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Lecture 20 The Hertzsprung-Russell Diagram November 11, 2015 1 2 Hertzsprung-Russell Diagram • Hertzsprung and Russell found a correlation between luminosity and spectral type (temperature) 10000 Hot, bright stars 100 Luminosity (Solar units) Sun 1 Cool, dim stars 0.01 Main Sequence 0.0001 hot O B A F G K M cooler 3 4 • Most stars (~90%) are on the main sequence – The greater the temperature, the more luminous the star – M type stars are the most common. – O type stars are the least common. 5 Sizes of Stars • Size of star is related to the temperature and the luminosity by: Luminosity Temperature Radius 4 L T 4 R 4 2 2 – If you know the position on HR diagram you know the size of the star. 6 Mira is _____ than Barnard’s star because it is ______. A. B. C. D. hotter … larger smaller … more 88% luminous more luminous … larger more luminous … hotter 10% 1% A. 0% B. C. D. 7 100 R 10 R Supergiants 10000 100 1 R Luminosity (Solar units) Giants 1 0.01 0.0001 White Dwarfs Sun Cool, dim stars 0.1 R Main Sequence 0.01 R hot O B A F G K M cooler 8 • Supergiants -- cool, bright, red, large stars • Giants -- cool, bright red, less large stars • Main Sequence -- spans range from hot, bright stars to cool, dim stars. • White dwarfs -- hot, small, dim stars. • These classifications will give clues to stages in the evolution of stars. 9 Stars come in many sizes! 10 Sizes of Objects in our Universe Images from webisto.com/space 11 Sizes of Objects in our Universe 12 Sizes of Objects in our Universe 13 Sizes of Objects in our Universe 14 Sizes of Objects in our Universe 15 A star has a high luminosity (100 solar luminosities) and a surface temperature of 3500 K. What type of star is it? A. B. C. D. A high mass–main sequence star A low mass–main sequence star A red giant A white dwarf 75% 18% 6% 0% A. B. C. D. 16 Masses of Stars • We cannot directly measure the mass of an isolated star. • If something is orbiting the star, can use the general form of Kepler’s Third Law M 1 M 2 P 2 a 3 • Luckily, 2/3 of all stars are binary stars, two stars that orbit one another 17 A binary star consists of two identical stars orbiting each other. If they are 4 AU apart and orbit with a period of 4 years, how massive is each star? A. B. C. D. 4M 95% 2M 1M 0.5M 3% A. 2% B. C. 0% D. 18 A binary star consists of two identical stars orbiting each other. If they are 4 AU apart and orbit with a period of 4 years, how massive is each star? A. B. C. D. 4M 2M 1M 0.5M 2 3 M M P a 1 2 3 a 2M 2 P 4 AU a M 2M 2 2 2P 2 4 yr 3 3 19 Mass-Luminosity Relation • True ONLY for Main Sequence stars • As the mass increases, the luminosity increases rapidly Luminosity Mass3 20 HR Diagram: Main sequence stars labeled by mass in units of solar masses. 21 Which of the following entries is incorrect when comparing different spectral classes of main sequence stars with the Sun? Temperature* O Higher Mass A. A Higher B. B Size C. C Color* D. D Luminosity G Same 56% M Lower A Same Lower Smaller B Same Same C 24% Blue Yellow Red 11% Higher 9% Same A. Lower B. C. D D. 22 Which of the following entries is incorrect when comparing different spectral classes of main sequence stars with the Sun? Same M Lower A Same Lower Smaller B Same Same Color* Blue Yellow Red Luminosity Higher Same Lower A. A Temperature* B. B Mass C. C D. D Size O Higher Higher G C D 23 Apparent Brightness • The brightness an object appears to have. • The further away the object, the dimmer it looks Luminosity Apparent Brightness d = distance 2 4 d 24 Spectroscopic Parallax • If luminosity and apparent brightness are known, distance can be determined. m M 5log d 5 • Difficult to accurately measure luminosity for one star – Use spectra to get spectral type and class • Can use a cluster of stars • Distance to the cluster can be determined by comparing the HR diagram of the cluster with a template HR diagram 25 Spectroscopic Parallax Observations 10000 Template 100 1 0.01 hot O B A F G KM cooler 0.0001 hot O B A F G K M cooler 26 Spectroscopic Parallax 10000 100 1 0.01 0.0001 hot O B A F G K M cooler 27 The Distance Ladder • See Figure 16.17 on p. 416 • First rung: parallax • Second rung: spectroscopic parallax, or “main-sequence fitting” • Third rung: standard candles/inverse square law (Cepheids/supernovae) • Fourth rung: Hubble’s Law 28 The Distance Ladder We will be discussing the longer distance “rungs” of the ladder in subsequent lectures. Figure 26.12, Freedman and Kaufmann, 7th ed. Universe, © 2005 W. H. Freeman & Company