Lesson 1: Stellar Magnitudes


This activity requires approximately two 30-minute classes


Identify and control major variables in their investigations (e.g., predict what variables might affect the size of craters on the moon, using a flour and marble simulation)
Draw a conclusion, based on evidence gathered through research and observation, that answers an initial question (e.g., conclude that simulated flour craters are deeper and wider when the marble is heavier or is dropped from greater heights)

General Objectives

By examining a related applet, students will be able to associate stellar brightness with the magnitude scale, correctly identifying brighter stars with smaller numbers on the scale.

Curricular Connections

  • Mathematics
    • number lines
    • integers

Lesson Overview

In this lesson, students will view the applet Stellar Magnitudes in an attempt to understand the stellar magnitude scale.

Materials and Resources



  • Magnitude

Developing the Lesson

Open the lesson by showing students the applet Stellar Magnitudes without explaining it. Have students hypothesize as to what is happening in the applet. After discussing students' ideas briefly, demonstrate the applet again while explaining:

While the thousands of stars in the night sky appear to be very similar, they are more distinct from one another than their appearance from the Earth would suggest. Stars have various sizes, masses, temperatures, colours, luminosities (power), compositions and lifetimes. The largest stars are 300 times the diameter of the Sun and would engulf the orbit of the Earth, while the smallest stars are smaller than the Earth itself. The most noticeable distinction between stars, however, is the difference in their brightness. The apparent brightness of an object in the sky is denoted by its magnitude, a numeric scale established by Hipparchus around the year 160 BC. Hipparchus stated that the brightest stars in the sky were of first magnitude and the dimmest were of sixth magnitude, making smaller numbers correspond to brighter objects. The scale has been expanded and can now be applied to any object in the sky. The full moon has a magnitude of -12.7, Venus at its brightest is -4.1, the brightest star is -1.46 and the dimmest objects detectable through the largest telescopes are about +29.

Once students have grasped the magnitude scale, have them make educated guesses as to the Sun's magnitude from given points in space.

Sun's magnitude
Location Magnitude Distance (AU)
Mercury -30.0 0.4
Venus -29.0 0.7
Earth -28.0 1.0
Mars -27.0 1.5
Jupiter -24.4 5.2
Saturn -23.2 9.6
Uranus -21.5 19.2
Neptune -20.6 30.1
Pluto -20.5 39.5
Alpha Century (star) +0.4 272,647
Vega (star) +4.3 1,659,060
Deneb (star) +13.0 89,174,497

Give students the Sun's magnitude from Mercury and from Pluto, and instruct them to guess the magnitudes from the remaining planets, as well as from the nearest star outside our solar system.


When finished, reveal actual values and have students compare their answers with them. Discuss any surprises.


Assess students' understanding through observation and questioning.