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The Sun's Atmosphere
9-1 The photosphere is the (400 km thick) visible layer of the Sun: 5800 K blackbody: Granules are small-scale convection, with rising centers about 100K hotter than falling edges.
9-2 The chromosphere ( 500 km thick) is characterized by spikes of gas called spicules: rising at 2000 m/s to height of nearly 10,000 km.
9-3 The corona ejects some of its mass into space as the solar wind: at 400 km/s (compare to sound at 400 m/s). T~ 1e6 K (rare gas of fast particles) energized by magnetic field line tearing
The Active Sun
9-4 Sunspots reveal the solar cycle and the Sun's rotation. 22 yr cycle for pole reversals, 11 yr cycle for magnetic activity. Minima in 65, 76, 86, 97 - not many sunspot right now? Equatorial spots rotate faster.
9-5 The Sun's magnetic fields create sunspots: these are flux loops poking out of photosphere. Equilibrium: higher magnetic pressure -> lower gas pressure -> lower temperature -> dark spots. Polarity of leading spot = polarity of the hemisphere. Quadrupole moments during field reversal. Alpha-omega dynamo (inside Earth too).
9-6 Magnetic fields lead to plages and filaments on the Sun
9-7 X-ray images of the corona show a dramatically different Sun than we normally see. Coronal holes where field carries particles out, e.g. at poles
The Sun's Interior
9-8 Thermonuclear reactions in the core of the Sun produce its energy. E=mc2. Mass difference in fusion of 4H -> 1 He is on the order of 1e7 eV.
AN ASTRONOMERIS TOOLBOX 9-1 Thermonuclear Fusion
9-9 Solar models describe how energy escapes from the Sun's core. Hydrostatic equilibrium: gas presure out balances gravitational force in. Main sequence.
Radiation -> convection -> p-modes -> helioseismology is a tool for learning about internlal structure and dynamics.
p-modes yield velocity profiles: poles are slower on the edge, equatoria regions are faster outside than in (opposite requirement of Babcock dynamo)
9-10 The mystery of the missing neutrinos inspires speculation about the Sun's interior. Not a mystery anymore. Neutrino mass permits oscillation of electorn neutrino into muon and tau neutrino, accounting for obs/pred = 1/3.
nu + n -> p + e
nu + Cl -> Ar + e
Neutrino mass under 10 eV also accounts for dark matter implied by Vera Rubin's galactc rotation curves (see tour workshop on Jupiter's moons)
What If ... The Moon Didn't Exist? Moon induces large tides, which have slowed the earth's rotation and lengthened the day.
111-1 Distances to nearby stars are determined by stellar parallax: p(arcsec) = 1/d(parsec). This relation defines parsecs, which are about 3 LY.
111-2 Distances affect the brightnesses of stars as seen from Earth: Absolute magnitudes: Sun= - 26.7, Full Moon = -12.6, Venus <-4.4, Sirus = -1.5.
2(ma - mb) = 5 log [Ib/Ia]. Magnitude difference is proportional to log of intensity ratio. (Seeds p.25)
111-3 Absolute magnitudes consider the effects of distance on brightness:
m- M = 5 log [d/10] where m is the apparent magnitude, M is the absolute magnitude, and d=distance in parsecs.
AN ASTRONOMER'S TOOLBOX 111-1 The Distance-Magnitude Relationsbip
111-4 The brightnesses of stars helps us classify them and study their evolution
10-1 A star's color reveals its surface temperature: recall Wien's law from Ch.4.
the U index (ultraviolet) is lower for hotter stars , though they are more violet; and the V index (visible) is lower for cooler stars, though they are redder.
10-2 A star's spectrum also reveals its surface temperature: This is more complicated. In general, hotter stars show radiation from higher excited states (until an element is ionized) and very cool stars can even have molecular lines.
10-3 Stars are classified by their spectra: O B A F G K M in order of decreasing temperature
10-4 The Hertzsprung-Russel diagram identifies distinct groups of stars. Main sequence stars are near hydrostatic equilibrium. Red Giants are cool and bright, White dwarfs are hot and dim. Cepheids are on the instability strip.
