Objectives: Stars Have Several Properties At First

Objectives Stars Have A Number Of Properties Which At First Glance

Objectives : Stars have a number of properties which, at first glance, may appear to be unrelated. But further analysis shows that they are related after all. Two astronomers, independently of each other, plotted the luminosities of a number of stars versus their temperatures to create what is now known as the "Hertzsprung-Russell", or "H-R" diagram (named after the scientists who invented it), which allows us to study this relationship between properties. In this lab, you will find that for the majority of stars there is a definite relationship between temperature and luminosity. You will also find out that other properties of stars are also related.

In fact, the H-R diagram contains a surprisingly large amount of information in one simple graph. I. Plotting and understanding the H-R diagram In the two tables and graph that follow, Temperatures are in Kelvins (K), and Luminosities and Masses are given in units relative to the Sun; the units are called: "Solar Luminosities" and "Solar Masses" respectively, which are abbreviated as L☉ and M☉. Note that the values for the Sun in these units are 1.0 L☉ and 1.0 M☉ by definition. You may ignore the column "Spectral Type" in the tables.

First is a list of the 25 nearest stars in the sky , starting with the nearest: Next is a list of the 25 visually brightest stars in the sky, starting with the brightest (this list shares a few entries in common with the previous list): Finally, here is a graph showing each star's Temperature versus Luminosity. Data for the nearest stars have been plotted using a box symbol, while data for the brightest stars have been plotted using an x. For the FIVE stars that appear on both lists, they are plotted on the graph using BOTH symbols -- so you will see a box with an x inside, which looks rather like a filled-in box. Print out or save a copy of this H-R Diagram (image opens in new browser window, or you can download a PDF of the graph here ) so you can draw and write on it in order to answer the following questions.

You can either draw on a paper copy, or digitally annotate a copy of the file on your computer. Whether you choose the paper or computer drawing method, you will be handing in your drawing at the end, so be sure to follow all instructions that tell you to write or draw on it! Instructions to write or draw on the diagram will be indicated by an underline, like this. 1. You should see a general trend among a majority of the stars.

This trend is called the "Main Sequence". Circle the main sequence on your diagram -- that is, circle the stars which follow the trend that you observe; the stars are called "main sequence stars", and the trend is the main sequence. How many stars does it include? Count the stars and type a whole number: stars. 2.

Describe this trend in one brief but complete sentence in your own words , making sure to include the properties of the stars that are represented (see the graph axes!) and HOW they are related to one another? 3. Find and label by name the following seven stars on your paper graph: The Sun, Barnard's Star, Sirius A, Sirius B, Regulus A, Deneb, and Betelgeuse. Note that the tickmarks on the axes (shown on the top and sides) are not evenly spaced. Nonetheless, the small tick marks between each large tick mark represent equal sized numerical steps.

E.g. on the vertical axis, between 1 and 10 are eight small tick marks, representing 2, 3, 4, 5, 6, 7, 8, and 9. And on the horizontal axis, between 3,000 and 10,000 there are six vertical dashed lines marking intervals of 1000, so representing 4000, 5000, 6000, 7000, 8000, and 9000. Moreover, between the dashed vertical lines for 3000 and 4000, there are nine small tick marks on the top axis (only) marking intervals of 100, so representing 3100, 3200, 3300... 3900. Which of those stars are on the main sequence?

Check all that apply. A) The Sun B) Barnard's Star C) Sirius A D) Sirius B E) Regulus A F) Deneb G) Betelgeuse H) None of these are on the main sequence 4. Which of those seven stars is closest to the top right corner of the graph? (Stars in this area are called Red Giants because their temperature is cool and thus their color is red, and they are swelled up to a huge "giant" size, making their luminosity very large.) Highlight correct answer below. The Sun Barnard's Star Sirius ASirius BRegulus ADeneb Betelgeuse 5. Which of those seven stars is closest to the bottom left corner of the graph? (Stars in this area are called white dwarfs.

They have high temperatures, glowing white or even blue, because they are the hot leftover core of a dead star. Their sizes are very small though, "dwarfs" in fact, because they are undergoing no nuclear fusion to keep them swelled up. Their small size gives them a small luminosity.) Highlight correct answer below. The Sun Barnard's Star Sirius ASirius BRegulus ADeneb Betelgeuse 6. For the main sequence stars you have already labeled on your paper copy of the graph, write their Mass (given in the table) next to their label on the graph.

Note: Barnard's Star's mass is 0.16 -- this is missing from the table. Now, label the location AND mass of the following additional four main sequence stars: "L726-8A" (the eighth closest star), "Ross 154" (the 12th closest), "61 Cyg A" (16th closest), and "Spica" (16th brightest; Mass = 10, which is missing from the table). List the order that ALL of your labeled main sequence stars (these four plus those from the previous questions) appear on the main sequence, from highest to lowest luminosity. 7. Look at the progression of masses that you just labeled along the main sequence.

