Measuring astronomical distancesEdexcel International A Level Physics: Subtopic test

1. 1
   Astronomers measure the position of a nearby star against much more distant background stars on two occasions six months apart. The star appears to shift position, and the observations give a parallax angle of 0.050 arcsecond for the star.

   (a)

   Which statement defines the parallax angle of the star?

   [1 mark]

   • AThe full angular shift of the star between the two observations
   • BHalf the angular shift between the two observations, equal to the angle subtended at the star by a distance of 1 AU
   • CThe angle through which the star moves each year because of its own motion through space
   • DThe angle between the star and the Sun as seen from the star at the first observation

   (b)

   The distance to a star in parsecs equals 1 divided by its parallax angle in arcseconds. What is the distance to this star in parsecs?

   [1 mark]

   • A0.050 pc
   • B2.0 pc
   • C20 pc
   • D200 pc

   (c)

   Calculate the distance to the star in metres. 1 parsec = 3.09 × 10¹⁶ m.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A type Ia supernova in a distant galaxy is used as a standard candle. Its peak luminosity is known to be 2.0 × 10³⁶ W. Telescopes on Earth measure the intensity of its light, and assume that no light is absorbed on the way.

   (a)

   What is meant by a standard candle?

   [1 mark]

   • AAn object of known luminosity
   • BAn object at a known distance from Earth
   • CAn object whose intensity at Earth is always the same
   • DAn object with the same surface temperature as the Sun

   (b)

   The same supernova could be observed from a position twice as far away. What would the intensity received then be, compared with the original intensity?

   [1 mark]

   • AHalf as much
   • BTwice as much
   • CThe same
   • DOne quarter as much

   (c)

   The intensity received at Earth is 1.6 × 10⁻¹³ W m⁻². Calculate the distance to the supernova in metres.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Astronomers want to find the distance to two objects: a star X about 50 pc from Earth, and a type Ia supernova Y in a galaxy about 5.0 × 10⁷ pc away. Their telescope can measure a parallax angle reliably only if it is at least 1.0 × 10⁻³ arcsecond. A type Ia supernova has a known peak luminosity.

   (a)

   Use calculations to deduce whether trigonometric parallax can be used to find the distance to each object.

   [3 marks]

   (b)

   Describe how the distance to supernova Y can be found using a standard candle, and state one reason why the result may be uncertain.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A university observatory is preparing teaching material on how astronomers measure distances in space, from the nearest stars to distant galaxies.

   (a)

   Describe how trigonometric parallax is used to measure the distance to a nearby star.

   [6 marks]

   (b)

   Evaluate the use of type Ia supernovae as standard candles, rather than parallax, for measuring the distance to a very distant galaxy.

   [6 marks]

   Total for question 4: 12 marks

End of questions