Measuring astronomical distancesEdexcel A-Level Physics: Subtopic test

1. 1
   An astronomer measures the position of a nearby star against the background of very distant stars. She repeats the measurement six months later and finds that the position of the nearby star appears to have shifted. The parallax angle of the star is 0.25 arcseconds.

   (a)

   What is meant by the parallax angle of a star?

   [1 mark]

   • AHalf of the angle through which the star appears to move against the background stars over six months
   • BThe whole angle through which the star appears to move against the background stars over six months
   • CThe angle between the star and the Sun, seen from the Earth
   • DThe angle subtended at the star by the diameter of the Earth's orbit

   (b)

   Calculate the distance to the star.

   [1 mark]

   • A0.25 pc
   • B0.063 pc
   • C4.0 pc
   • D8.0 pc

   (c)

   Explain why the astronomer repeats the measurement six months later.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A Type Ia supernova is observed in a distant galaxy. All Type Ia supernovae reach the same peak luminosity, 2.0 × 10³⁶ W. At its peak the intensity received at the Earth from this supernova is 1.0 × 10⁻¹⁴ W m⁻².

   (a)

   Why can a Type Ia supernova be used as a standard candle?

   [1 mark]

   • AIts parallax angle is large and easy to measure
   • BIts peak luminosity is known, so its distance can be found from the intensity received
   • CIt gives the same intensity at the Earth however far away it is
   • DIt is always found in our own galaxy

   (b)

   Calculate the distance to the supernova.

   [1 mark]

   • A1.4 × 10²⁵ m
   • B1.6 × 10⁴⁹ m
   • C5.6 × 10²⁴ m
   • D4.0 × 10²⁴ m

   (c)

   Suggest why the distance to this supernova cannot be found using trigonometric parallax, and why a standard candle can be used instead.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A star, X, has a parallax angle of 0.050 arcseconds. The intensity of the light received from it at the Earth is 2.4 × 10⁻¹⁰ W m⁻². One parsec is 3.09 × 10¹⁶ m, and the luminosity of the Sun is 3.85 × 10²⁶ W.

   (a)

   Calculate the distance to star X in parsecs and in metres.

   [3 marks]

   (b)

   Calculate the luminosity of star X and compare it with the luminosity of the Sun.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Astronomers want to measure the distance to a star in our own galaxy and the distance to a distant galaxy 25 Mpc from the Earth. Their telescope can measure angles no smaller than 0.01 arcseconds.

   (a)

   Describe how trigonometric parallax is used to find the distance to the star in our own galaxy, and state a limitation of the method.

   [6 marks]

   (b)

   Show that trigonometric parallax cannot be used to find the distance to the galaxy, and explain how a standard candle in the galaxy could be used instead.

   [6 marks]

   Total for question 4: 12 marks

End of questions