Do you notice a trend? (If not, you may wish to label the masses of a few additional main sequence stars until you do notice a trend.) Describe in a sentence or two the trend that you notice with mass, making sure to include the other two properties of the stars -- as labeled on the graph axes -- as well in your description. II. Comparing Two Populations Remember that the H-R diagram above has two different populations of stars shown on it: the 25 nearest stars, and the 25 visually brightest stars, or those that appear the brightest to us as viewed from Earth. Draw a legend on your graph labeling which symbol represents each population. (See the description preceding the graph above.) 8. Look at how the two groups of stars are distributed on the H-R diagram.

Carefully describe any differences you notice between these two groups. Aim for 1-2 complete sentences. 9. Our galaxy is a collection of a couple hundred billion stars (and intervening material) all gravitationally bound to each other and orbiting their collective center of mass. If you could measure all of the stars in our galaxy and plot their properties on an H-R diagram, which set of stars on your paper (the nearest or the brightest) do you think would be a more representative sample of stars in the galaxy at large?

Explain WHY. Hint: As an example, use people as an analogy to stars. Think about last time you were in a big crowd of people, say at a large sporting event. Let's say you made a list of the 25 fans sitting closest to your seat, and another list of the 25 fans that were the loudest you could hear. Which list of people -- closest or loudest -- would be more representative of most of the fans in the stadium?

Just to complete the analogy: stars=people, distance=distance, visual brightness=perceived loudness. Just for fun: maybe luminosity=obnoxiousness, temperature=propensity for cursing, and mass (the underlying property that determines the others)=alcohol consumption?? 10. If a star were measured to have a temperature of 3,500 K, predict, by examination of your H-R diagram, the luminosity which you think this star most likely has. In a couple of sentences, explain how you made your prediction, and any assumptions you made.

III. Star radius on the H-R diagram There is another property of stars that is related to their temperature and luminosity: the star's radius (or physical size). The most luminous stars have either a very large temperature (hot things glow brightly), or a very large radius (the more glowing surface area there is, the more the total luminosity given off), or both. The most luminous stars have either a very large temperature (hot things glow brightly), or a very large radius (the more glowing surface area there is, the more the total luminosity given off), or both. The following exercise involves estimating star sizes based on their position on the H-R diagram. 11. The Sun's location on the H-R diagram should fall along one of the lines of constant radius you drew. How does the Sun's radius compare to Star Alpha? Highlight correct answer below. The sun is 100 times smaller. The sun is 10 times smaller. The sun is the same size as star Alpha. The sun is 10 times bigger. The sun is 100 times bigger. 12. You should now have three lines of constant radius on your H-R diagram. Each line represents a radius that is ten times larger than the previous one. Label the lines accordingly, e.g., "Radius = 1xAlpha", "Radius = 10xAlpha", and "Radius = 100xAlpha". Given these, in which direction on the H-R diagram do the radii of stars increase? Your answer does NOT need to be a complete sentence; a few words is fine. 13. Continue drawing several more lines of constant radius (or size) based on the pattern, until all labeled stars are encompassed. Label each line with the corresponding radius factor. Estimate the size of Betelgeuse relative to the Sun, based on your lines, entering a decimal number (e.g., 0.001, 1, 10000, etc.). Betelgeuse is about times ??? the size of the SUN. Do the same for Sirius B, relative to the Sun. Sirius B is ???? times the size of the SUN. 14. In this section, count the number of supergiants, red giants, and white dwarfs on your diagram. Calculate their percentages of the total stars plotted. Explain your calculations clearly. 15. Based on the fractions of stars in each group, what can you conclude about the relative durations of the main sequence, red giant, and white dwarf phases in a star's life? Use specific reasoning. 16. Using the star lifetime table, estimate the Sun's main sequence lifetime. Enter the number and label the unit ("million" or "billion"). 17. Which statement correctly describes the relationship between star mass and main sequence lifetime? Highlight the correct answer. 18. How does the star lifetime vary along the main sequence? Explain in your own words, considering mass, luminosity, temperature, and the diagram's properties. 19. If all stars in a cluster are formed simultaneously, how will the main sequence change after 5 billion years? After 25 billion years? Explain how these changes help estimate the age of a star cluster. 20. Order the star clusters from oldest to youngest. 21. Summarize the key concepts and conclusions from this lab, including properties of stars, trends in the data, different star groups, and how the H-R diagram illustrates stellar evolution. Be sure to include your annotated H-R diagram in your submission, either by scanning or taking a picture.