HR Diagram Star Inspector

Aim: To investigate whether hotter stars are always brighter.

Star Inspector

No star selected
Surface temperature —
Absolute magnitude —

Click a star, or focus the star field and use the arrow keys to cycle through the twelve stars.

Reading the Inspector

Surface temperature is in kelvin (K). Absolute magnitude measures true brightness: the lower the number (more negative), the brighter the star. The colour swatch shows the star's real colour, which comes from its temperature.

Data Table

Star Surface temperature (K) Absolute magnitude Clear row

Aim: To investigate whether hotter stars are always brighter.

Part A — Is hotter always brighter?

  1. Select the Sun and record its surface temperature and absolute magnitude in the table.
  2. Do the same for Rigel and Betelgeuse.
  3. Of these three, which star is the hottest? Which is the brightest? (Remember: a lower magnitude means a brighter star.)
  4. From these three stars alone, could you argue that hotter stars are always brighter? Which star already causes trouble for that claim?

Part B — Collect the full data set

  1. Work through the remaining nine stars until every row of your table is complete.
  2. Find the hottest star in your table. Is it also the brightest?
  3. Find the brightest star in your table. Is it also the hottest?
  4. Pick out one star that is hot but dim, and one that is cool but bright. What does each of them do to the claim "hotter is always brighter"?

Part C — Plot it on paper

  1. On graph paper, plot every star as a labelled point, with surface temperature on the horizontal axis and absolute magnitude on the vertical axis. Set the axes up exactly as your worksheet instructs.
  2. Do the twelve points scatter at random, or do they gather into groups?
  3. How many distinct groups can you see, and which stars belong to each one?
  4. Does the largest group form any kind of line or band across your diagram?

Part D — What the groups mean

  1. Sirius A and Sirius B orbit each other, so they are the same distance from us — yet their absolute magnitudes are enormously different. What must be different about the stars themselves?
  2. Betelgeuse and Proxima Centauri have similar temperatures but sit at opposite ends of your brightness axis. What does that suggest about their sizes?
  3. Which group on your diagram does the Sun belong to? Using what you know about the life cycle of a star like the Sun, mark where it may sit on your diagram in the far future.
  4. Write one sentence answering the aim: are hotter stars always brighter? Quote two stars from your diagram as evidence.