PENN STATE ASTRONOMY LABORATORY #10

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1 PENN STATE ASTRONOMY LABORATORY #10 THE LIVES OF THE STARS I. Objective In the labs you have done so far, you have examined the physical properties and classifications of stars. All of these properties and classifications were based on a single instant in the star s lifetime. However, stars evolve and change over their lives. In this lab you will be examining the ages, life-cycles, and evolutionary states of stars. You will use two different groups of stars: one an open star cluster called the Pleiades, and the other a globular star cluster called M15. By examining a luminosity-temperature diagram for each of these clusters, you will see which of their stars have already completed their lives as main sequence stars. This will tell you how long the stars in these clusters have existed until now, so you can determine the ages of the Pleiades and M15. II. Exercises: Open Star Clusters, the Pleiades To see the Pleiades in the fall, you need to look well after midnight. In the winter and spring, this group is conspicuous, high in the sky. The seven brightest stars in the Pleiades have names, which originate in Greek mythology: #1...Merope #5...Maia #2... Alcyone #6... Calaeno #3... Electra #7... Pleione #4... Taygeta These seven stars are often nicknamed the Seven Sisters. With exceptional eyesight, you could see the seven brightest stars without a telescope; most people can only see five. However, the Pleiades actually contains several hundred stars. The following table lists the luminosities and spectral types of 24 stars in the Pleiades cluster: Star Solar Spectral Luminosity Type A 1,940 B7 B 1,150 B6 C 1,940 B8 D 1,340 B6 E 929 B6 F 738 B8 G 160 B9 H 138 B9 J 105 B9 K 52.4 A1 L 35.6 A5 M 25.6 A3 Star Solar Spectral Luminosity Type N 19.0 A6 P 14.2 A5 Q 9.20 F1 R 6.98 A9 S 4.78 F2 T 3.43 F3 U 3.05 F6 V 2.11 F6 W 1.43 F8 X 1.18 G2 Y 0.87 G6 Z 0.67 G6 63

2 a. Using the table of stars on page 63, plot an H-R diagram using the graph given on page 68. Your instructor will remind you how to plot a luminosity-temperature diagram, and how the spectral types listed here relate to the temperature scale you have already learned about in the H-R diagram lab. Label each point with the appropriate letter. Also label the axes on the graph (including units). b. The solid line already plotted on your Pleiades H-R Diagram represents the location of the main sequence. Compare the luminosities and temperatures of the Pleiades stars to those of the main sequence. Determine from your own results which of the Pleiades stars no longer appear to be main sequence stars and list their letters, explaining why you conclude they are not main sequence stars. For what follows, you need this information on the lengths of time that different kinds of stars can survive as main sequence stars: Spectral Main Sequence Spectral Main Sequence Type Lifetime Type Lifetime B0 12 million years A5 1.4 billion years B3 34 million years F0 7 billion years B5 100 million years G0 11 billion years B8 290 million years G5 18 billion years A0 540 million years K0 26 billion years A2 860 million years c. Study your diagram and decide where the main sequence terminates ( turns off ). Using the information above, estimate the age of the Pleiades cluster. Explain the reasoning behind your estimate. 64

3 III. Exercises: Globular Star Clusters, M15 Globular clusters contain some of the oldest stars known. They can be comprised of several thousand to a million stars. Like we did for the Pleiades, an H-R diagram can be used to determine the approximate age of a globular cluster, by finding where the main sequence terminates ( turns off ). Below is an H-R Diagram of M15 1 a globular cluster orbiting the center of our Milky Way Galaxy d. Do you notice anything different about this H-R Diagram, compared to the one you made of the Pleiades? Be as complete as possible! You already found the age of the Pleiades. The Pleiades is an open star cluster in the disk of our galaxy. Now, we will find the approximate age of M15, which is a globular cluster in the halo of our galaxy. e. Using the information given, determine the age of the globular cluster M15. Explain your answer! 1 The H-R Diagram of M15 is adapted from Durrell & Harris (1993). 65

4 Now, if we assume that the entire universe has a finite age, the universe has to be at least as old as M15, and is probably older. Let s assume that M15 formed about a billion years after the universe formed. f. From the information given, what is your estimate for the age of the universe? Explain your answer. Check to see if this answer makes sense by comparing it to the estimated age of the Earth, which is 4.6 billion years would you expect the universe to be older or younger than the Earth? Do the estimated ages agree with your expectations? IV. Exercises: Stellar Remnants A type B7 star like Alcyone in the Pleiades will end its stellar life by ejecting most of its mass into a planetary nebula. The remaining mass will collapse into a white dwarf. A white dwarf is a member of a class of objects called compact objects that also includes neutron stars and black holes. Compact objects are incredibly dense. g. Given that the density of a white dwarf is 10 9 kg/m 3 and that the density of a neutron star is kg/m 3, determine the weights, in tons, of: 1. a cubic centimeter (cm 3 ) of white dwarf matter, and 2. a cubic centimeter (cm 3 ) of neutron star matter. (1 ton = 900 kg) 66

5 Stars that are brighter than type B0 end their lives in huge explosions known as supernovae. A neutron star or a black hole is the remnant that is left after this event. The Schwarzchild radius (R) of a black hole (the radius of its event horizon ) is related to its mass (M) by the formula: R = 2GM c 2 where G = m 3 /kg/s 2 and c = m/s. h. Given that the mass of the Earth is kg, determine its Schwarzchild radius if it somehow collapses into a black hole. Do the same for the Sun (the mass of the Sun is kg). i. Summarize the facts and ideas presented, including any additional questions you may have. 67

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