10.2.1 Hertzsprung-Russell diagram and the life cycle of stars Quiz: Pearson Edexcel Physics, Unit 10

The 20 questions

1. On a Hertzsprung-Russell diagram, what are the usual axes?

   • Mass on the vertical axis and radius on the horizontal axis, both on linear scales.
   • Wavelength on the vertical axis and distance on the horizontal axis, both on linear scales.
   • Redshift on the vertical axis and luminosity on the horizontal axis, both on log scales.
   • Luminosity up the vertical axis and surface temperature across, decreasing to the right.

2. What is the main energy source of a main sequence star?

   • Burning of carbon compounds in its atmosphere.
   • Radioactive decay of iron in its core.
   • Gravitational collapse of its outer layers.
   • Fusion of hydrogen into helium in its core.

3. What happens when a star leaves the main sequence to become a red giant?

   • Its core hydrogen runs out and its outer layers expand and cool.
   • Its outer layers contract and become hotter.
   • It stops fusing and its luminosity becomes constant.
   • Its core gains hydrogen and its luminosity falls.

4. What is a white dwarf?

   • A large, cool star that is still fusing hydrogen in its core.
   • A cloud of gas and dust that has not yet started to collapse.
   • A small, hot, dense remnant of a low-mass star that no longer undergoes fusion.
   • A star much more massive than the Sun that is still on the main sequence.

5. Where on an HR diagram would a very hot and very luminous star lie?

   • Top right, at high luminosity and low surface temperature.
   • Bottom left, at low luminosity and high surface temperature.
   • Top left, at high luminosity and high surface temperature.
   • Bottom right, at low luminosity and low surface temperature.

6. Where on an HR diagram would a red dwarf lie?

   • Bottom left, with low luminosity and high surface temperature.
   • Bottom right, with low luminosity and low surface temperature.
   • Top right, with high luminosity and low surface temperature.
   • Top left, with high luminosity and high surface temperature.

7. A star is cool but very luminous. What must be true about its radius?

   • Its radius must be very small, since cool stars are dense.
   • Its radius must be very large, since luminosity depends on surface area.
   • Its radius must equal the radius of the Sun.
   • Its radius cannot be determined from luminosity and temperature.

8. A star has the same surface temperature as the Sun but 100 times the Sun's luminosity. What is the ratio of its radius to the Sun's radius?

   • 100
   • 10
   • 1/10
   • 50

9. A star has the same radius as the Sun but twice its surface temperature. How many times its luminosity?

   • 16 times
   • 8 times
   • 2 times
   • 4 times

10. A star has radius 10 times the Sun's radius and surface temperature 3000 K, compared with the Sun at 5800 K. Approximately how does its luminosity compare with the Sun's?

    • about 1400 times
    • about 7 times
    • about 100 times
    • about 0.07 times

11. What is the effect on a star's luminosity and surface temperature as it moves from the main sequence to the red giant branch?

    • Both its luminosity and surface temperature decrease.
    • Both its luminosity and surface temperature increase.
    • Its luminosity decreases while its surface temperature increases.
    • Its luminosity increases while its surface temperature decreases.

12. Why do higher-mass main sequence stars have shorter lives than the Sun?

    • They have less hydrogen fuel per unit mass than lower-mass stars, which is their main limit.
    • They lose mass by radiation only and so keep burning longer than the Sun does.
    • They burn fuel much faster, since higher core temperatures and luminosity drive fusion.
    • They are cooler at the surface, so their fuel is consumed more slowly over their lives.

13. Why do most stars lie on the main sequence of the HR diagram?

    • They are in the final stage of their lives with no internal energy source at all.
    • They are dense remnants that have stopped emitting radiation.
    • They have just formed from a cloud of gas and dust and have not yet started fusing.
    • They spend most of their lives fusing hydrogen in stable core equilibrium.

14. Why is a white dwarf hot but faint on an HR diagram?

    • It has a low surface temperature and a very small surface area, so it is very faint.
    • It has a low surface temperature and a very large surface area, so it is very bright.
    • It is very hot but has a tiny surface area, so its luminosity is low.
    • It has a high surface temperature and a very large surface area, so it is very bright.

15. A star of radius 1.0 x 10^9 m has surface temperature 10000 K. What is its luminosity? Use σ = 5.67 x 10^-8 W m^-2 K^-4.

    • 7.1 x 10^27 W
    • 1.4 x 10^28 W
    • 7.1 x 10^25 W
    • 5.7 x 10^27 W

16. Explain why a red giant appears in the top right of the HR diagram.

    • It is hot and dense, so its luminosity is high despite its small size.
    • It is cool and small, which gives a high luminosity from its rapid fusion.
    • It is hot and small, so its luminosity is high because of its dense core.
    • It is cool, but its very large surface area gives it a high luminosity.

17. Which statement describes how the HR diagram is linked to stellar evolution?

    • Stars are fixed at one point on the diagram for their whole life, so it shows no change at all.
    • Only the Sun appears on the diagram, so it cannot show other stars or their life cycles.
    • The diagram shows only the distances of stars, not their evolution or changing properties.
    • Stars move across the diagram as their size and temperature change over their lives.

18. A star is observed with luminosity 1.0 x 10^26 W and surface temperature 5800 K. Which change would move it to the top of the HR diagram without changing its temperature?

    • Increasing its distance from Earth, which makes it appear brighter in the sky.
    • Increasing its radius, which increases its luminosity at the same temperature.
    • Decreasing its radius, which increases its surface temperature without changing luminosity.
    • Decreasing its mass without changing its radius, which lowers its temperature.

19. A massive star ends its life in a supernova. Which statement about its end state is correct?

    • It never leaves any remnant behind.
    • It always becomes a white dwarf, whatever its mass.
    • Depending on its mass, it may leave a neutron star or a black hole.
    • It always leaves a red dwarf that lasts for longer than the age of the universe.

20. Why is a main sequence star more luminous than a red dwarf of the same radius?

    • It contains less hydrogen fuel, so it burns more intensely.
    • Its surface temperature is higher, and luminosity scales with T^4 at fixed radius.
    • Its radius is smaller, which makes it emit more energy per second.
    • Its distance from Earth is less, so its luminosity is higher.

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