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Guiding Questions

Guiding Questions. How far away are the stars? What evidence do astronomers have that the Sun is a typical star? What is meant by a “first-magnitude” or “second-magnitude” star? Why are some stars red and others blue? What are the stars made of?

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Guiding Questions

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  1. Guiding Questions • How far away are the stars? • What evidence do astronomers have that the Sun is a typical star? • What is meant by a “first-magnitude” or “second-magnitude” star? • Why are some stars red and others blue? • What are the stars made of? • As stars go, is our Sun especially large or small? • What are giant, main-sequence, and white dwarf stars? • How do we know the distances to remote stars? • How does our Sun evolve? • How can we find the temperature, power, and size of stars from their spectra?

  2. The brightness of a star is not a good indicator of distance. e.g., Polaris is closer than Betelgeuse but Betelgeuse appears brighter. Distances to nearby stars can be measured using parallax. Parallax is the apparent change in the position of an object do to a change in observing position. Careful measurements of the parallaxes of stars reveal their distances.

  3. 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

  4. 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/L = (d/d)2 x (b/b) Where L = the Sun’s luminosity

  5. Luminosity Function As stars go, our Sun is neither extremely luminous nor extremely dim. It is somewhat more luminous than most nearby stars – of the 30 stars within 4 pc, only three have a greater luminosity. Luminosity of Sun = L = 3.86 X 1026 W

  6. Greater distances can be measured with Cepheid variables

  7. Cepheids compress, heat up, and brighten

  8. Periods reveals Luminosity of Cepheids

  9. Measure a star’s Luminosity -> find its distance from its apparent 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) Intensity = Power/Area

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  12. Historically, the apparent magnitude scale runs from 1 (brightest) to 6 (dimmest). Today, the apparent magnitude scale extends into the negative numbers for really bright objects and into the 20s and 30s for really dim objects. Absolute magnitude, on the other hand is how bright a star would look if it were 10 pc away. Astronomers often use the magnitude scale to denote brightness.

  13. Astronomers often use the magnitude scale to denote brightness.

  14. A star’s color depends on its surface temperature. Wien’s law: l(m) = 3 x 10-3 T(K) The hotter the object, the shorter the wavelength of its brightest light

  15. UBV photometry is the process of systematically looking at intensity emitted by a star in three wavelength (color band) regions. [U: ultraviolet, B: blue, V: visual]

  16. The spectra of stars reveal their chemical compositions as well as surface temperatures. • In the late 19th Century, Harvard astronomers obtained spectra for hundreds of thousands of stars. • Annie Jump Cannon grouped stellar spectra into a classification scheme of spectral types A through O. • Today we recognize the spectral types O, B, A, F, G, K, and M as running from hottest to coolest.

  17. The spectra of stars reveal their chemical compositions as well as surface temperatures.

  18. The spectra of stars reveal their chemical compositions as well as surface temperatures. • O B A F G K M • hottest to coolest • bluish to reddish • Further refined by attaching an integer, for example: F0, F1, F2, F3 … F9 where F1 is hotter than F3 • An important sequence to remember: • Our Best Astronomers Feel Good Knowing More • Oh Boy, An F Grade Kills Me • Oh Be a Fine Girl (or Guy), Kiss Me

  19. Strengths of absorption lines (our Sun is a G2 and has strong FeII and Ca II lines)

  20. Stefan-Boltzmann law relates a star’s energy output, called LUMINOSITY, to its temperature and size. Flux = Intensity = Power/Area = sT4 LUMINOSITY = Power = Flux * Area = 4pR2 sT4 LUMINOSITYis POWER, or Energy/time, measured in joules per second The Stefan-Boltzman constant, s = 5.67 X 10-8 W m-2 K-4 Small stars have low luminosities unless they are very hot. Cool stars must be very large in order to have large luminosities (e.g. Red Giants). Stars come in a wide variety of sizes

  21. Hertzsprung-Russell (H-R) diagrams reveal the different kinds of stars. HR DIAGRAM Absolute magnitude vs temperature or luminosity vs spectral type

  22. Hertzsprung-Russell (H-R) diagrams reveal the different kinds of stars. • Main sequence stars • Stars in hydrostatic equilibrium found on a line from the upper left to the lower right. • Hotter is brighter • Cooler is dimmer • Red giant stars • Upper right hand corner (big, bright, and cool) • White dwarf stars • Lower left hand corner (small, dim, and hot)

  23. Determining the Sizes of Stars from an HR 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.

  24. Details of a star’s spectrum reveal whether it is a giant, a white dwarf, or a main-sequence star. Luminosity classes • Class I includes all the supergiants. • Class V includes the main sequence stars. • The Sun is a G2 V

  25. Luminosity increases with mass, in main-sequence stars. Bigger is brighter!

  26. Luminosity increases with temperature, in main-sequence stars. Bigger is hotter!

  27. When core hydrogen burning ceases, a main-sequence star becomes a red giant . • When all of the hydrogen in the core has been depleted, the interior can no longer repel the inward pull of gravity. • The core heats under pressure, causing the outer layers to expand and swell. • These outer layers get farther from the hot core and cool, resulting in a red color.

  28. H-R diagram shows Sun’s evolution.

  29. Guiding Questions • How far away are the stars? • What evidence do astronomers have that the Sun is a typical star? • What is meant by a “first-magnitude” or “second-magnitude” star? • Why are some stars red and others blue? • What are the stars made of? • As stars go, is our Sun especially large or small? • What are giant, main-sequence, and white dwarf stars? • How do we know the distances to remote stars? • How does our Sun evolve? • How can we find the temperature, power, and size of stars from their spectra?

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