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Name: _________________________________________ Astronomy Lab: The Hertzsprung-Russell (H-R) Diagram Adapted from Astronomy Through Practical Investigations No. 31 L.S.W. Publications Inc. © 1988 Sometimes the student of astronomy starts to become overwhelmed trying to understand the many measurements and observations astronomers make. Data concerning distance, brightness, color, spectral class, mass, temperature, motion, etc. all seem to be gathered in an attempt to impress the student with the astronomer’s cleverness. This is a false impression, however, for the gathering of such information is not the ultimate goal of the astronomer. Astronomy’s major goal is an understanding of the nature of stars; the large collection of stars called galaxies; and ultimately, the universe itself. For example, in the 20th century, it has become apparent that stars change or evolve over time, and it is appropriate to talk in terms of stellar birth, life, and death. The very atoms which we are composed of were created in stars. Moreover, since our planet earth is subject to the “whims” of a star called the sun, it becomes most practical to learn about the life cycle of stars (which we will be discussing in class in the coming days). Some of the greatest advances concerning the nature of stars have come about by comparing their properties as expressed in graphs. In the early 1900’s, while studying the spectral classification work of the Harvard Observatory, Ejnar Hertzsprung noticed some systematic differences among spectra that were related to their proper motion (the observed motion of a star in the sky over time as the star moves through space). In particular, he noticed differences in the later spectral classes (K and M red stars) that roughly separated into two groups: (1) one with small proper motions and (2) another with large proper motions. Since on average the amount of proper motion is an indication of the distance of the star, he suggested there were differences in luminosity among stars of the same spectral class. That is, some stars were at very great distances (small proper motion) but still visible; therefore, they must be much brighter than other stars of the same spectral class that were nearby (large proper motion). Soon thereafter, both Hertzsprung and a famous American astronomer named Henry N. Russell developed a graphical representation comparing the absolute magnitude (luminosity) and spectral class (temperature) of stars. This diagram is named in their honor as the Hertzsprung-Russell diagram or the H-R diagram. It is in terms of this diagram that much of the insight and proof of the modern theory of stellar evolution has developed. On the original H-R diagram, the vertical axis represents stellar brightness in absolute magnitude (M) and the horizontal axis, spectral class. This is one of three common representations found in use today. The other two are (1) absolute magnitude on the vertical axis versus color (as measured by the color index) on the horizontal axis and (2) the luminosity in units of solar luminosity on the vertical axis versus effective temperature on the horizontal axis. The magnitude-color diagram is used for stars too faint to record their spectra and the luminosity-effective temperature diagram is used by theoretical astronomers calculating the properties of stellar models. The important property in common among all three is that they compare the luminosity or total energy radiated (absolute magnitude) versus the surface temperature (spectral class or color index). It is not unexpected to find these quantities related. The following diagram shows schematically four of the most interesting regions in which stars are found. Notice stellar properties tend to cluster in certain regions rather than form a random or scattered pattern all over the diagram. This result tells us there exists a relation between luminosity and temperature, although for each temperature, there may be more than one relation. The roughly diagonal strip running across the diagram is called the main sequence and contains most of the stars. Answer the following questions in complete sentences. Use only the information from the reading above. You may want to highlight information in the reading that applies to the questions below. 1. Why do astronomers collect data on stars such as their distance, brightness, color, mass and temperature? 2. When looking at the spectral class K and M stars, Ejnar Hertzsprung observed the proper motion of these stars. What did he conclude about the luminosity of these stars when observing their proper motions (and thus their distance)? 3. What two properties do all H-R diagrams compare? 4. Do stars plot randomly or in specific locations on the diagram? Explain you answer. Directions: 1. The data in Tables 1 and 2 represent the apparent magnitude (m), absolute magnitude (M), distance and spectral class for 25 of the 100 brightest stars and 25 of the 100 nearest stars. Notice these represent two different ways of selecting star samples for an astronomical study. Many of the stars in the sample of nearest stars are double or multiple stars, and the companions are indicated by using A, B, and C, by the names of the stars. An H-R diagram has been provided to you in which 75 stars from each of the samples have already been plotted − Plot the stars in Table 1 and 2 in the diagram to complete the samples of 100 brightest and 100 nearest stars. − Use a dot (•) for the brightest stars and an open dot (○) for the nearest stars. − If two stars fall on the same spot, shift one slightly. − BE NEAT, YOU WILL NEED TO REFER TO THIS GRAPH TO ANSWER LATER QUESTIONS. 