Classifying Stars
3 marks · NormalA student is looking at the night sky and notices some stars appear brighter than others. Explain why two stars of the same size and type can appear to have very different brightness as observed from the Earth.
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Two stars of the same size and type can appear to have different brightness from Earth because their apparent brightness depends on their distance from us, not just their actual luminosity. A nearby star appears brighter than a similar, more distant star. Also, anything between the star and Earth could affect how bright it appears.
Must show: distance from Earth, apparent brightness vs actual brightness
Also accept: Light absorption by space dust/blocking mentioned but not required
Common mistakes: Confuses size/type with distance alone; Does not mention both actual brightness and distance
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Marking points
M1 · 1 mark · States that distance from Earth affects apparent brightness.
M2 · 1 mark · States that actual luminosity or size/type is the same (so not the cause here).
A1 · 1 mark · Relates apparent brightness to both luminosity and distance in reasoning.
Worked solution
Apparent brightness is how bright a star looks from Earth. If two stars are the same actual size and luminosity, but one is much farther away, it will appear dimmer because its light spreads out more by the time it reaches us. The difference is only due to distance, not the stars' actual properties.