The Nature of the StarsGuiding QuestionsParallaxCareful measurements of the parallaxes of stars reveal their distances Barnard’s star has a parallax of 0.54 arcsec If a star’s distance is known, its luminosity can be determined from its brightnessThe Population of StarsStellar MotionsAstronomers often use the magnitude scale to denote brightness – a scale that was introduced by the ancient Greeks about 300 BA star’s color depends on its surface temperature - recall Wien’s LawPhotometry and Color RatiosThe spectra of stars reveal their chemical compositions as well as surface temperatures Full Spectral Typing Spectral Class and Luminosity ClassRelationship between a star’s luminosity, radius, and surface temperature Flowchart of Key Stellar ParametersThe Hertzsprung-Russell (H-R) DiagramPathway to Spectroscopic ParallaxA Binary Star SystemBinary StarsBinary Star Systems and Stellar MassesMass-Luminosity Relation for Main-Sequence StarsSpectroscopy makes it possible to study binary systems in which the two stars are close togetherLight curves of eclipsing binaries provide detailed information about the two starsKey WordsThe Nature of the StarsChapter 19Guiding Questions1. How far away are the stars?2. What evidence do astronomers have that the Sun is a typical star?3. What is meant by a “first-magnitude” or “second magnitude” star?4. Why are some stars red and others blue?5. What are the stars made of?6. As stars go, is our Sun especially large or small?7. What are giant, supergiant, and white dwarf stars?8. How do we know the distances to remote stars?9. Why are binary star systems important in astronomy?10.How can a star’s spectrum show whether it is actually a binary star system?11.What do astronomers learn from stars that eclipse each other?ParallaxCareful measurements of the parallaxes of starsreveal their distances• Distances to the nearer stars can be determined by parallax, theapparent shift of a star against the background stars observed as the Earth moves along its orbit• Parallax measurements made from orbit, above the blurring effects of the atmosphere, are much more accurate than those made with Earth-based telescopes• Stellar parallaxes can only be measured for stars within a few hundred parsecsBarnard’s star has a parallax of 0.54 arcsecIf a star’s distance is known, its luminosity can bedetermined from its brightness• A star’s luminosity (total light output), apparent brightness, and distance from the Earth are related by the inverse-square law• If any two of these quantities are known, the third can be calculatedThe Population of Stars• Stars of relatively low luminosity are more common than more luminous stars• Our own Sun is a rather average star of intermediate luminosityStellar MotionsAstronomers often use the magnitude scaleto denote brightness – a scale that was introduced by the ancient Greeks about 300 BC• The apparent magnitude scale is an alternative way to measure a star’s apparent brightness• The absolute magnitude of a star is the apparent magnitude it would have if viewed from a distance of 10 parsecsA star’s color depends on its surface temperature- recall Wien’s LawPhotometry and Color Ratios• Photometry measures the apparent brightness of a star• The color ratios of a star are the ratios of brightness values obtained through different standard filters, such as the U, B, and V filters• The color ratios are a measure of the star’s surface temperatureThe spectra of stars reveal their chemicalcompositions as well as surface temperatures • Stars are classified into spectral types– divisions of the spectral classes• O, B, A, F, G, K, and M– Subclasses• 0, 1, 2, 3, 4, 5, 6, 7, 8, 9• The original letter classifications originated from the late 1800s and early 1900s•The spectral class of a star is directly related to its surface temperature– O stars are the hottest– M stars are the coolest• Brown dwarfs are in even cooler spectral classes now called L and T– Unlike true stars, brown dwarfs are too small to sustain thermonuclear fusionFull Spectral TypingSpectral Class and Luminosity Class•The Sun– Classified as a G2 V• Luminosity classes (use Roman numerals)– I – Giant– II – Giant– III – Giant– IV – Sub-giant– V – Main SequenceRelationship between a star’s luminosity, radius, and surface temperature• Stars come in a wide variety of sizes• Recall Stefan-Boltzmann LawFlowchart of Key Stellar ParametersThe Hertzsprung-Russell (H-R) Diagram• The H-R diagram is a graph plotting the absolute magnitudes of stars against their spectral types—or, equivalently, their luminosities against surface temperatures• The positions on the H-R diagram of most stars are along the main sequence, a band that extends from high luminosity and high surface temperature to low luminosity and low surface temperatureOn the H-R diagram, giant and supergiantstars lie above the main sequence, while white dwarfs are below the main sequenceBy carefully examining a star’s spectral lines, astronomers can determine whether that star is a main-sequence star, giant, supergiant, or white dwarfUsing the H-R diagram and the inverse square law, the star’s luminosity and distance can be found without measuring its stellar parallaxPathway toSpectroscopic ParallaxA Binary Star SystemBinary Stars• Binary Stars– Two stars held in orbit around each other by their mutual gravitational attraction• Surprisingly common• Visual Binary– Those binary star systems that can be resolved into two distinct star images by an Earth-based telescope are called visual binaries• Each of the two stars in a binary system moves in an elliptical orbit about the center of mass of the systemSampleBinaryStarSystemBinary Star System AnalogyBinary Star Systems and Stellar Masses• Binary stars are important because they allow astronomers to determine the masses of the two stars in a binary system• The masses can be computed from measurements of the orbital period and orbital dimensions of the systemMass-Luminosity Relation forMain-Sequence Stars• Main sequence stars are stars like the Sun but with different masses• The mass-luminosity relation expresses a direct correlation between mass and luminosity for main-sequence stars• The greater the mass of a main-sequence star, the greater its luminosity (and also the greater its radius and surface temperature)The H-R Diagram View of theMain