Week 13 - Creating your own H-R Diagram

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Mt San Antonio College *

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Astronomy

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Dec 6, 2023

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Name: Key Creating your Own H-R Diagram In this lab we are going to measure the luminosity and temperature for 12 stars and use these results to create a Hertzsprung-Russell Diagram! We will complete this lab in a few steps! Background on Apparent and Absolute Magnitude a) We use apparent magnitude to describe how bright a star looks in the sky. The smaller the number the brighter the star looks. b) We use absolute magnitude to describe the luminosity of a star. The smaller the number, the higher the star’s luminosity. 1: Apparent Magnitude a) Question: Two stars are seen in the sky. Star A has an apparent magnitude of -2. Star B has an apparent magnitude of +3. Which star looks brighter in the sky? Star A because it has the smaller apparent magnitude. The smaller the apparent magnitude the brighter the star looks. b) On thje last page of the lab you will find star charts containing the 12 stars we will study today. The size of the circle marking a star indicates the star’s apparent magnitude. Match the size of the circle marking the star with the stellar magnitude grid on each page to determine the star’s apparent magnitude. Only whole number (5, 6, 7, etc.) stellar magnitude circles are given, so if you find that a star’s circle is in between two stellar magnitude circles then give the magnitude as a half (5.5, 6.5, 7.5, etc.). Record the apparent magnitudes for all 12 stars on the Table. Answers are on the chart at the end of the answer key. c) Questions: From your data, which star(s) appear brightest in the sky? Smallest apparent magnitude = brightest, so HD37767, HD27685 From your data, which star(s) appear faintest in the sky? Largest apparent magnitude = faintest, so HD107399, BD+63137, Feige 40, HD5351 Based on your data so far, can you tell which star has the highest luminosity? If so, which star is it? If not, what additional information do you need to determine the highest luminosity star? No – in order to tell luminosity you have to know the distance to the star. 1

2: Distance a) The parallax measured for each star is included in the Table. Before going any farther, answer the following questions: Which star is located closest to us? Explain your answer. BD +63137 because it has the largest parallax. Which star is farthest from us? Explain your answer. HD 221741 because it has the smallest parallax. b) Calculate the distance to each star using the following equation and record your answers in the Table: Distance (in light years) = 3260 / Parallax (in milliarcseconds) Note that this equation is different from the previous lab because the parallax is in milliarcseconds instead of arcseconds. c) Question: Which star is at the greatest distance? Does this match your answer to question a? HD 221741 - yes Which star is the closest? Does this match your answer to question a? BD +63137 - yes 3. Changing From Apparent to Absolute Magnitude a) Using the distance we are going to change the apparent magnitudes that you recorded into absolute magnitudes. b) For each star, calculate the magnitude offset and record it in the Table. If you need help finding the Log button on the calculator, just let me know! Magnitude Offset = 5 x log(Distance) – 7.57 2

c) The offset you entered is how much smaller a star’s absolute magnitude is compared with its apparent magnitude. Question: As you increase the distance to a star, what happens to the size of the offset? The greater the distance, the larger the offset. d) Calculate the absolute magnitude for each star using: Absolute magnitude = Apparent magnitude – Offset e) Question: Which star has the largest luminosity? Explain your answer. HD 221741 because it has the smallest absolute magnitude. Which star has the smallest luminosity? Explain your answer. BD +63137 because it has the largest absolute magnitude. 4. Stellar Type / Temperature Now that we have measured the luminosity / absolute magnitude of each star we also want to measure the spectral type and surface temperature of each star. At the end of this lab you will find a set of spectra – these are spectra of different spectral type main sequence stars. Look at these spectra to answer the following questions: a) Which of the 13 spectra types included is the hottest? O5V b) Which of the 13 spectra types included is the coldest? M5V c) Look at the spectra of the hottest and coldest main sequence stars in the set. Describe the differences you see between the spectral lines of the two. The spectral lines are in different locations. d) The right side of each spectrum represents red and the left side of each spectrum is blue. How does the amount of blue and red differ between the hottest and coldest main sequence stars? Explain why this makes sense based on the temperature of the stars. The hot star has more blue and less red, while the cold star has more red and less blue. This makes sense because the hot star is blue in color and the cold star is red in color. e) After the spectra of the main sequence stars, you will see spectra of the stars in our sample today. For each star in the sample, figure out which main sequence star best matches the spectrum of the star. This “best match” is the spectral type of the star. Record the spectral type of each of your stars in the Table. You can use a spectral type more than once! 5) Constructing an H-R Diagram 3

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