Magnitudes.doc - AST 114 – Spring 2002 Magnitudes and...

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AST 114 – Spring 2002 Magnitudes and Stellar Brightness MAGNITUDES AND STELLAR BRIGHTNESS What will you learn in this Lab? For the first time we’ll be looking at the stars tonight as something more than just pretty points of light. We’re going to take special note of their location, their relative brightnesses, and even their colors. These types of observations were the fundamental work of both ancient and modern amateur astronomers alike – giving us a unique record of the brightnesses of stars through the course of history. What do I need to bring to the Class with me to do this Lab? For this lab you will need: A copy of this lab script A pencil A scientific calculator Your flashlight Your field guide Your star charts Your star wheel © 2002 Arizona State University Page 1 of 8
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AST 114 – Spring 2002 Magnitudes and Stellar Brightness Introduction : This exercise is designed to help the student become familiar with the magnitudes ( i.e., apparent brightnesses) and colors of stars. While completing tonight's work, you should learn the naming convention used to identify stars. Magnitudes : To describe how bright individual stars are and to express differences in brightness, astronomers assign each star a magnitude according to its brightness. The magnitude system was first used by the Greek astronomer Hipparchus, who divided the stars into six categories. He called the brightest stars in the sky `first magnitude' and the faintest `sixth magnitude'. Stars of intermediate brightness were assigned a number between 1 and 6. We still use a modified version of this system, with extensions to encompass a larger range of brightnesses by using numbers smaller than 1 and larger than 6. The first thing you should notice about this system is that its ordering is backward: a small number (like 1, or even a negative number) denotes a brighter star than a large number (like 8 or 9). The brightest star in the night sky, Sirius, has a magnitude of -1.5 while the faintest star that can be seen with the unaided eye is about magnitude 6. For comparison, the magnitude of the Sun is -26.5. An easy way to remember that the magnitude system runs backward is to think of magnitudes in terms of rank. A `first-rank' star is brighter than a `second-rank' star, which in turn is brighter than a `third-rank' star, etc. Another important feature of the magnitude system is that it is a logarithmic scale. When the system was first setup, it was thought that 1st magnitude stars were about twice as bright as 2nd magnitude stars, which were twice as bright as the 3rd magnitude stars, etc. Following this pattern, one would expect a 1st magnitude star to be about 32 (= 2 x 2 x 2 x 2 x 2) times brighter than a 6th magnitude star, but careful measurements have shown that this is not correct. Instead, the jump from 1st magnitude to 6th magnitude actually represents a change of 100 times in brightness. Each increase of one magnitude corresponds to a decrease by a factor of about 2.512 ( 100 5 ) in brightness. For
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