Main-Sequence Fitting Find a star and measure its apparent brightness ( F ).

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Main-Sequence Fitting Find a star and measure its apparent brightness ( F ).

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Main-Sequence Fitting Find a star and measure its apparent brightness ( F ).

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Main-Sequence Fitting

Find a star and measure its apparent brightness (F).

Take its spectrum, identify its spectral type.

Use its spectral type to look up its luminosity

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!

MethodObjectsDistanceCalibrated

Radarsolar system1 AU

Parallaxnearby stars1,000 light-yearsRadar

Main-sequence fittingdistant stars10,000 light-yearsParallax

Cepheid Variables

Find a Cepheid Variable star and measure its apparent brightness (F).

Record its light-curve, and measure the period of variation.

Use its period to look up its luminosity.

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

Find a spiral galaxy and measure its apparent brightness (F).

Measure its rotation velocity from the Doppler Effect.

Use its rotation velocity to look up its luminosity.

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

Find a Type Ia supernova explosion and measure its apparent brightness (F).

Measure the light curve and record the decay rate.

Use its decay rate to look up its luminosity.

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!!

Find a whatever and measure its apparent brightness (F).

Measure some property of the whatever that relates to its luminosity in a known way.

Use that property to look up its luminosity.

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!

MethodObjectsDistanceCalibrated

Radarsolar system1 AU

Parallaxnearby stars1,000 light-yearsRadar

Main-sequence fittingdistant stars10,000 light-yearsParallax

Cepheid variablesnearby galaxies108 light-yearsM.S.F.

Tully-Fisher relationdistant galaxies109 light-yearsCepheids

Supernovaedistant galaxiesfew 109 light-yearsCepheids

Hubble Lawwhole universe1010 light-yearsCepheids

Methods 3, 4, 5, and 6 are all Standard Candle methods!

Scale of the Universe: Tour with Numbers Attached:

Distances between stars: few light-years.

Distances to galactic objects: nebula, star clusters, etc.: few thousand light years

Size of entire Galaxy: 100,000 light years, 100 billion stars

Distances to nearest galaxies: few million light years

Distances to distant galaxies: few billion light years

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.”