Physics 11–12 · Year 12

The Hertzsprung-Russell diagram from real star data, with spectral classes

Module 8: From the Universe to the Atom (Origins of the Elements)

Calculation and dataLow risk

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The idea

Plotting luminosity against surface temperature sorts stars into a main sequence, giants and white dwarfs, and the position of a star reveals its size and its stage of life.

What you need

CSIRO Australia Telescope National Facility HR diagram activities and spectral class pages (senior astrophysics education resource)
Spreadsheet

How to do it

Open the CSIRO Pleiades colour-magnitude activity (linked from the ATNF HR diagram activities page); compute the colour index B - V for each star and plot V magnitude (brightest at the top) against B - V (bluest, hottest stars on the left).
Label the main sequence, then use the ATNF HR diagram page to mark where giants, supergiants, white dwarfs and the Sun lie.
Use L = 4 pi R^2 sigma T^4 to compute the radius of a giant and a white dwarf at the same temperature as the Sun.
Match each star's spectral class (O B A F G K M) to its temperature using the CSIRO spectral class page and mark the class boundaries on the axis.

What you should see

The Pleiades stars fall mostly on a band from hot and bright to cool and dim, the main sequence; the Sun (5772 K, class G2) lies part way along it. A star at the Sun's temperature but 100 times its luminosity has 10 times its radius; a white dwarf at 10 000 K with 0.001 solar luminosity has a radius of about 0.01 of the Sun's. The Sun's own luminosity from its radius and temperature is 3.828e26 W, matching the NASA fact sheet.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Brighter stars are always hotter; a cool giant can outshine a hot dwarf because it is far larger.
The main sequence is a path stars move along during life; a star's position on it is set by its mass.

Safety card

Low riskLearners carry it out

Hazards

No hazard is listed.

Controls

No control is listed.

Note

Screen activity only.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-15 PH11/12-5
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-04 PY-12WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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