Edexcel iGCSE Physics (4PH1) 8.4 Cosmology Exam Style Question Paper 1B - New Syllabus
Question
This question is about red-shift.
(a) The diagram shows two identical stars, A and B, orbiting each other.
Light from each star arrives at Earth. As the stars move in their orbit, the wavelength of the light observed at Earth changes due to the Doppler effect. Add an X to the diagram to show the position of star B when the light emitted from it shows maximum red-shift when detected on Earth.
(b) Light is received on Earth from a distant galaxy. The longest wavelength of light arriving at Earth from the galaxy is \(561\,\mathrm{nm}\). A lamp on Earth produces the same light with a wavelength of \(550\,\mathrm{nm}\). Calculate the velocity of the galaxy.
[speed of light = \(3.0\times10^8\,\mathrm{m\,s^{-1}}\)]
Syllabus Topic Codes (Edexcel International GCSE Physics 4PH1):
• 3.8: Doppler Effect — part (a)
▶️ Answer/Explanation
(a) Maximum red-shift [1 mark]
Maximum red-shift occurs when star B is moving directly away from Earth. At this position, the component of its velocity away from Earth is greatest.
Therefore, X should be placed at the 3 o’clock position on the orbit.
(b) Velocity of the galaxy [4 marks]
First calculate the change in wavelength:
\(\Delta\lambda=561-550\)
\(\Delta\lambda=11\,\mathrm{nm}\)
Use the red-shift relationship:
\(\displaystyle \frac{\Delta\lambda}{\lambda}=\frac{v}{c}\)
Substitute the values:
\(\displaystyle \frac{11}{550}=\frac{v}{3.0\times10^8}\)
Rearranging:
\(\displaystyle v=\frac{11}{550}\times3.0\times10^8\)
\(v=6.0\times10^6\,\mathrm{m\,s^{-1}}\)
Therefore:
\(\boxed{v=6.0\times10^6\,\mathrm{m\,s^{-1}}}\)
The wavelength observed from the galaxy is greater than the wavelength measured on Earth, so the light is red-shifted. This indicates that the galaxy is moving away from Earth.
Question
The diagram is an incomplete Hertzsprung-Russell diagram which astronomers use to compare stars.
(a) The y-axis label is missing. Which of these is the correct label for the y-axis?
A absolute magnitude
B apparent magnitude
C scalar magnitude
D vector magnitude
(b) The box contains words to identify the shaded areas P, Q and R on the Hertzsprung-Russell diagram.
Use words from the box to identify P, Q and R.
(c) Explain which side of the diagram contains stars with the highest surface temperature.
Syllabus Topic Codes (Edexcel International GCSE Physics 4PH1):
• 8.11P: Absolute Magnitude — part (a)
• 8.12P: Hertzsprung-Russell Diagram — parts (a), (b) and (c)
▶️ Answer/Explanation
(a) Y-axis label [1 mark]
Answer: A, absolute magnitude
- A is correct because absolute magnitude is used to compare the intrinsic brightness of stars on an H-R diagram.
- B is incorrect because apparent magnitude depends on how bright a star appears from Earth and therefore depends on distance.
- C is incorrect because scalar magnitude is not the quantity used to label this axis.
- D is incorrect because vector magnitude is not the quantity used on an H-R diagram.
(b) Identifying the shaded regions [3 marks]
The shaded regions are:
- P: white dwarfs
- Q: main sequence
- R: red giants
The main sequence is the region containing stars such as the Sun, while white dwarfs and red giants occupy different regions of the H-R diagram because of their different luminosities and surface temperatures.
(c) Surface temperature [2 marks]
- The stars with the highest surface temperatures are on the left-hand side of the H-R diagram.
- Blue and white stars have higher surface temperatures than red and orange stars.
Therefore, the temperature scale on an H-R diagram decreases from left to right.