* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Universe 8e Lecture Chapter 17 Nature of Stars
Aries (constellation) wikipedia , lookup
Canis Minor wikipedia , lookup
Observational astronomy wikipedia , lookup
Dyson sphere wikipedia , lookup
Auriga (constellation) wikipedia , lookup
Corona Borealis wikipedia , lookup
Cassiopeia (constellation) wikipedia , lookup
Corona Australis wikipedia , lookup
Type II supernova wikipedia , lookup
Star of Bethlehem wikipedia , lookup
Cygnus (constellation) wikipedia , lookup
Star catalogue wikipedia , lookup
Canis Major wikipedia , lookup
Stellar classification wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Malmquist bias wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Stellar kinematics wikipedia , lookup
Hayashi track wikipedia , lookup
Star formation wikipedia , lookup
Stellar evolution wikipedia , lookup
Roger A. Freedman • William J. Kaufmann III Universe Eighth Edition CHAPTER 17 The Nature of Stars M 39 is an Open or Galactic Cluster Review of Previously Covered Concepts The distance (d) to a star can be determined from a measurement of the star’s parallax (p). Stellar Parallax As Earth moves from one side of the Sun to the other, a nearby star will seem to change its position relative to the distant background stars. d=1/p d = distance to nearby star in parsecs p = parallax angle of that star in arcseconds Some Nearby Stars Proxima Centauri: p = 0.772 arcsec, d = 1/p = 1.3 pc Barnard’s Star: p = 0.545 arcsec, d = 1/p = 1.83 pc Sirius A/B : p = 0.379 arcsec, d = 1/p = 2.64 pc 1 pc = 206,265 AU = 3.26 LY Review of Previously Covered Concepts The distance (d) to a star can be determined from a measurement of the star’s parallax (p). The “intrinsic brightness” or luminosity (L) of a star can be determined from a measurement of the star’s apparent brightness (b) and a knowledge of the star’s distance. If a star’s distance is known, its luminosity can be determined from its brightness. As you get farther and farther away from a star, it appears to get dimmer. Luminosity, L, doesn’t change Apparent brightness, b, does change following the inverse square law for distance. b = L / (4pd2) If a star’s distance is known, its luminosity can be determined from its brightness. A star’s luminosity can be determined from its apparent brightness if its distance is known: L = 4p d 2 b L = 4p d2 b L/L = (d/d)2 (b/b) Where L = the Sun’s luminosity Example: The Sun d = 1 AU = 1.51011 m b = 1370 W/m2 (Solar Constant) L = 4p d2 b = 1.256 101 2.251022 m2 1.37103 W/m2 L = 3.871026 W Example: e Eridani d = 3.22 pc = 3.22206,265 AU = 6.65105 AU b = 6.7310-13 b L/L = (6.65105)2 6.7310-13 = 0.3 e Eri has a luminosity equal to 30% of the solar luminosity. Review of Previously Covered Concepts The distance (d) to a star can be determined from a measurement of the star’s parallax (p). The “intrinsic brightness” or luminosity (L) of a star can be determined from a measurement of the star’s apparent brightness (b) and a knowledge of the star’s distance. The surface temperature (T) of a star can be determined from a measurement of the star’s color (or spectral type). Today we will learn 17-7 How H-R diagrams summarize our knowledge of the stars 17-6 How stars come in a wide variety of sizes 17-8 How we can deduce a star’s size from its spectrum Let’s pause to examine the spread of “L” and “T” values among the stars that are nearest to us (Appendix 4). Plot “L vs. T” for 27 Nearest Stars Data drawn from Appendix 4 of the textbook. L and T appear to be Correlated Nearest Stars L and T appear to be Correlated A few of the brightest stars in the night sky Hertzsprung-Russell (H-R) Diagram Hertzsprung-Russell (H-R) Diagram More complete mapping of stars onto the H-R Diagram Stars come in a wide variety of sizes Stefan-Boltzmann law relates a star’s energy output, called LUMINOSITY, to its temperature and size. LUMINOSITY = 4pR2sT4 LUMINOSITY is measured in joules per square meter of a surface per second and s = 5.67 X 10-8 W m-2 K-4 Small stars will have low luminosities unless they are very hot. Stars with low surface temperatures must be very large in order to have large luminosities. Determining the Sizes of Stars from an H-R Diagram Main sequence stars are found in a band from the upper left to the lower right. Giant and supergiant stars are found in the upper right corner. Tiny white dwarf stars are found in the lower left corner of the HR diagram. Hertzsprung-Russell (H-R) diagrams reveal the different kinds of stars. Main sequence stars Red giant stars Stars in hydrostatic equilibrium found on a line from the upper left to the lower right. Hotter is brighter Cooler is dimmer Red Dwarfs (on MS) & Brown Dwarfs (not on MS): lower right corner (small, dim, and cool) Upper right hand corner (big, bright, and cool) White dwarf stars Lower left hand corner (small, dim, and hot) Details of a star’s spectrum reveal whether it is a giant, a white dwarf, or a main-sequence star. Both of these stars are spectral class B8. However, star a is a luminous super giant and star b is a typical main-sequence star. Notice how the hydrogen absorption lines for the more luminous stars are narrower. LUMINOSITY CLASS Based on the width of spectral lines, it is possible to tell whether the star is a supergiant, a giant, a main sequence star or a white dwarf. These define the luminosity classes shown on the left occupying distinct regions on the HR diagram. The complete spectral type of the Sun is G2 V. The “G2” part tells us Teff, the “V” part tells us to which sequence or luminosity class the star belongs. Example: M5 III is a red giant with Teff ~ 3500K, M=0 (or L=100 Lsun). HR Diagram This template will be used in the upcoming test. Please become familiar with it. We will do a few examples in class of how to read off the temperature, luminosity and size of a star given a full spectral type. HR Diagram I expect you to know which of the gray sequences is which luminosity class. From top to bottom: Ia, luminous supergiants Ib, supergiants III, giants V, main sequence Examples: G2V The Sun M5III B4Ib M5Ia