Chapter 16--Properties of Stars

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• How does the magnitude of a star relate to its apparent brightness? The magnitude scale runs backward, so a star of magnitude 5 is brighter than a star of magnitude 18. 16.3 Stellar Surface Temperature • How are stars classified into spectral types? From hottest to coolest, the major spectral types are O, B, A, F, G, K, and M. These types are futher subdivided into numbered categories. For example, the hottest A stars are type A0 and the coolest A stars are type A9, which is slightly hotter than F0. • What determines a star’s spectral type? The main factor in determining a star’s spectral type is its surface temperature. Spectral type does not depend much on composition, because the compositions of stars—primarily hydrogen and helium—are nearly the same. 16.4 Stellar Masses • What is the most important property of a star? A star’s most important property is its mass, which determines its luminosity and spectral type at each stage of its life. • What are the three major classes of binary star systems? A visual binary is a pair of orbiting stars that we can see distinctly through a telescope. An eclipsing binary reveals its binary nature because of periodic dimming that occurs when one star eclipses the other as viewed from Earth. A spectroscopic binary reveals its binary nature when we see the spectral lines of one or both stars shifting back and forth as the stars orbit each other. • How do we measure stellar masses? We can directly measure stellar mass only in binary systems for which we are able to determine the period and separation of the two orbiting stars. We can then calculate the system’s mass using Newton’s version of Kepler’s third law. 16.5 The Hertzsprung–Russell Diagram • What is the Hertzsprung–Russell (H–R) diagram? The H–R diagram is the most important classification tool in stellar astronomy. Stars are located on the H–R diagram by their surface temperature (or spectral type) along the horizontal axis and their luminosity along the vertical axis. Surface temperature decreases from left to right on the H–R diagram. • What are the major features of the H–R diagram? Most stars occupy the main sequence, which extends 540 part V • Stellar Alchemy diagonally from lower right to upper left. The giants and supergiants inhabit the upper-right region of the diagram, above the main sequence. The white dwarfs are found near the lower left, below the main sequence. • How do stars differ along the main sequence? All main-sequence stars are fusing hydrogen to helium in their cores. Stars near the lower right of the main sequence are lower in mass and have longer lifetimes than stars further up the main sequence. Lower-mass main-sequence stars are much more common than higher-mass stars. • What determines the length of time a star spends on the main sequence? A star’s mass determines how much hydrogen fuel it has and how fast it fuses that hydrogen into helium. The most massive stars have the shortest lifetimes because they fuse their hydrogen at a much faster rate than do lower-mass stars. • What are Cepheid variable stars, and why are they important to astronomers? Cepheid variables are very luminous pulsating variable stars that follow a period–luminosity relation, which means we can calculate luminosity by measuring pulsation period. Once we know a Cepheid’s luminosity, we can calculate its distance with the luminosity–distance formula. This technique enables us to measure distances to many other galaxies in which we have observed these variable stars. 16.6 Star Clusters • What are the two major types of star cluster? Open clusters contain up to several thousand stars and are found in the disk of the galaxy. Globular clusters are much denser, containing hundreds of thousands of stars, and are found mainly in the halo of the galaxy. Globular-cluster stars are among the oldest stars known, with estimated ages of up to 12–14 billion years. Open clusters are generally much younger than globular clusters. • Why are star clusters useful for studying stellar evolution? The stars in star clusters are all at roughly the same distance and, because they were born at about the same time, are all about the same age. • How do we measure the age of a star cluster? The age of a cluster is equal to the main-sequence lifetime of the hottest, most luminous main-sequence stars remaining in the cluster. On an H–R diagram of the cluster, these stars sit farthest to the upper left and define the main-sequence turnoff point of the cluster.
True Statements? Decide whether each of the following statements is true or false and clearly explain how you know. 1. Two stars that look very different must be made of different kinds of elements. 2. Sirius is the brightest star in the night sky, but if we moved it 10 times farther away it would look only one-tenth as bright. 3. Sirius looks brighter than Alpha Centauri, but we know that Alpha Centauri is closer because its apparent position in the sky shifts by a larger amount as Earth orbits the Sun. 4. Stars that look red-hot have hotter surfaces than stars that look blue. 5. Some of the stars on the main sequence of the H–R diagram are not converting hydrogen into helium. 6. The smallest, hottest stars are plotted in the lower left-hand portion of the H–R diagram. 7. Stars that begin their lives with the most mass live longer than less massive stars because it takes them a lot longer to use up their hydrogen fuel. 8. Star clusters with lots of bright, blue stars are generally younger than clusters that don’t have any such stars. 9. All giants, supergiants, and white dwarfs were once mainsequence stars. 10. Most of the stars in the sky are more massive than the Sun. Problems 11. Similarities and Differences. What basic composition are all stars born with? Why do stars differ from one another? 12. Across the Spectrum. Explain why we sometimes talk about wavelength-specific (e.g., visible-light or X-ray) luminosity or apparent brightness, rather than total luminosity and total apparent brightness. 13. Determining Parallax. Briefly explain how we calculate a star’s distance in parsecs by measuring its parallax angle in arcseconds. 14. Magnitudes. What is the magnitude system? Briefly explain what we mean by the apparent magnitude and absolute magnitude of a star. 15. Deciphering Stellar Spectra. Briefly summarize the roles of Annie Jump Cannon and Cecilia Payne-Gaposchkin in discovering the spectral sequence and its meaning. 16. Eclipsing Binaries. Describe why eclipsing binaries are so important for measuring masses of stars. 17. Basic H–R Diagram. Draw a sketch of a basic Hertzsprung– Russell (H–R) diagram. Label the main sequence, giants, supergiants, and white dwarfs. Where on this diagram do we find stars that are cool and dim? Cool and luminous? Hot and dim? Hot and bright? 18. Luminosity Classes. What do we mean by a star’s luminosity class? On your sketch of the H–R diagram from problem 17, identify the regions for luminosity classes I, III, and V. 19. Pulsating Variables. What are pulsating variable stars? Why do they vary periodically in brightness? 20. H–R Diagrams of Star Clusters. Explain why H–R diagrams look different for star clusters of different ages. How does the location of the main-sequence turnoff point tell us the age of the star cluster? 21. Stellar Data. Consider the following data table for several bright stars. M v is absolute magnitude, and m v is apparent magnitude. Spectral Luminosity Star M v m v Type Class Aldebaran 0.2 0.9 K5 III Alpha Centauri A 4.4 0.0 G2 V Antares 4.5 0.9 M1 I Canopus 3.1 0.7 F0 II Fomalhaut 2.0 1.2 A3 V Regulus 0.6 1.4 B7 V Sirius 1.4 1.4 A1 V Spica 3.6 0.9 B1 V Answer each of the following questions, including a brief explanation with each answer. a. Which star appears brightest in our sky? b. Which star appears faintest in our sky? c. Which star has the greatest luminosity? d. Which star has the least luminosity? e. Which star has the highest surface temperature? f. Which star has the lowest surface temperature? g. Which star is most similar to the Sun? h. Which star is a red supergiant? i. Which star has the largest radius? j. Which stars have finished burning hydrogen in their cores? k. Among the main-sequence stars listed, which one is the most massive? l. Among the main-sequence stars listed, which one has the longest lifetime? 22. Data Tables. Study the spectral types listed in Appendix F for the 20 brightest stars and for the stars within 12 lightyears. Why do you think the two lists are so different? Explain. *23. The Inverse Square Law for Light. Earth is about 150 million km from the Sun, and the apparent brightness of the Sun chapter 16 • Properties of Stars 541
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Chapter 16--Properties of Stars

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