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Keep your voting card ready Phys1403 – Introductory Astronomy Instructor: Dr. Goderya Topics for Today’s Class Foundations of Astronomy | 13e Seeds • H-R Diagram • Luminosity Classes Chapter 9 • Worksheet Family of the Stars © Cengage Learning 2016 Luminosity Equation The Hertzsprung-Russell Diagram Part 1 • We know that stars have different: Quantitatively: L = 4 R2 T4 Surface area of the star Surface flux due to a blackbody spectrum The Heart of Astrophysics – Temperatures – Luminosities – Sizes • To order them, we use the HertzsprungRussell (H-R) diagram – Luminosity (or absolute magnitude) versus temperature (or spectral type) 1 The Hertzsprung-Russell Diagram • In an H–R diagram, a star is represented by a dot at a position that shows the star’s luminosity and temperature. • The background color in this diagram indicates the temperature of the stars. • The Sun is a yellow-white G2 star. • Most stars including the Sun have properties along the mainsequence strip running from hot high-luminosity stars at upper left to cool low-luminosity stars at lower right. Fig. 9-8, p. 179 The Hertzsprung-Russell Diagram The Hertzsprung-Russell Diagram Same temperature, but much brighter than MS stars Must be much larger Giant Stars Same Luminosity but hotter than Sun The Radii of Stars in the H-R Diagram • Observations with interferometers can resolve the size and shape of some nearby stars. • The stars of the upper main sequence are indeed larger than the Sun, as predicted by the H–R diagram. • The examples shown here are flattened by rapid rotation, but most stars rotate slower and are more nearly spherical. • On the scale of this diagram, the supergiant Betelgeuse would have a diameter of about 7 meters ( 23 feet ) . ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) 2 The Relative Sizes of Stars in the HR Diagram • An H–R diagram showing the luminosity and temperature of many well-known stars. • Individual stars that orbit each other are designated A and B, for example Spica A and Spica B. • The dashed lines are lines of constant radius; star sizes on this diagram are not to scale. • To visualize the size of the largest stars, imagine that the Sun is the size of a tennis ball. • Then, the largest supergiants would be the size of a sports stadium and white dwarfs the size of grains of sand. ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) Masses of Stars in the H-R Diagram • The more massive a star is, the brighter it is – High-mass stars have much shorter lives than low-mass stars L = M3.5 ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) Comparison of Spectral Lines • These model spectra show how the widths of spectral lines reveal a star’s luminosity class. • Supergiants have very narrow spectral lines, and main-sequence stars have broad lines. Certain spectral lines are more sensitive to this effect than others; • Careful inspection of a star’s spectrum can determine its luminosity classification. Luminosity Class • See Figure 9-12 – Ia Bright Supergiants – Ib Supergiants – II Bright Giants – III Giants – IV Subgiants – V Main-Sequence Stars • Our Sun: G2 star on the Main Sequence: G2V 3 Example Luminosity Classes Luminosity Classes and H-R Diagram Ia Bright Supergiants • Our Sun: G2 star on the Main Sequence: Ia Ib G2V Ib Supergiants II III IV • Polaris: G2 star with Supergiant luminosity: II Bright Giants III Giants IV Subgiants V G2Ib V Main-Sequence Stars Attendance ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) Campus.kellerisd.net ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) 4 HR-Diagram Worksheet • Please work on the worksheet provided to you for the next 15-20 minutes. 5