Topic 10: Space — Edexcel A-Level Physics
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Topic 10: Space explained
A Hertzsprung-Russell (H-R) diagram plots stellar luminosity on the vertical axis against surface temperature on the horizontal axis, with temperature decreasing to the right.
Read the full explanation
Sketching it means drawing labelled axes, marking the main sequence as a diagonal band from hot, luminous stars (upper left) to cool, dim stars (lower right), and placing the giants (upper right) and white dwarfs (lower left). Interpreting it means reading a star's position to infer its luminosity and temperature, and recognising that a star of given temperature can have very different luminosity depending on whether it is a dwarf, main-sequence star or giant. Luminosity is often given in solar units or as a multiple of the Sun's luminosity.
Your focus
- Sketch an H-R diagram with correctly labelled and oriented axes.
- Place the main sequence, giants and white dwarfs in their correct regions.
- Interpret a star's position to state its luminosity and surface temperature.
Topic 10: Space exam tips
Marking Points
- Axes correctly labelled: luminosity (vertical) and surface temperature (horizontal), with temperature decreasing left to right.
- Main sequence drawn as a diagonal band running from hot, high-luminosity stars to cool, low-luminosity stars.
- Giants/supergiants placed in the upper-right region (cool but highly luminous).
- White dwarfs placed in the lower-left region (hot but low luminosity).
- Interpretation: a star's position gives both its luminosity and its surface temperature, and stars of the same temperature can differ greatly in luminosity.
- Recognition that luminosity may be expressed in solar units or as a multiple of the Sun's luminosity.
Examiner Tips
- 💡Always label both axes and state the direction of decreasing temperature before plotting any points.
- 💡Use the diagonal main sequence as an anchor, then place giants above-right and white dwarfs below-left relative to it.
- 💡When interpreting, quote both coordinates (luminosity and temperature) rather than just one.
Common Mistakes
- Plotting temperature increasing to the right: the convention is that temperature decreases to the right, so reverse the axis.
- Placing white dwarfs in the upper-left because they are hot: hot but dim stars belong in the lower-left, so correct the vertical position.
- Treating the main sequence as a single line rather than a band: draw it as a broad diagonal region.
- Assuming all stars of one temperature have the same luminosity: position on the vertical axis distinguishes dwarfs, main-sequence stars and giants.