Labs from YSI 95:
How Bright are the Stars?

Todd Duncan
CARA Yerkes Summer Institute, August 1995

This is the staff copy of the lab and other brainstorming/reflections.

Teacher's Guide:

(Note that this lab is to be used in conjunction with the "Energy from the Sun" lab, also from the 1995 YSI.)

Materials:

Outline of Class


Student Worksheet

Earlier today we measured the amount of energy flowing from one star (the sun) to the earth. We did this by measuring the amount of heat we got from the sunlight. But energy can be observed in many different forms, so there are many different ways to measure energy. Tonight we're going to learn how to use how bright a star looks, as a way to measure how much energy is reaching us from that star.

The apparent brightness of a star is just how bright that star appears to be. Our eyes can do a pretty good job of ordering stars by brightness. Early sky-watchers developed a system for classifying stars on the basis of how bright they looked. The brightest stars were referred to as stars of the "first magnitude," the next brightest were "second magnitude," and so on down the line. This classification system has evolved into the very precise magnitude system that astronomers use today. It assigns a specific magnitude number to each star, indicating how bright it is. The smaller the magnitude, the brighter the star. For example, the sun is very bright at magnitude -26.7, while the bright star Vega is +0.04.

Using what we've talked about in class, and your results from the daytime lab, how much would the light from Vega have raised the temperature of your calorimeter?


Observing procedure:

On the next page, you have a chart with several stars on it. Find each of these stars in the sky, and rank them in order of brightness (1 for the star you think is brightest, down through 6 for the one you think is faintest).

alpha Cygni     _________

pi Herculis       _________

gamma Aquilae  ________

alpha Aquilae     ________

alpha Lyrae     __________

theta Lyrae      __________

Now we're going to try the same thing with a device called a photoelectric photometer. It does essentially what your eyes do: measures brightness. But it does this by converting the light that hits it into a current we can read out: the more light that hits it, the more current, and the bigger the number on the readout. For this part of the lab, record the readout you obtain on the photometer when you point it at each star:

alpha Cygni     _________

pi Herculis       _________

gamma Aquilae  ________

alpha Aquilae     ________

alpha Lyrae     __________

theta Lyrae      __________

Does the order of brightness match with what you found using your eyes as the detectors?

If we have time, we'll also convert the photometer measurements into magnitudes, and see how closely they match with "standard" values.


Important Disclaimers and Caveats