A stars: Vega, Sirius A, Altair... Are any of these peculiar?
EYES ON... Star Names
10-5, Stars are also classified by their luminosity: I-V in order of (generally) decreasing brightness.
10-6 Binary stars provide information about stellar masses
AN ASTRONOMER'S TOOLBOX 10-1 Kepler's Third Law and Stellar Masses (We discussed this in Ch.3, when Zita derived Kepleršs 3d law from F=ma. Just put the sum of masses in place of the central mass, for a system of two comparable masses.)
10-7 The orbital motion of binary stars affects the wavelengths of their spectral lines
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Ch.9 RQ 1, 2, 3 (p.231)
I Describe the Sun's atmosphere. Be sure to mention some of the phenomena that occur at various altitudes above the solar surface.
2 Describe the three main layers in the solar atmosphere and how you would best observe them.
3 When will the next sunspot maximum and minimum occur? Explain your reasoning.
4 Why is the solar cycle said to have a period of 22 years, even though the sunspot cycle is only 11 years long?
5 How do astronomers detect the presence of a magnetic field in hot gases, such as those in the solar photosphere?
6 Describe the dangers in attempting to observe the Sun. How have astronomers learned to circumvent these hazards?
7 Give an everyday example of hydrostatic equilibrium.
8 Give some everyday examples of heat transfer by convection and radiative diffusion.
9 What do astronomers mean by "a model of the Sun"?
10 Why do thermonuclear reactions in the Sun take place only in its core?
11 What is hydrogen burning? Why is hydrogen burning fundameiitally unlike the burning of a log in a fireplace?
12 Describe the Sun's interior, including the main phvslcal processes that occur at various depths within the Sun.
13 What is a neutrino, and why are astronomers so interested in detecting neutrinos from the Sun?
Ch.10 RQ 1, DQ 18 (p.247)
Review Questions
1 How and why is the spectrum of a star related to its surface temperature?
2 Describe UBV filters and how an astronomer uses them to measure a star's surface temperature.
3 Explain why the color index of a star is related to its surface temperature.
4 What is the primary chemical component of most stars?
5 Which is the hottest star listed in Table 10-1? Which is the coolest?
6 Draw an H-R diagrai-n and sketch the regions occupied by main sequence stars, red giants, and white dwarfs. Briefly discuss the different ways in which you could have labeled the axes of your graph.
7 How can observations of a visual binary lead to information about the masses of its stars?
8 What is a radial-velocity curve? What kinds of stellar svstems exhibit such curves?
Discussion Question
18 How might a star's rotation affect the appearance of its spectral lines?
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OBSERVING PROJECTS
19 Locate the star Betelgeuse in Orion. Observe it both by eye and through a small telescope. What color is it in each viewing? It helps to compare its color to those of its neighbors.
20 Locate one or more of the following double star systems: Regulus in Leo, Algleba in Leo, Mirak in Bootes, Ras Algethi in Hercules, Albireo in Cygnus, Vega in Lyra, Polaris in Ursa Minor, Rigel in Orion, Antares in Scorpius, Sirius in Canis Major, and Schedar in Cassiopeia. View them with and without a telescope. What are their colors?
21 Observe the eclipsing binary Algol (p Persei) using nearby stars to judge its brightness during the course of an eclipse. I Algol has an orbital period of 2 days, 20 hours, and 53 minutites. When the eclipse begins, its apparent magnitude drops from 2.1 to 3.4. It remains this faint for about 2 hours. The cntire eclipse, from start to finish, takes about 10 hours. Consult the "Celestial Calendar" section of the current issue of Skv & Telescope for the predicted dates and times of minimum brightness of Algol. Note that the schedule is given in Universal Time (the same as Greenwich Mean Time), so you will have to convert to your own time zone. Algol is normally the second brightest star in the constellation Perseus. Because of its northerly position (right ascension = 3h 08.2"', declination +40' 57'), Algol is readily visible from northern latitudes during the fall and winter months.
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Maintained by E.J. Zita