Investigation 3: Production of Light - Activity 5

Blackbody Properties

Overview: Students discover that they can determine sizes of stars, if they are modeled accurately by blackbodies.

  • Lecture Tutorials on Introductory Astronomy (Adams, et al.): Tutorial on blackbody properties: Students read and complete the hot plate questions and luminosity questions, continuing to Part II about stars and the HR diagram.
  • Introduce the HR diagram with the sizes of stars represented. 
  • Introduce the relation for a spherical blackbody: L= sigma * T4*4pi*R2, which relates the luminosity (L in Watts= Joules/sec), temperature (T, in Kelvin) and Radius ofthespherical blackbody (R in meters). Sigma= Stefan-Boltzmann constant = 5.67 x 10-8 Joules / (seconds x meters2 x Kelvin4

Teacher tips/tricks:

  • Timing: about 1 hour for lecture-tutorial alone


  • Compare temperatures and/or sizes of other pairs of objects on HR diagram.
  • Ask students to read off properties of selected stars on the HR diagram.

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