The Cosmic Distance Ladder Rung 1 Radar Ranging

The Cosmic Distance Ladder

Rung 1: Radar Ranging - Distances in the solar system - Send out radio pulses - Measure how long it takes for them To return - Distance = cy/2

Rung 2: Parallax - The stars within a few hundred light years

Beyond the third rung: Standard Candles d L

Rung 3: Main sequence of clusters as S. C. – Stars in our galaxy

1. Find a star cluster close enough to measure the distance from parallax e. g. Hyades (46 pc) 2. Compare apparent magnitude of MS with cluster farther away (e. g. Pleiades) > distance! Side note: Pleiades 7. 5 times farther away, but more visible? > it’s younger, so its most luminous, massive stars still exist

Rung 4: Cepheid variables as S. C. – Works up to 10 s of millions of parsecs away

CVs: (population I) -Pulsating yellow supergiants (F 6 -K 2), periods days-months, -Driven by doubly-ionized He; at dimmest part of the cycle, the stars radiation heats He III in the outer layers > expansion (increase in L) > cooling and recombination (He III > He) > contraction

Mean luminosity depends on the period of pulsation!

Mean luminosity depends on the period of pulsation! Determine this relation for nearby Cepheids (for which distance, hence luminosity, is known) and use it to calculate distance to more distant ones

Rung 5: The Tully-Fisher relation – Works up to billions of parsecs away

Rung 5 take II: Type Ia supernovae as standard candles White dwarf accretes matter from companion star OR two white dwarfs merge > thermonuclear explosion from runaway Carbon burning results when its mass is tipped over the Chandrasekhar limit

Technically, Type Ia SNe are standardisable candles: use the width of the lightcurves to calibrate to a single, intrinsic brightness

Rung 6: Hubble’s law – works out to the edge of the known universe • Galaxies are moving away from us at a speed proportional to their distance V=H×d • H is called Hubble’s constant

The Cosmic Distance Ladder