Chapter 16--Properties of Stars

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apparent brightness period 0 25 50 75 100 125 150 175 200 Pulsating Variable Stars time (days) Figure 16.12 A typical light curve for a Cepheid variable star. Cepheids are giant, whitish stars whose luminosities regularly pulsate over periods of a few days to about a hundred days. The pulsation period of this Cepheid is about 50 days. Not all stars shine steadily like our Sun. Any star that significantly varies in brightness with time is called a variable star. A particularly important type of variable star has a peculiar problem with achieving the proper balance between the power welling up from its core and the power being radiated from its surface. Sometimes the upper layers of such a star are too opaque, so energy and pressure build up beneath the photosphere and the star expands in size. However, this expansion puffs the upper layers outward, making them too transparent. So much energy then escapes that the underlying pressure drops, and the star contracts again. In a futile quest for a steady equilibrium, the atmosphere of such a pulsating variable star alternately expands and contracts, causing the star to rise and fall in luminosity. Figure 16.12 shows a typical light curve for a pulsating variable star, with the star’s brightness graphed against time. Any pulsating variable star has its own particular period between peaks in luminosity, which we can discover easily from its light curve. These periods can range from as short as several hours to as long as several years. Most pulsating variable stars inhabit a strip (called the instability strip) on the H–R diagram that lies between the main sequence and the red giants (Figure 16.13). A special category of very luminous pulsating variables lies in the upper portion of this strip: the Cepheid variables,or Cepheids (so named because the first identified star of this type was the star Delta Cephei). Cepheids fluctuate in luminosity with periods of a few days to a few months. In 1912, another woman astronomer at Harvard, Henrietta Leavitt, discovered that the periods of these stars are very closely related to their luminosities: The longer the period, the more luminous the star. This period– luminosity relation holds because larger (and hence more luminous) Cepheids take longer to pulsate in and out in size. Once we have measured the period of a Cepheid variable, we can use the period–luminosity relation to determine its luminosity. We can then calculate its distance with 536 part V • Stellar Alchemy luminosity (solar units) 10 6 10 5 10 4 10 3 10 2 10 10 2 Figure 16.13 An H–R diagram with the instability strip highlighted. the luminosity–distance formula. In fact, as we’ll discuss in Chapter 20, Cepheids provide our primary means of measuring distances to other galaxies and thus teach us the true scale of the cosmos. The next time you look at the North Star, Polaris, gaze upon it with renewed appreciation. Not only has it guided generations of navigators in the Northern Hemisphere, but it is also one of these special Cepheid variable stars. astronomyplace.com 1 0.1 10 3 10 4 10 5 10 Solar Radii 1 Solar Radius 0.1 Solar Radius 10 2 Solar Radius 10 3 Solar Radius 10 2 Solar Radii Stellar Evolution Tutorial, Lessons 1, 4 16.6 Star Clusters Cepheids with periods of days instability strip All stars are born from giant clouds of gas. Because a single interstellar cloud can contain enough material to form many stars, stars almost inevitably form in groups. In our snapshot of the heavens, many stars still congregate in the groups in which they formed. These groups are of two basic types: modest-size open clusters and densely packed globular clusters. Open clusters of stars are always found in the disk of the galaxy (see Figure 1.18). They can contain up to several thousand stars and typically span about 30 light-years (10 parsecs). The most famous open cluster is the Pleiades, a prominent clump of stars in the constellation Taurus (Figure 16.14). The Pleiades are often called the Seven Sisters, although only six of the cluster’s several thousand stars are easily visible to the naked eye. Other cultures have other names for this beautiful group of stars. In Japanese it is called Subaru, which is why the logo for Subaru automobiles is a diagram of the Pleiades. Globular clusters are found primarily in the halo of our galaxy, although some are in the disk. A globular clus- Sun Polaris surface temperature (Kelvin) 10 3 Solar Radii variable stars with periods of hours (called RR Lyrae variables) 30,000 10,000 6,000 3,000
ter can contain more than a million stars concentrated in a ball typically from 60 to 150 light-years across (20 to 50 parsecs). Its innermost part can have 10,000 stars packed within a region just a few light-years across (Figure 16.15). The view from a planet in a globular cluster would be marvelous, with thousands of stars lying closer than Alpha Centauri is to the Sun. Because a globular cluster’s stars nestle so closely, they engage in an intricate and complex dance choreographed by gravity. Some stars zoom from the cluster’s core to its outskirts and back again at speeds approaching the escape velocity from the cluster, while others orbit the dense core more closely. When two stars pass especially close to each other, the gravitational pull between them deflects their trajectories, altering their speeds and sending them careening off in new directions. Occasionally, a close encounter boosts one star’s velocity enough to eject it from the cluster. Through such ejections, globular clusters gradually lose stars and grow more compact. Star clusters are extremely useful to astronomers for two key reasons: 1. All the stars in a cluster lie at about the same distance from Earth. 2. Cosmically speaking, all the stars in a cluster formed at about the same time (i.e., within a few million years of one another). Astronomers can therefore use star clusters as laboratories for comparing the properties of stars, as yardsticks for measuring distances in the universe [Section 20.3], and as timepieces for measuring the age of our galaxy. We can use clusters as timepieces because we can determine their ages from H–R diagrams of cluster stars. Figure 16.14 A photo of the Pleiades, a nearby open cluster of stars. The most prominent stars in this open cluster are of spectral type B, indicating that the Pleiades are no more than 100 million years old, relatively young for a star cluster. The region shown here is about 11 light-years across. Figure 16.15 This globular VIS cluster, known as M 80, is over 12 billion years old. The prominent reddish stars in this Hubble Space Telescope photo are red giant stars nearing the ends of their lives. The region pictured here is about 15 light-years across. VIS chapter 16 • Properties of Stars 537
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Chapter 16--Properties of Stars

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