1. The Sun: Part 3 and Measuring the Stars: Part 1

2. Net result: 4 protons → 4 He + 2 neutrinos + energy Hydrostatic Equilibrium: pressure from fusion reactions balances gravity. Sun is stable. Mass of end products is less than mass of 4 protons by 0.7%. Mass converted to energy. 600 millions of tons per second fused. Takes billions of years to convert p's to 4 He in Sun's core. Process sets lifetime of stars. Nuclear Fusion of H -> He in the Sun

3. Fusion as an Energy Source Can we build fusion reactors on Earth to generate clean (no carbon dioxide) energy? Maybe. 2 H + 3 H → 4 He + neutron + energy Trouble is 1) Confinement of the reaction 2) safely stopping the neutrons Methods: 1) Magnetic confinement (tokomaks) JET => ITER => DEMO 2) Inertial confinement (lasers) JET tokomak

4. Sunspots Roughly Earth-sized Last ~2 months Usually in pairs Follow solar rotation

6. Sunspots They are darker because they are cooler (4500 K vs. 5800 K). Related to loops of the Sun's magnetic field. radiation from hot gas flowing along magnetic field loop at limb of Sun.

7. Filament Ejection Movie

8. Sunspot numbers vary on a 11 year cycle.

9. 0.1% variation from maximum to minimum

10. Sun's magnetic field changes direction every 11 years. Maximum sunspot activity occurs about halfway between reversals.

11. Above the photosphere, there is the chromosphere and... The Corona Best viewed during eclipses. T = 10 6 K Density = 10 -15 g/cm 3 only!

12. We expect X-rays from gas at this temperature. X-ray brightness varies over 11-year Solar Cycle: coronal activity and sunspot activity go together. Yohkoh X-ray satellite

13. The Solar Wind At top of corona, typical gas speeds are close to escape speed => Sun losing gas in a solar wind. Wind escapes from "coronal holes", seen in X-ray images. Wind speed 500 km/sec (takes a few days to reach Earth). 10 6 tons/s lost. But Sun has lost only 0.1% of its mass from solar wind.

14. Space Weather Today’s forecast: solar wind velocity = 331.5 km/s density = 4.3 protons/cm 3 Sunspot number: 1 days without a sunspot since: For update see www.spaceweather.com List of recent and upcoming Near-miss encounters and space related news.

15. Active Regions Prominences: Loops of gas ejected from surface. Anchored in sunspot pairs. Last for hours to weeks. Flares: A more energetic eruption. Lasts for minutes. Less well understood. Prominences and flares occur most often at maximum of Solar Cycle.

16. Space weather and solar science ● Coronal Mass Ejections: solar science and ultimately predicting space weather

17. Solar Probe in 2018

18. Spectral Classes Strange lettering scheme is a historical accident. Spectral Class Surface Temperature Examples OBAFGKMOBAFGKM 30,000 K 20,000 K 10,000 K 7000 K 6000 K 4000 K 3000 K Rigel Vega, Sirius Sun Betelgeuse Further subdivision: BO - B9, GO - G9, etc. GO hotter than G9. Sun is a G2.

19. Classification of Stars Through Spectroscopy Remember: stellar spectra show black-body radiation and absorption lines. Pattern of absorption lines depends on temperature (mainly) and chemical composition. Spectra give most accurate info on these as well as: density in atmosphere gravity at surface velocity of star towards or from us Ionized helium. Requires extreme UV photons. Only hottest stars produce many of these.

20. Stellar Sizes - Direct Measurement For a few nearby giant stars we can image them directly using HST or the VLA. Almost all other stars are too far away

21. Stellar Sizes - Indirect Method Almost all stars too far away to measure their radii directly. Need indirect method. For blackbodies, use Stefan's Law: Luminosity  (temperature) 4 x (surface area) Energy radiated per cm 2 of area on surface every second  T 4 (T = temperature at surface) Determine luminosity from apparent brightness and distance, determine temperature from spectrum (black-body curve or spectral lines), then find surface area, then find radius (sphere surface area is 4  R 2 ) And: Luminosity = (energy radiated per cm 2 per sec) x (area of surface in cm 2 ) So:

22. The Wide Range of Stellar Sizes

23. How Massive are Stars? 1. Binary Stars. Orbital period depends on masses of two stars and their separation. 2. Theory of stellar structure and evolution. Tells how spectrum and color of star depend on mass.