1.2 Distance Ladder

Question 1

How many steps are on the distance ladder?

Answer 1

4

Question 2

What is the zeroth step on the distance ladder?

Answer 2

1au = 1.5E11m

Earth-sun distance

Question 3

What is the first step on the distance ladder?

Answer 3

Geometric/trigonometric parallaxes

1pc = 3.09E16m = distance at which an object has a parallax of 1”

Question 4

How is a parsec calculated?

Answer 4

d = 1au / 1 arc second

1 arc second = pi/180 * 1/3600 = 4.8E-6 rad

Question 5

Why does the parsec calculation use 1 arcsecond?

Answer 5

That is the turbulence in the Earth’s atmosphere, and it limits observation

Question 6

What is the parallax on the ground of Earth?

Answer 6

Parallax > 0.01”, d< 100pc

Question 7

What is step two on the distance ladder?

Answer 7

Spectroscopic parallaxes and main sequence fitting

Question 8

What is the hypothesis involved when using step 2, Spectroscopic parallaxes and main sequence fitting?

Answer 8

• Have a calibrated HR diagram
• Know spectra/colours of objects on the HR diagram
• Colours don’t change with distance, only flux does

Question 9

Which hypothesis was incorrect for step 2, Spectroscopic parallaxes and main sequence fitting?

Answer 9

That colours do actually change with distance due to extinction

Question 10

What is step 3 on the distance ladder?

Answer 10

Cepheid pulsations - relies on a standard candle

Question 11

How does the brightness of a cepheid change over time?

Answer 11

It changes periodically due to pulsations

M = -1.3 -3.5logP where P is in days and M is the absolute magnitude

Question 12

What is the absolute magnitude?

Answer 12

M = magnitude of a source if placed at 10pc

Question 13

What is the rough diameter of the milkyway?

Answer 13

100kpc