Chapter 12

Question 1

What does the brightness of a star depend on?

Answer 1

1. Luminosity (how much light energy it gives out in given time)
2. Distance from observer

Question 2

What is a star’s apparent magnitude?

Answer 2

How bright a star looks when seen from Earth

Question 3

What is a star’s absolute magnitude?

Answer 3

How bright a star really is regardless of observed brightness.

Question 4

How is the distance from the earth to a star measured?

Answer 4

Using the apparent magnitude and absolute magnitude of a star.

Question 5

What are standard candles and why are they significant?

Answer 5

Objects that you can measure the brightness of directly.

The method used for finding distances to stars from the earth using their apparent and absolute magnitude only works for standard candles.

Question 6

Give an example of a standard candle

Answer 6

Cepheid variable stars whose brightness changes in a certain pattern.

Finding a Cepheid variable within a galaxy allows us to measure the distance of that galaxy from us.

Question 7

Kepler’s Third Law

Answer 7

T^2 is proportional to R^3

Question 8

Radar velocity ratio

Answer 8

v/c = Tback - Tout/ Tback + Tout

v/c =T2 - T1/ T2 +T1

Question 9

Two way radar pulses derivation

Answer 9

v/2 (T2 +T1) =c/2 (T2- T1)

T2/T1 = 1 + (v/c) / 1 - (v/c)

Question 10

What is the k factor?

Answer 10

One way doppler shift.

Factor by which wavelength is shifted one way.

Question 11

What is the k^2 factor

Answer 11

2 way doppler shift.

Factor by which wavelength is shifted in 2 ways.

Question 12

What is the doppler effect?

Answer 12

Change in frequency of a wave due to the relative motion of the source and the observer.

Question 13

Difference between cosmological and doppler red shift

Answer 13

Cosmological: Light from distant galaxies is redshifted due to wavelength stretching in an expanding universe. Space itself is expanding.

Doppler: Due to relative motion of observer and source.
- moving through space.

Question 14

What does the redshift of spectra from galaxies give?

Answer 14

The recessional velocity

Question 15

Hubble’s law

Answer 15

V = H d

Question 16

Unit of H0 in hubble’s law?

Answer 16

S^-1

Question 17

What is the significance of hubble’s constant?

Answer 17

Rate of expansion of the universe depends on this constant.

Question 18

How to determine the age of the universe

Answer 18

t = 1/H

assuming that universe has been expanding at the same rate for its whole life.

Question 19

What is the size of the observable universe?

Answer 19

Absolute size is unknown and changing.

Observable universe is a sphere with Earth at the centre with a radius that equal to the maximum distance that light can travel during the universe’s age.

Question 20

Redshift as evidence of big bang

Answer 20

Redshift shows that most celestial objects are moving away from us.

Therefore, extrapolating backwards, universe must have started at a single point.

Question 21

Cosmic Microwave Background Radiation

Answer 21

After 300,000 years the universe became transparent since photons no longer lost energy through collisions as the temperature dropped significantly allowing nuclei and electrons to form.

Because the universe has expanded, the wavelengths of this CMBR have been stretched and are now in the microwave region.

Question 22

CMBR as evidence for big bang

Answer 22

Uniform radiation which shows evidence of a uniform and HOT source.