luminosity scale ofSystematic error in luminosity scale of

main sequence leads to systematic error in main sequence leads to systematic error in measuring distances to clustersmeasuring distances to clusters

Cluster DistancesCluster Distances

Stars are all at the same distanceStars are all at the same distanceClusters contain Cepheid VariablesClusters contain Cepheid Variables

Cepheid VariablesCepheid Variables

Long Period variables are very brightLong Period variables are very bright Individual ones can be seen in nearby Individual ones can be seen in nearby

galaxiesgalaxiesCan be used to determine Galaxy Can be used to determine Galaxy

distancesdistancesHubble Space Telescope greatly extends Hubble Space Telescope greatly extends

the range of distances out to which the range of distances out to which Cepheids are observedCepheids are observed

Cepheid Calibration of Tully Fisher Cepheid Calibration of Tully Fisher RelationRelation

Tully Fisher RelationTully Fisher Relation

Can be extended to very large distance Can be extended to very large distance because galaxy rotational velocities are because galaxy rotational velocities are easy to measureeasy to measure

Can reach the undisturbed Hubble flow to Can reach the undisturbed Hubble flow to measure the true expansion parameter.measure the true expansion parameter.

All systematic errors accumulate to this All systematic errors accumulate to this pointpoint

Expansion parameter tells us expansion Expansion parameter tells us expansion ageage