Astrophysics Questions

Question 1

Explain how Earth-based parallax measurements can be used to determine the distance of star clusters from the Earth.

Answer 1

1. The star is viewed from two positions at 6 month intervals.
2. The diameter/radius of the Earth’s orbit about the Sun must be known.
3. The change in angular displacement of the star against background of fixed stars is measured.
4. Trigonometry is used to calculate the distance to the star.

Question 2

State what is meant by a standard candle.

Answer 2

A standard candle is an astronomical object in space of known luminosity

Question 3

Describe how astronomers use standard candles

Answer 3

1. Standard candle’s brightness (on Earth) is measured.
2. Use inverse square law ( F=L/4pid^2)
   where:
   F= Intensity /flux
   L= luminosity of standard candle
   d^2= distance from object to source
   3.(Hence) distance to standard candle is calculated/determined.

Question 4

Explain why stars have to be within a certain distance from the Earth for trigonometric parallax to be useful.

Answer 4

1. trigonometric parallax is the change in position of a star against the background of more distant stars.
2. If star is too distant the angle is too small to measure

Question 5

Explain what is meant by redshift.

Answer 5

1. Redshift is the (fractional) increase in wavelength received (by an observer)
2. Due to source and observer receding (from each other).

Question 6

Explain how redshift can be used to determine the velocity of a galaxy relative to the Earth.

Answer 6

1. Measure frequency/wavelength of light (from the galaxy) .
2. Compare (measured) frequency/wavelength to the frequency/wavelength for a source on the Earth.
3. redshift formula use it ta calculate velocity by rearranging.

Question 7

Explain how Hubble’s law can be used to find a value for the age of the universe.

Answer 7

1. See v = H0d and v = d/t

2. Therefore t = 1/ H0

Question 8

Describe how you would expect the average density of matter in the universe to affect its ultimate fate.

Answer 8

1. If density less than critical value, expansion would continue for ever. ( open universe)
2. If density greater than critical, expansion would stop and universe would
   contract again. ( closed universe)
3. If density equals critical value, expansion rate would decrease to zero but universe would not contract again ( flat universe)

Question 9

It is difficult for scientists to estimate the average density of the universe reliably. Explain why.

Answer 9

1. The mass of the universe is uncertain because of the amount of dark
   matter is uncertain.
2. The value of the Hubble constant is uncertain since measurements of distances to distant galaxies are uncertain.

Question 10

Outline how parallax measurements are used to determine the distance to nearby stars
and explain how the use of a standard candle enables the distance to more distant objects
to be determined.

Answer 10

Parallax:

1. The star is viewed from two positions at 6 month intervals.
2. The (change in) angular position of the star relative to a background of fixed/distant stars is measured.
3. The radius of the Earth’s orbit about the Sun must be known and trigonometry is used (to calculate the distance to the star.

Standard Candle:

1. intensity of standard candle is measured
2. Luminosity of standard candle is known
3. Inverse square law is used (to calculate distance to standard candle)

Question 11

State the origin of the force that maintains the black holes in an orbit.

Answer 11

gravity

Question 12

Explain how standard candles are used in astronomy, and suggest how the existence of a layer of dust around a Cepheid variable star might affect the conclusions drawn by astronomers.

Answer 12

1. Standard candles are (stellar) objects of known luminosity.
2. Standard candle’s brightness on earth is measured.
3. Use inverse square law [F=L/4πd^2} to calculate distance.
4. Dust layer will reduce flux of Cepheid
5. Cepheid will appear to be further away than it is.

Question 13

The conditions needed for fusion to occur make it difficult to replicate outside of a star.
State and explain:
1. how the process of fusion is able to release energy
2. The conditions necessary for fusion to occur
3. Why the conditions are difficult to replicate outside of a star

Answer 13

Point 1:
a) In nuclear fusion small nuclei fuse to produce a larger nucleus. Energy is released as deltaE=deltam c^2.

