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How Far Away Are The Stars?

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How Far Away Are The Stars?

- Kepler’s Third Law relates period and distance
- Defines a relative distance scale
- One accurate distance determines everything

- Nearby Lights Appear to Change Position as the Observer Moves
- Can Triangulate to get distance
- Can determine true brightness of lights

- Stars appear identical all over Earth
- They do show slight parallax shift from opposite sides of Earth’s orbit

- 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

- 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

- 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- All pre-Hipparcos distance data is obsolete

- 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

- 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.

- 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

- 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

- 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

- 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.)

- 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

- 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)