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Astronomy 263/264
Constellation Structure and Stellar Motion

Summary

In this lab, we try to give you a three dimensional view of a constellation that demonstrates the distances between stars in a constellation. You will also learn how stars move over the course of many thousands of years and see how the Big Dipper will look 100,000 years from today! You will also gain a better understanding of the concepts of apparent vs. absolute magnitude, and radial vs. tangential velocity.

Concepts

It is possible to determine distance by measuring the apparent brightness of a star. For instance, if you were looking at a 100 Watt light-bulb from a distance of 3 meters away, the bulb would appear brighter to you than if it were at a distance of 10 meters. Astronomers use quantities called magnitudes to determine the distance to stars and other luminous celestial bodies.

There are two kinds of magnitude associated with the brightness of a star: absolute and apparent magnitude. The absolute magnitude of a star is a measure of its intrinsic brightness, and is used as an absolute "measuring stick" to compare stellar energy output. You can think of the power rating (100 Watts) of the light bulb in the previous paragraph as a measure of its absolute magnitude; the energy output of the bulb stays constant no matter where you are. Astronomers define the absolute magnitude as that magnitude a star would have if it was 10 parsecs (about 32 light years) away. This differs from a star's apparent magnitude -- the magnitude it has because of its distance from us, i.e. how bright the star appears in our sky. In our light bulb example, when we note how bright the bulb appears to be, we are measuring its apparent magnitude.

The magnitude of a star is a pure number and has no units, and the brighter the star is, the smaller the magnitude of the star. Objects brighter than the first magnitude are assigned negative numbers. For instance, the apparent magnitude of the Sun is -26 (very bright!) while the apparent magnitude of the star Deneb is +1.26 (a bright evening star, but very dim compared to the Sun).

The velocity of the star relative to the Sun is called space velocity. For ease of understanding, this velocity is often resolved into two components; one directed along the line of sight called radial velocity and the component perpendicular to the line of sight called tangential velocity. The radial velocity of a star can be calculated from the well-known Doppler shift of the spectral lines. The tangential velocity of a star can be measured from photographs taken several years apart. The radial velocity is responsible for the change in appearance of a constellation over many thousands of years.

Last updated on November 5, 2000.