Topic 10 - Space

Question 1

How do astronomers use standard candles to determine distances to galaxies? (6 marks)

Answer 1

• Use (stellar) parallax to determine distance to a nearby standard candle
• Measure the intensity of radiation from the standard candle Or Measure λmax and use Wien’s law to determine the (surface) temperature of the standard candle
• Use inverse square law to calculate the luminosity of the standard candle Or Use the Hertzsprung-Russell diagram to determine the luminosity of the standard candle
• Locate standard candle (in nearby galaxy)
• Standard candle has a known luminosity
• Measure intensity of radiation from the standard candle and use inverse square law to calculate distance to nearby galaxy

Question 2

What is meant by redshift? (2 marks)

Answer 2

Redshift is the (fractional) increase in the wavelength received
Due to the source of radiation moving away from the observer.

Question 3

Q: Explain the changes in the appearance of the HR diagram as a star cluster ages (6 marks)

Answer 3

• Main sequence stars evolve into red giants when hydrogen fusion ends
• This occurs first for the most massive stars (upper main sequence)
• Red giants appear above the main sequence with larger surface area and lower temperature
• When helium fusion ends, red giants evolve into white dwarfs
• White dwarfs appear below the main sequence with smaller surface area and higher temperature

Question 4

The ultimate fate of the universe may be a closed universe, but astronomers cannot
be sure what their current models predict.

Explain why astronomers cannot be sure that the universe is closed. (2 marks)

Answer 4

• (For a closed universe) the density of the universe must be greater than the critical density
• And the (average) density of the universe is uncertain

Question 5

Describe how trigonometric parallax can be used to determine the distance to a nearby star. (3)

Answer 5

• Find (angular) displacement of the star (as Earth moves around the Sun) over a 6
  month period
• Measurements are made against the background of (more) distant stars
• Radius/diameter of the Earth’s orbit about the Sun must be known/measured (to
  calculate the distance to the star)

Question 6

How are gravitational potential energy, mass, and gravitational potential changes related?

Answer 6

The change in gravitational potential energy (ΔEp) is given by:
ΔEp = m × ΔVgrav

Question 7

Describe what is meant by dark matter.

Answer 7

• Dark matter has mass
• Dark matter does not emit electromagnetic radiation

Question 8

How do you calculate the approximate age of the universe using the Hubble constant?

Answer 8

Formula: t = 1/H₀