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Chapter 16

Chapter 16. The Nature of Stars. UVU Survey of Astronomy. Astro1010-lee.com. twlee2016@gmail.com. Chapter 16. The Nature of Stars. Stellar Distance Scales. Light Year = the distance that light travels in one year Parsec = the distance to a point where 1 AU subtends one second of arc.

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Chapter 16

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  1. Chapter 16 The Nature of Stars UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  2. Chapter 16 The Nature of Stars Stellar Distance Scales Light Year = the distance that light travels in one year Parsec = the distance to a point where 1 AU subtends one second of arc UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  3. Chapter 16 The Nature of Stars Remember that nearby stellar distances can be measured using parallax UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  4. Chapter 16 The Nature of Stars Nearest star to the Sun is Proxima Centauri which is a member of a 3-star system: Alpha Centauri complex Model of distances: Sun is a marble, Earth is a grain of sand orbiting 1 m away Nearest star is another marble 270 km away Solar system extends about 50 m from Sun; rest of distance to nearest star is basically empty UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  5. Chapter 16 The Nature of Stars The 30 closest stars to the Sun UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  6. Chapter 16 The Nature of Stars Brightest stars were known to, and named by, the ancients (Procyon) In 1604, stars within a constellation were ranked in order of brightness, and labeled with Greek letters (Alpha Centauri) In the early 18th century, stars were numbered from west to east in a constellation (61 Cygni) As more and more stars were discovered, different naming schemes were developed (G51-15, Lacaille 8760, S 2398) Now, new objects are simply labeled by their celestial coordinates UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  7. Chapter 16 The Nature of Stars Brightness Scales • Apparent magnitude • Hippachus • 1st to 6th • Spica (1st Mag), Vega (0 Mag) • Brightness (Luminocity) • measured by light meter • An Apparent Magnitude difference of 5 represents a Brightness ratio of 100/1 UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  8. Chapter 16 The Nature of Stars Apparent Magnitudes of several objects UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  9. Chapter 16 The Nature of Stars The Absolute Magnitude (M) of stars is defined as the apparent magnitude that the star would have if were at 10 parsecs distance. Then the following ratio holds: UVU Survey of Astronomy Astro1010-lee.com Note: This is not exact – there are constants left out. twlee2016@gmail.com

  10. Chapter 16 The Nature of Stars Luminosity, or absolute brightness, is a measure of the total power radiated by a star. Apparent brightness is how bright a star appears when viewed from Earth; it depends on the absolute brightness but also on the distance of the star UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  11. Chapter 16 The Nature of Stars Two stars that appear equally bright might be a dimmer, nearer star and a brighter, farther star UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  12. Chapter 16 The Nature of Stars The color of a star is indicative of its temperature. Red stars are relatively cool, while blue ones are hotter. UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  13. Chapter 16 The Nature of Stars Spectral classes make up a Temperature Sequence • O, B, A, F, G, K, M • O hottest • M coolest • Oh Be A Fine Girl Kiss Me UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  14. Chapter 16 The Nature of Stars These are representative spectra of each class. UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  15. Chapter 16 For the vast majority of stars that cannot be imaged directly, size must be calculated knowing the luminosity and temperature: Supergiant stars are more than 100 solar radii Giant stars are between 10 and 100 solar radii Upper main sequence stars are 8 to 100 solar radii Average stars are 0.5 to 8 solar radii Dwarf stars are 0.1 to 0.5 solar radii UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  16. Chapter 16 The Nature of Stars Stellar radii vary widely. In the chart, note the great changes in scale required to show the different sizes. UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  17. Chapter 16 The Nature of Stars EjnarHertzsprung (8 Oct, 1873 - 21 Oct, 1967) was a Danish chemist and astronomer. UVU Survey of Astronomy Astro1010-lee.com Henry Norris Russell (Oct 25, 1877 – Feb 18, 1957) was an American Astronomer. Together they invented one of most useful graphs in Astronomy twlee2016@gmail.com

  18. Chapter 16 The Nature of Stars The Hertzsprung– Russell Diagram The H–R diagram plots stellar luminosity against surface temperature. This is an H–R diagram of a few prominent stars UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  19. Chapter 16 The Nature of Stars Once many stars are plotted on an H–R diagram, a pattern begins to form. These are the 80 closest stars to us; note the dashed lines of constant radius. The darkened band is called the main sequence, as this is where most stars are. UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  20. Chapter 16 The Nature of Stars An H–R diagram of the 100 brightest stars looks quite different, These stars are all more luminous than the Sun. Two new categories appear here – the red giants and the blue giants. Clearly, the brightest stars in the sky appear bright because of their enormous luminosities, not their proximity. UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  21. Chapter 16 The Nature of Stars Major Sections of the H-R Diagram. They start with the Main Sequence , then the two Giant Stages then finally the White Dwarf Stage UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  22. Chapter 16 The Nature of Stars This is an H–R plot of about 20,000 stars. The main sequence is clear, as is the red giant region. About 90% of stars lie on the main sequence; 9% are giants and 1% are white dwarfs. UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  23. Chapter 16 • Spectroscopic parallax: has nothing to do with parallax, but does use spectroscopy to extend our ability to determine the distance to a star • Measure the star’s apparent magnitude (brightness) and spectral class (temperature) • Use temperature to estimate luminosity • Apply inverse-square law to find distance UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  24. Chapter 16 Spectrographic Parallax can be seen on this H-R Diagram and the definition of Absolute Magnitude UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  25. Chapter 16 The Nature of Stars Spectroscopic parallax can extend the cosmic distance scale to several thousand parsecs. UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  26. Chapter 16 The Nature of Stars The spectroscopic parallax calculation can be misleading if the star is not on the main sequence. The width of spectral lines can be used to define luminosity classes UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  27. Chapter 16 Determination of Stellar Masses Many stars are in binary pairs; measurement of their orbital motion allows determination of the masses of the stars. Visual binaries can be measured directly; this is Kruger 60: UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  28. Chapter 16 The Nature of Stars Study of spectral lines reveals the motion of spectroscopic binaries and hence their spacing. From that the masses are calculated. UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  29. Chapter 16 The Nature of Stars The mass of a star is also correlated with its radius, and very strongly correlated with its luminosity. UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  30. Chapter 16 The Nature of Stars Mass is also related to stellar lifetime UVU Survey of Astronomy Astro1010-lee.com Using the mass–luminosity relationship twlee2016@gmail.com

  31. Chapter 16 The Nature of Stars So the most massive stars have the shortest lifetimes – they have a lot of fuel but burn it at a very rapid pace. On the other hand, small red dwarfs burn their fuel extremely slowly, and can have lifetimes of a trillion years or more. UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  32. Chapter 16 The Nature of Stars In –Class Quiz Spectroscopic parallax: has nothing to do with parallax, but does use spectroscopy to extend our ability to determine the ______________ to a star UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

  33. Chapter 16 The Nature of Stars End of Chapter 16 UVU Survey of Astronomy Astro1010-lee.com twlee2016@gmail.com

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