Chandra :: Educational Materials

Variable Stars (teacher's version)

Stars appear to shine with a constant light; however, thousands of stars vary in brightness. The brightness that a star appears to have (apparent magnitude) from our perspective here on Earth depends upon its distance from Earth and its actual intrinsic brightness (absolute magnitude.) The behavior of stars that vary in magnitude (brightness) - known as variable stars - can be studied by measuring their changes in brightness over time and plotting the changes on a graph called a light curve. Amateur astronomers around the world observe variable stars and assist professional astronomers by sending their data to variable star organizations, such as the American Association of Variable Star Observers (AAVSO) in Cambridge, Massachusetts. The behavior of some variable stars can be observed with the unaided eye or binoculars. Measuring and recording the changes in apparent magnitude and drawing the resulting light curves will allow you to begin to unravel the stories of the often turbulent and always exciting lives of variable stars. The collection and study of variable star data requires the ability to estimate the apparent magnitudes of stars. The two activities that follow will assist you in acquiring the skill of estimating the magnitudes of variable stars.

The two activities, Stellar Heartbeats and A Variable Star in Cygnus, have been adapted from the "Hands-On-Astrophysics, Variable Stars in Math, Science, and Computer Education" curriculum project developed and published by the American Association of Variable Star Observers (AAVSO.)

Stellar Heartbeats html pdf
A Variable Star in Cygnus html | pdf

Engaging Activity for Your Students:
Question students about what they think may cause a star's brightness to change. Continue questioning until the students settle on ideas that resemble the following possibilities:

Cepheid type variables vary due to the expansion/contraction of the surface of the star, i.e. a large stellar surface area translates to a brighter star, a small surface area yields a dimmer star.
Stars in orbit around one another, such as in a binary star system, may eclipse one another.
Novae -- A single star may explosively respond to material dumped onto it by a companion star. The explosive response is enough to re-stabilize the star, but temporarily maikes the star very bright.

Useful Resources:

Types of Variable Stars (at AAVSO)
Estimating Magnitudes Using Interpolation (at AAVSO)
Brief Explanation of Magnitudes (from Hands-On-Astrophysics)
Brief Explanation of Julian Day System (from Hands-On-Astrophysics)
Backyard Astronomers Trigger Multi-satellite Observing Campaign on SS Cygni and Astronomers Team Up for Chandra Observations of SS Cygni (Chandra Chronicles Articles describing how the AAVSO amateur observers assisted the Chandra X-Ray Observatory)
Hands-On-Astrophysics, Variable Stars in Math, Science, and Computer Education (at AAVSO)
HOAFUN (at AAVSO - an interactive and easy-to-use tutorial for estimating the magnitudes of variable stars and understanding the light curves of different types of variable stars.)

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Revised: 03/18/03