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How Far Away Are The Stars?. Distances in the Solar System. Kepler’s Third Law relates period and distance Defines a relative distance scale One accurate distance determines everything. The Streetlight Analogy: What can the prisoner learn about the outside world?. Parallax.
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Distances in the Solar System • Kepler’s Third Law relates period and distance • Defines a relative distance scale • One accurate distance determines everything
The Streetlight Analogy: What can the prisoner learn about the outside world?
Parallax • Nearby Lights Appear to Change Position as the Observer Moves • Can Triangulate to get distance • Can determine true brightness of lights
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
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
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
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• Stars out to 200-300 l.y. are known to within 10% • 30,000 more accurate to 20% • All pre-Hipparcos distance data is obsolete
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
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
In nearby towns, lights of known type and brightness can be observed Use brightness to estimate distance
We know how much light a town emits per block Can estimate the distance of towns even when individual lights cannot be seen
Once we have a good idea how big and bright a typical city is, we can estimate the distance to faraway cities
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.
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
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
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
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.)
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
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)