2. Run a smooth diagonal band (or outline) through the middle of the diagram that approximates the largest number of stars. Label this region the Main Sequence. 3. Circle the area in the upper right hand corner that includes the largest number of stars and label this Group 1. 4. Circle the stars that cluster in the lower left hand corner and label this Group II. 5. Along the bottom of the diagram above the spectral class, color in a band corresponding to the appropriate color of each spectral class: O - blue B - light blue A - blue/white F - white G - yellow K - orange M - red Questions: 1. The luminosity of a star is a measure of the total amount of energy a star radiates per second into space. The luminosity thus depends on how much surface area a star has and how much energy the star emits from each square unit of the surface area. The radius of a star will determine its surface area and the temperature will determine how much each square unit of surface radiates. Therefore, if we consider two stars with the same surface temperature but different luminosities, they must have different amounts of surface area, or they differ in radius. Complete parts a-e applying these ideas to interpreting the H-R diagram. a. Comparing Group I in the H-R diagram you completed with the same temperature Main Sequence Stars, which group is most luminous? b. Considering the two factors that determine luminosity, why are stars of the answer in part a. most luminous? c. Comparing Group II in the H-R diagram you completed with Main Sequence Stars of the same temperature, which group is least luminous? d. Considering the two factors that determine luminosity, why are stars of the answer in part c. least luminous? e. In what portion of the H-R diagram are hot, bright stars found? Cool, faint stars? 2. It is common in astronomy to use descriptive terms for regions on the H-R diagram. In particular, stellar sizes are grouped as Supergiants, Giants, and Dwarfs. Using the labeled letters on the following H-R diagram, complete the parts a-I of this question. a. Which lettered region would most likely represent the region of the Red Supergiants? b. Which lettered region would most likely represent the region of the White Dwarfs? c. Which lettered region would most likely represent the region of the Red Giants? d. Which lettered region would most likely represent the region of the Blue Supergiants? e. Which lettered region would most likely represent the location of the Sun, a G2 Main Sequence star? f. Comparing regions Z and W, which region represents the hottest stars? g. Comparing regions Z and W, which region represents the largest diameter stars? h. Comparing regions S and X, which region represents the hottest stars? i. Assuming stars of S and X have the same luminosity, which region would have the larger diameter stars? 3. Refer to the plotted H-R diagram you completed to answer parts a-e of this question. a. Considering the two factors that influence apparent brightness (luminosity and distance), which is more important in determining why the 100 brightest stars appear bright? b. Why don’t most of the nearest stars have proper names? c. Why aren’t there any White Dwarfs among the 100 brightest stars? d. Which sample, the 100 nearest or the 100 brightest, has more Main Sequence Stars? e. Which star, a K0 Supergiant (KOIa) or a K0 Main Sequence Star (KOV), would be visible the greatest distance from the Earth? CONCLUSION: Why are the nearest stars to our solar system so faint as we see them, but the brightest stars in the sky are so far away? TABLE 1 25 of the 100 Brightest Stars Star m M Distance Spectral (pc) Class Polaris 2.0 -4.5 200 F8 Mira 2.0 -1.0 40 M6 Algol 2.1 -0.5 31 B8 Aldebaran 0.8 -0.8 21 K5 Capella 0.1 -0.6 14 G8 Rigel 0.1 -7.0 270 B8 Bellatrix 1.6 -4.1 140 B2 Mintaka 2.2 -6.0 450 B0 Betelguese 0.4 -5.9 180 M2 Sirius -1.4 1.4 2.7 A1 Castor 1.6 0.8 14 A1 Procyon 0.4 2.7 3.5 F5 Pollux 1.2 1.0 10.7 K0 Regulus 1.3 -0.8 26 B7 Merak 2.4 0.6 23 A1 Dubhe 1.8 -0.6 30 G9 Denebola 2.1 1.6 13 A3 Mizar 2.1 0.0 26 A2 Spica 1.0 -3.1 65 B1 Arcturus -0.1 -0.2 11 K1 Antares 0.9 -4.7 180 M1 Vega 0.0 0.5 8.1 A0 Altair 0.8 2.2 5.0 A7 Deneb 1.2 -7.3 500 A2 Markab 2.5 0.0 32 B9 TABLE 2 25 of the 100 Nearest Stars Star m M Distance (pc) 15 1915 A 8.9 11.2 3.5 15 1915 B 9.7 12.0 3.5 L347-14 13.7 14.8 5.9 αDra 4.7 5.9 5.7 Altair 0.8 2.2 5.0 δPav 3.6 4.7 5.8 HR 7703 A 5.3 6.5 5.8 HR 7703 B 11.5 12.7 5.8 -45 13677 8.0 9.0 6.3 61 Cyg 5.2 7.5 3.4 -39 14192 6.7 8.7 3.9 -49 13515 8.9 10.6 4.6 ε Ind 4.7 7.0 3.5 DO Cep A 9.8 11.8 4.0 DO Cep B 11.4 13.4 4.0 L789-6 12.6 14.9 3.4 -21 6267 A 9.3 11.0 4.6 -21 6267 B 11.0 12.7 4.6 43 4305 10.0 11.5 5.0 Ross 780 10.2 11.7 4.9 -36 15693 7.4 9.6 3.7 56 2966 5.6 6.5 6.6 Ross 248 12.2 14.8 3.2 1 4774 8.9 10.0 6.1 γCeti 3.5 5.7 3.6 Spectral Class M4 M5 M7 K0 A7 G7 K4 M5 M0 K5 M0 M3 K5 M3 M4 M6 M2 M3 M5 M5 M2 K3 M6 M2 G8