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CHAPTER 6 THE FAMILY OF STARS Topic Summaries stellar parallax (p) The small 6. 1 Star Distances apparent shift in position of a nearby How far away are the stars? • Distance is critical in astronomy. Finding the luminosities, diameters, and masses of stars requires first finding their distances. • Astronomers can measure the distance to nearby stars by observing their parallaxes. Stellar distances are commonly expressed in parsecs. One parsec is 206,265 AU- the distance to an imaginary star whose parallax is 1 arc second. star relative to distant background objects due to Earth's o rbital motion. parsec (pc) The distance to a hypothetical star whose parallax is 1 second of arc. 1 pc = 206,265 AU = 3.26 ly. Stars farther away than about 170 pc have parallaxes too small to measure from ground-based observatories. intrinsic brightness A measure of the amount of light a star produces. 6. 2 Apparent Brightness, Intrinsic Brightness, and Luminosity flux A measure of the flow of energy How much energy do stars make? light. out of a surface. Usually applied to absolute visual magnitude (Mu) Intrinsic brightness of a star. The Once you know the distance to a star, you can find its intrinsic brightness, which can be expressed as its absolute magnitude. The absolute magnitude of a star equals the apparent magnitude it would have if it w ere 10 pc away. apparent visual magnitude the star would have if it were 10 pc away. lumi osity (L) The total amount of energ(' a star radiates per second at all wavelengths. 6. 3 Star Temperatures How can you tell a star's temperature using its spectrurt:~? ~ spectral class A sta r's position in the temperature classification system 0, B, A, F, G, K, M, based on the The hydrogen Balmer lines are weak in cool stars because atoms are not excite(! out of the ground state. In hot stars, the Balmer lines are weak because atoms are excited to higher orbits or are ionized. Only at intermediate temperatures are the Balmer lines strong. spectral sequence The The strength s of many spectral lines in a star's spectrum can be used to tell you its temperature. Stars are classified in the temperature spectral sequence: 0, B, A, F, G, K, M. Hertzsprung-Russell (H-R) diagram A plot of the intrinsic Long after the spectral sequence was created, astronomers discovered LandT objects with temperatures even cooler than th e M stars. temperature of stars. It separates appearance of the star's spectrum. arrangement of spectral classes (0, B, A, F, G, K, M) ranging from hot to cool. brightness versus the surface the effects of temperature and 6. 4 Star Sizes How big are stars? The H-R diagram is a plot of luminosity versus surface temperature. It is an important graph in astronomy becau se it sorts the stars into categories by size. Roughly 90 percent of normal stars, including the sun, fall on the main sequence, with the hotter main-sequence stars being more luminous. The giants and supergiants, however, are much larger and lie above the main sequence in the diagram. Some of the white dwarfs are hot stars, but they fall below the main sequence because they are so small. The large size of the giants and supergiants means their atmospheres have low densities. Giant stars, luminosity class Ill, have narrow spectral lines, and supergiants, class I, have extremely narrow lines. Class V main-sequence stars have relatively broad spectral lines. surface area on stellar luminosity and is commonly plotted as absolute magnitude versus spectral type but also as luminosity versus surface temperature or color. main sequence The region of the H-R diagram running from upper left to lower right, which includes roughly 90 percent of all stars generating energy by nuclear fusion. giant Large, cool, highly luminous star in the upper right of the H- R diagram, typically 10 to 100 times the diameter of the sun. Key Terrns Topic Surnrnaries supergiant Exceptionally luminous • star whose diameter is 100 to 1,000 Stars so far away that their parallaxes are too small to measure can have their distances estimated by the technique of spectroscopic parallax. times that of the sun. red dwarf A faint, cool, low-mass, 6. 5 Star Masses-Binary Stars main-sequence star. How much matter do stars contain? white dwarf Dying star at the collapsed to the size of Earth and is • The only direct way you can find the mass of a star is by studying binary stars. When two stars orbit a common center of mass, astronomers find their masses by observing the period and sizes of their orbits. slowly cooling off. • Few binary star systems are easy to analyze; most are spectroscopic binaries in which the component stars are known only by the alternating Doppler shifts of their spectral lines. • Eclipsing binary star systems allow measurement not just of the component stars' masses but also independent checks on their temperatures and diameters. lower left of the H-R diagram that has luminosity class A category of stars of similar luminosity, determined by the widths of lines in their spectra. spectroscopic parallax The method of determining a star's distance by comparing its apparent 6. 6 'TYpical Stars magnitude with its absolute magnitude as estimated from its What is the typical star like? spectrum. binary stars Pairs of stars that orbit around their common center of mass. visual binary system A binary star system in which the two stars are separately visible in the telescope. spectroscopic binary system A star system in which the stars are too close together to be visible separately. We see a single point of light, and only by taking a spectrum can we determine that there are two stars. eclipsing binary system A binary star system in which the stars cross in front of each other as seen from Earth. light curve A graph of brightness versus time commonly used in analyzing variable stars and eclipsing binaries. mass-luminosity relation The more massive a main-sequence star is, the more luminous it is. Review Guestions 1. Why are Earth-based parallax measurements limited to the nearest stars? 7. What observations would you make to classify a star according to its luminosity? Why does that method work? 2. What does luminosity measure that is different from what absolute visual magnitude measures? 8. Why does the orbital period of a 3. Why are hydrogen Balmer lines strong in the spectra of medium-temperature stars and weak in the spectra of hot and cool stars? 9. Why don't astronomers know the inclination of a spectroscopic binary? How do they know the inclination of an eclipsing binary? 4. Why does the luminosity of a star depend on both its radius and its temperature? 5. How can you be sure that giant stars really are larger than mainsequence stars? 6. Why do astronomers conclude that white dwarfs must be very small? binary star depend on its mass? 10. If all of the stars in the photo to the left are members of the same star cluster, then they all have about the same distance. Then why are three of the brightest much redder than the rest? What kind of star are they?