3
Calibrating
the Period-Luminosity Relationship
using Proper Motion
•Suppose
all planes fly at 500 MPH = 733 ft/sec
–Observe the angle that a plane shifts in
1 second of time
–A plane that moves 10 in 1 sec (900 in 90 sec) is at 42,000 ft
–A plane that moves 20 in 1 sec (900 in 45 sec) is at 21,000 ft
•Works
for stars too: closer stars have
faster “proper motion”
–Can
only get average distances using average proper motion, since any given star might be moving faster or
slower than average
•Harlow
Shapley found 11 Cepheids with proper motion
–Use
average proper motion, average distance, to find average (M-m)
–Let
him replace Leavitt’s relative “m” axis with absolute
“M”
–Now
period Þ M then M-m Þ d
