Main-Sequence Fitting
1. Find a star and measure its apparent brightness (F).
2. Take its spectrum, identify its spectral type.
3. Use its spectral type to look up its luminosity
4. Apply the inverse-square law to calculate the distance.
The relation between spectral type and luminosity
had to be established by stars with parallax
distances (bootstrapping)!
Astronomical Distance Ladder!
Method
Objects
Distance
Calibrated
1)
Radar
solar system
1 AU
2)
Parallax
nearby stars
1,000 light-years
Radar
3)
Main-sequence fitting distant stars
10,000 light-years
Parallax
Cepheid Variables
1. Find a Cepheid Variable star and measure its apparent
brightness (F).
2. Record its light-curve, and measure the period of
variation.
3. Use its period to look up its luminosity.
4. Apply the inverse-square law to calculate the distance.
The relation between period and luminosity had to
be established by stars with main-sequence fitting
distances!
Tully-Fisher
1. Find a spiral galaxy and measure its apparent brightness
(F).
2. Measure its rotation velocity from the Doppler Effect.
3. Use its rotation velocity to look up its luminosity.
4. Apply the inverse-square law to calculate the distance.
The relation between rotation velocity and
luminosity had to be established by galaxies with
Cepheid distances!
Supernovae
1. Find a Type Ia supernova explosion and measure its
apparent brightness (F).
2. Measure the light curve and record the decay rate.
3. Use its decay rate to look up its luminosity.
4. Apply the inverse-square law to calculate the distance.
The relation between light curve decay and
luminosity had to be established by galaxies with
Cepheid distances!
Any Standard Candle Method!!
1. Find a whatever and measure its apparent brightness
(F).
2. Measure some property of the whatever that relates to its
luminosity in a known way.
3. Use that property to look up its luminosity.
4. Apply the inverse-square law to calculate the distance.
The relation between the property and luminosity
had to be established by objects with known
distances!
Astronomical Distance Ladder!
Method
Objects
Distance
1)
Radar
solar system
1 AU
2)
Parallax
nearby stars
1,000 light-years
Radar
3)
Main-sequence fitting distant stars
10,000 light-years
Parallax
4)
Cepheid variables
nearby galaxies
108 light-years
M.S.F.
5)
Tully-Fisher relation
distant galaxies
109 light-years
Cepheids
6)
Supernovae
distant galaxies
few 109 light-years
Cepheids
7)
Hubble Law
whole universe
1010 light-years
Cepheids
Methods 3, 4, 5, and 6 are all Standard Candle
methods!
Calibrated
Scale of the Universe: Tour with Numbers Attached:
1. Distances between stars: few light-years.
2. Distances to galactic objects: nebula, star clusters, etc.: few
thousand light years
3. Size of entire Galaxy: 100,000 light years, 100 billion stars
4. Distances to nearest galaxies: few million light years
5. Distances to distant galaxies: few billion light years
6. Limit of observable universe: 14 billion light years
Distances throughout universe measured with 10%
accuracy!!
“The universe is vastly larger than any known human
purpose could justify.”