Point 2:

a) A very high temperature: To overcome the electrostatic repulsion between nuclei.
b) A very high density: to maintain a high collision rate.

Point 3:

a) very high temperatures lead to confinement problems and contact with container causes temperature to fall.
b) strong magnetic fields required

Question 14

Discuss how astronomers were led to propose the existence of dark matter and the consequences of its existence for the ultimate fate of the universe.

Answer 14

1. Dark matter has mass but does not emit e-m radiation
2. Dark matter proposed when observations of galaxies indicated that they must contain more matter than could be seen.
3. The existence of dark matter will increase the (average) density of the universe
4. This may make it more likely that the universe ends with a ‘Big Crunch’

Question 15

Explain why the observable universe has a finite size

Answer 15

1. The universe started from a small initial point [ Big Bang]
2. Idea that universe has a finite age

Question 16

A student states that blue stars are cooler than yellow stars. Use Wiens law to comment on the
accuracy of this statement.

Answer 16

1. wavelength of yellow stars>wavelength of blue stars.
2. lambda max is inversely proportional to temperature
3. Therefore temperature of blue star is greater than temperature of yellow star
4. Statement incorrect

Question 17

State and explain the conditions necessary for fusion to occur in a star

Answer 17

1. VERY high temperatures to overcome the electrostatic force of repulsion between nuclei.
2. Very high density to give a sufficient collision rate

Question 18

The Sun is approximately 4.5 billion years old. Describe the life-cycle of the Sun starting from its present position.

Answer 18

1. The Sun is fusing hydrogen into helium in its core.
2. When hydrogen fusion stops, the core of the Sun cools and the core contracts under gravitational forces.
3. The Sun then becomes hot enough for helium nuclei to fuse together in its core, this causes the Sun to expand into a red giant.
4. Helium begins to run out and the core collapses again under gravitational forces.
5. The outer layers are ejected into space and the core becomes a white dwarf star

Question 19

In the early 20 th century, Edwin Hubble analysed the light from the stars in distant clusters of galaxies and deduced that these clusters of galaxies are receding.

(a) Explain how Hubble was able to deduce that the distant clusters of galaxies are receding.

Answer 19

1. Light from these galaxies were redshifted,

2. Due to the doppler effect they must be receding.

Question 20

Massive stars can only produce elements up to iron (Fe) in their cores by fusion. Why?

Answer 20

1. Fe has the highest binding energy per nucleon.
2. If nuclei were to be formed with A > 56, the B.E./nucleon would decrease
3. This would require a net input of energy (and so does not occur)

Question 21

State what is meant by redshift, and explain how it allows the distance to the galaxy to be determined.

Answer 21

1. Redshift is the fractional increase in wavelength observed due to the source and the observed receding.
2. We use redhsift to find v =zc
3. We then use Hubbles law v=H0d to find d

Question 22

Scientists are uncertain about the age and ultimate fate of the Universe.
Explain why.

Answer 22

1. It is difficult to make accurate measurements of the distance of distant galaxies.
2. So Hubbles constant has a large uncertainty
3. The existence of dark matter also means that the average density of the universe has a large uncertainty
4. Therefore comparing the density of the universe to the critical density and therefore determining the ultimate fate would have a large uncertainty

Question 23

Explain why, in the absence of fusion, the luminosity of the

star would decrease over time.

Answer 23

1. The star cools, so temperature T reduces
2. The star contracts (under gravitational forces), so area A reduces
3. L =4 pi r^2 σ T ^4 therefore L decreases

Question 24

The telescope was named in honour of Edwin Hubble who measured the red shift of
light from a number of galaxies and related it to their distance from the Earth.

Explain what is meant by the term red shift in this context and state the inference
that Hubble made from his measurements.

Answer 24

1. The observed wavelength is longer than the actual wavelength
2. so the universe is expanding
   (All distant) galaxies are moving apart