Chapter 16--Properties of Stars
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2396_AWL_Bennett_Ch16 6/26/03 1:58 PM Page 521 16 Properties of Stars LEARNING GOALS 16.1 Snapshot of the Heavens 16.5 The Hertzsprung–Russell Diagram • How can we learn about the lives of stars, which last • What is the Hertzsprung–Russell (H–R) diagram? millions to billions of years? • What are the major features of the H–R diagram? • What are the two main elements in all stars? • How do stars differ along the main sequence? • What two basic physical properties do astronomers • What determines the length of time a star spends on use to classify stars? the main sequence? • What are Cepheid variable stars, and why are they 16.2 Stellar Luminosity important to astronomers? • What is luminosity, and how do we determine it? • How do we measure the distance to nearby stars? 16.6 Star Clusters • How does the magnitude of a star relate to its • What are the two major types of star cluster? apparent brightness? • Why are star clusters useful for studying stellar evolution? 16.3 Stellar Surface Temperature • How do we measure the age of a star cluster? • How are stars classified into spectral types? • What determines a star’s spectral type? 16.4 Stellar Masses • What is the most important property of a star? • What are the three major classes of binary star systems? • How do we measure stellar masses? 521 2396_AWL_Bennett_Ch16 6/26/03 1:58 PM Page 522 “All men have the stars,” he answered, mysterious points of light in the sky. We now know that all “but they are not the same things for stars form in great clouds of gas and dust. Each star begins different people. For some, who are its life with roughly the same chemical composition: About travelers, the stars are guides. For others three-quarters of the star’s mass at birth is hydrogen, and they are no more than little lights in the about one-quarter is helium, with no more than about 2% sky. For others, who are scholars, they consisting of elements heavier than helium. During most are problems. For my businessman they of any star’s life, the rate at which it generates energy de- were wealth. But all these stars are pends on the same type of balance between the inward pull silent. You—you alone—will have the of gravity and the outward push of internal pressure that stars as no one else has them.” governs the rate of fusion in our Sun. Despite these similarities, stars appear different from Antoine de Saint-Exupéry, from The Little Prince one another for two primary reasons: They differ in mass, and we see different stars at different stages of their lives. The key that finally unlocked these secrets of stars was n a clear, dark night, a few thousand stars an appropriate classification system. Before the twentieth are visible to the naked eye. Many more century, humans classified stars primarily by their bright- ness and location in our sky. The names of the brightest become visible through binoculars, and O stars within each constellation still bear Greek letters desig- with a powerful telescope we can see so many stars nating their order of brightness. For example, the brightest that we could never hope to count them. Like indi- star in the constellation Centaurus is Alpha Centauri, the second brightest is Beta Centauri, the third brightest is vidual people, each individual star is unique. Like the Gamma Centauri, and so on. However, a star’s brightness human family, all stars share much in common. and membership in a constellation tell us little about its true Today, we know that stars are born from clouds nature. A star that appears bright could be either extremely luminous or unusually nearby, and two stars that appear of interstellar gas, shine brilliantly by nuclear fusion right next to each other in our sky might not be true neigh- for millions or billions of years, and then die, some- bors if they lie at significantly different distances from Earth. times in dramatic ways. This chapter outlines how Today, astronomers classify a star primarily according to its luminosity and surface temperature. Our task in this we study and categorize stars and how we have come chapter is to learn how this extraordinarily effective classi- to realize that stars, like people, change over their fication system reveals the true natures of stars and their lifetime. life cycles. We begin by investigating how to determine a star’s luminosity, surface temperature, and mass. ypla om ce n . o c r o t m s a Measuring Cosmic Distances Tutorial, Lesson 2 16.1 Snapshot of the Heavens Imagine that an alien spaceship flies by Earth on a simple 16.2 Stellar Luminosity but short mission: The visitors have just 1 minute to learn A star’s luminosity is the total amount of power it radiates everything they can about the human race. In 60 seconds, into space, which can be stated in watts. For example, the they will see next to nothing of each individual person’s Sun’s luminosity is 3.8 ϫ 1026 watts [Section 15.2].We can- life. Instead, they will obtain a collective “snapshot” of hu- not measure a star’s luminosity directly, because its bright- manity that shows people from all stages of life engaged in ness in our sky depends on its distance as well as its true their daily activities. From this snapshot alone, they must luminosity. For example, our Sun and Alpha Centauri A piece together their entire understanding of human beings (the brightest of three stars in the Alpha Centauri system) and their lives, from birth to death. are similar in luminosity, but Alpha Centauri A is a feeble We face a similar problem when we look at the stars. point of light in the night sky, while our Sun provides Compared with stellar lifetimes of millions or billions enough light and heat to sustain life on Earth. The differ- of years, the few hundred years humans have spent study- ence in brightness arises because Alpha Centauri A is ing stars with telescopes is rather like the aliens’ 1-minute about 270,000 times farther from Earth than is the Sun. glimpse of humanity. We see only a brief moment in any More precisely, we define the apparent brightness of star’s life, and our collective snapshot of the heavens con- any star in our sky as the amount of light reaching us per sists of such frozen moments for billions of stars. From this unit area (Figure 16.1). (A more technical term for appar- snapshot, we try to reconstruct the life cycles of stars while ent brightness is flux.) The apparent brightness of any light also analyzing what makes one star different from another. source obeys an inverse square law with distance, similar Thanks to the efforts of hundreds of astronomers to the inverse square law that describes the force of grav- studying this snapshot of the heavens, stars are no longer ity [Section 5.3].Ifwe viewed the Sun from twice Earth’s 522 part V•Stellar Alchemy 2396_AWL_Bennett_Ch16 6/26/03 1:58 PM Page 523 COMMON MISCONCEPTIONS Luminosity is the total amount of power (energy per second) Photos of Stars the star radiates into space. Photographs of stars, star clusters, and galaxies convey a great deal of information, but they also contain a few arti- facts that are not real. For example, different stars seem to have different sizes in photographs, but stars are so far away that they should all appear as mere points of light. Stellar sizes in photographs are an artifact of how our instruments record light. Because of the problem of overexposure, brighter stars tend to appear larger than dimmer stars. Overexposure can be a particular problem for photo- graphs of globular clusters of stars and photographs of galaxies. These objects are so much brighter near their centers than in their outskirts that the centers are al- most always overexposed in photographs that show the Apparent brightness is the amount of starlight outskirts. That is why globular clusters and galaxies often Not to scale! reaching Earth (energy look in photographs as if their central regions contain a per second per square single bright blob, when in fact the centers contain many meter). individual stars separated by vast amounts of space. Figure 16.1 Luminosity is a measure of power, and apparent Spikes around bright stars in photographs, often brightness is a measure of power per unit area. making the pattern of a cross with a star at the center, are another such artifact. These spikes are not real but rather are created by the interaction of starlight with the supports holding the secondary mirror in the telescope [Section 7.2]. The spikes generally occur only with point sources of light like stars, and not with larger objects like galaxies. When you look at a photograph showing many galaxies (for example, Figure 20.1), you can tell which objects are stars by looking for the spikes. 2 distance, it would appear dimmer by a factor of 2 ϭ 4. If 1 AU we viewed it from 10 times Earth’s distance, it would ap- 2 AU pear 102 ϭ 100 times dimmer. From 270,000 times Earth’s distance, it would look like Alpha Centauri A—dimmer 3 AU by a factor of 270,0002,or about 70 billion. Figure 16.2 shows why apparent brightness follows an inverse square law. The same total amount of light must pass through each imaginary sphere surrounding the star.