Space physics

A student says, ‘A star in the top-left of a Hertzsprung–Russell diagram must be physically above and left of another star in space.’ Which response is best?

1. A

   Correct, because every scientific diagram is a map

2. B

   Correct, but only for stars in our galaxy

3. C

   Incorrect; the axes show stellar properties, not positions in space

4. D

   Incorrect; all stars are the same distance from Earth

1. a

   Select and explain Select the best response and name two properties represented on an H–R diagram.

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   Correct choiceC

   1. 1

      Selects option C: the H–R diagram axes represent stellar properties rather than positions in space.

   2. 2

      Names luminosity or absolute brightness as one represented property.

   3. 3

      Names surface temperature, colour, or spectral class as the other represented property.

For nearby stars, distance in parsecs can be estimated using d = 1 ÷ p, where p is the parallax angle in arcseconds.

1. a

   Calculate Calculate the distance to stars K and L.

   2
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   1. 1

      Calculates dK = 1 ÷ 0.50 = 2.0 pc.

   2. 2

      Calculates dL = 1 ÷ 0.20 = 5.0 pc.

2. b

   Analyse Identify which distance is least reliable and justify your choice from the table.

   2
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   1. 1

      Identifies star M as having the least reliable calculated distance.

   2. 2

      Explains that ±0.01 arcsec is the largest percentage uncertainty for M: 0.01 ÷ 0.05 = 20%.

3. c

   Explain Explain why parallax becomes difficult to use for very distant stars.

   2
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   1. 1

      States that parallax angle becomes smaller as distance increases.

   2. 2

      Explains that the angle approaches the instrument's resolution or becomes comparable with measurement uncertainty.

Spectral lines from most distant galaxies are observed at longer wavelengths than the same atomic lines measured in a laboratory. More distant galaxies generally show a larger shift. A media post claims this single observation ‘proves every detail of the Big Bang.’

1. a

   Distinguish Distinguish the observation from the inference made from it.

   2
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   1. 1

      Identifies shifted spectral-line wavelengths and the distance trend as observations.

   2. 2

      Identifies recession or expansion of space as an interpretation or inference from those observations.

2. b

   Explain Explain how the distance–redshift pattern supports an expanding-universe model.

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   1. 1

      Links longer observed wavelength to redshift.

   2. 2

      Links greater redshift, in this context, to greater recession rate.

   3. 3

      Explains that the general increase with distance matches the prediction that large-scale separations grow over time.

3. c

   Evaluate Evaluate the wording of the media claim and propose a more scientifically responsible conclusion.

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   1. 1

      Rejects ‘proves every detail’ because one evidence line does not uniquely establish all parts or parameters of a model.

   2. 2

      Recognises the need for independent evidence, uncertainty, calibration, and consideration of alternative explanations or model limits.

   3. 3

      Proposes wording such as ‘provides strong evidence for cosmic expansion and is consistent with a hot early-universe model when combined with other evidence.’