The Hertzsprung-Russell Diagram Lab Instructions

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Clemson University *

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Apr 3, 2024

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The Hertzsprung-Russell Diagram Lab Instructions These lab activities have evolved over years of use in Clemson University’s Department of Physics and Astronomy general astronomy laboratory. Contributors include Tom Collins, Mark Leising, Neil Miller, Peter Milne, Grant Williams, Donna Mullenax, Jessica Crist, Keith Davis, Amber Porter, Lea Marcotulli, and David Connick. Please direct all questions, complaints, and corrections to David Connick (dconnic@clemson.edu) who is responsible for all errors and omissions. I. Introduction In 1911 Einar Hertzsprung, a Danish astronomer, compared the stars in a cluster by plotting their apparent magnitudes against their colors. In 1913 the American astronomer Henry Norris Russell made a similar investigation of stars (of known distance) by plotting their absolute magnitudes versus their spectral classes, we will look at spectral classes in detail in the next lab. Hertzsprung and Russell made the very important discovery that the luminosities of most stars have a direct relation to their colors or spectral classes and therefore to their temperatures. The vertical axis (y axis) of a Hertzsprung-Russell (HR) diagram can be apparent magnitude, absolute magnitude, or luminosity. The horizontal axis (x axis) may be spectral class, temperature, or color index. Whichever combination of the above is plotted, most HR diagrams have similar characteristics. If all of the stars whose absolute magnitudes are known are plotted on the same HR diagram a pattern of preferred locations can be seen. Most stars will fall along a roughly diagonal band from upper left to lower right. This band is called the main sequence. Very hot stars and very luminous stars (large negative absolute magnitude) are at the upper left end of this band while relatively cool stars with low luminosities (large positive absolute magnitudes) lie at the lower right end. We now know that these stars are in the stable "adult" part of their lives during which their energy comes primarily from core hydrogen burning. The Sun is one of these stable core hydrogen-burning stars. The following table shows the lifetime of a star on the main sequence (hydrogen burning lifetime) as determined by its spectral type. Spectral Type Color Index (B-V) Lifetime (years) O -0.4 <10 6 B -0.2 3x10 7 A 0.2 4x10 8 F 0.5 4x10 9 G 0.7 1x10 10 K 1.0 6x10 10

M 1.6 >10 11 Fully developed stars that are not on the main sequence have exhausted their core hydrogen and are further along in their evolution. The cores of these stars are in a more advanced stage in which the fusion of elements heavier than hydrogen may be involved. Stars that lie above the main sequence are more luminous than main sequence stars of the same temperature and are therefore larger (giants or supergiants). Stars below the main sequence are less luminous than main sequence stars of the same temperature and are therefore smaller (e.g., the white dwarfs). We can use our knowledge about the spectral classes of stars and the HR diagram to determine what stars in the rest of the Galaxy are like. HR diagrams can tell us quite a bit about stars but we must interpret them carefully; there are logical pitfalls in drawing general conclusions using observations involving a limited number of stars. II. Figure 1 Analysis Figure 1 in the ‘H-R Diagram lab figures’ document shows an H-R Diagram with some example stars given in the major categories. Study Figure 1 and answer questions 1-5 on the worksheet. III. Figure 2 Analysis Hipparcos was a satellite that made precise measurements of stars over a period of years. Among the measurements it made were parallaxes, the apparent wobbles of stars due to the Earth’s motion around the Sun, which provides excellent distances for nearby stars. With precise distances the location of a star on an H-R diagram can be shown very accurately. Figure 2 shows a plot of the brightest stars we see from earth combined with a plot of the closest stars to earth using Hipparcos data. Use figure 2 to answer questions 6-8 on the worksheet. IV. Structure of our Galaxy You will need to open the Starry Night program. Once the program is open you should load the “HRgalaxy.skyset” file. You should see the night sky with an H-R diagram in the bottom left of the view screen. We will use the H-R diagram to compare the stars in the plane of our Milky Way galaxy with those outside the plane. The left vertical axis of the H-R diagram is the absolute magnitude while the bottom horizontal axis is the spectral class. With your FOV at about 20 degrees center the screen on a view of the Milky Way galaxy. The denser area you can find the better. In a moment you will move your view away from the Milky Way galaxy. Watch the H-R diagram as you change your view and answer questions 9-11 on the worksheet.

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