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


Parallax
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
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: 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
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
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  • All pre-Hipparcos distance data is obsolete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
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
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






Disasters
Disasters is, we can estimate the distance to faraway cities

  • 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
The Cosmic Distance Scale is, we can estimate the distance to faraway cities

  • 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
Distances in our Galaxy is, we can estimate the distance to faraway cities

  • 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


Spectroscopic parallax
Spectroscopic Parallax is, we can estimate the distance to faraway cities


Variable stars
Variable Stars is, we can estimate the distance to faraway cities

  • 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
Variable Stars as Yardsticks is, we can estimate the distance to faraway cities

  • 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
Extragalactic Yardsticks (“Standard Candles”) is, we can estimate the distance to faraway cities

  • 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)


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