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The Sun: Part 3 and Measuring the Stars: Part 1 Nuclear Fusion of H -> He in the Sun Net result: 4 protons → 4He + 2 neutrinos + energy 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 4He in Sun's core. Process sets lifetime of stars. Hydrostatic Equilibrium: pressure from fusion reactions balances gravity. Sun is stable. Fusion as an Energy Source Can we build fusion reactors on Earth to generate clean (no carbon dioxide) energy? Maybe. 2H JET tokomak + 3H → 4He + 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) Question 8 The solar neutrino problem refers to the fact that astronomers a) cannot explain how the Sun is stable. b) detect only one-third the number of neutrinos expected by theory. c) cannot detect neutrinos easily. d) are unable to explain how neutrinos oscillate between other types. e) cannot create controlled fusion reactions on Earth. Question 8 The solar neutrino problem refers to the fact that astronomers a) cannot explain how the Sun is stable. b) detect only one-third the number of neutrinos expected by theory. c) cannot detect neutrinos easily. d) are unable to explain how neutrinos oscillate between other types. e) cannot create controlled fusion reactions on Earth. Further experiments have shown that solar neutrinos can change into other types that were not initially detected. Sunspots Roughly Earth-sized Last ~2 months Usually in pairs Follow solar rotation 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. Filament Ejection Movie Sunspot numbers vary on a 11 year cycle. 0.1% variation from maximum to minimum Sun's magnetic field changes direction every 11 years. Maximum sunspot activity occurs about halfway between reversals. Clicker Question: Besides being darker relative to other parts of the photosphere, sunspots are characterized by what quality? A: They rotate faster than adjacent regions B: They have stronger magnetic fields than adjacent regions C: They have much greater density than adjacent regions D: They have much higher temperature than adjacent regions Above the photosphere, there is the chromosphere and... The Corona Best viewed during eclipses. T = 106 K Density = 10-15 g/cm3 only! We expect X-rays from gas at this temperature. Yohkoh X-ray satellite X-ray brightness varies over 11-year Solar Cycle: coronal activity and sunspot activity go together. 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). 106 tons/s lost. But Sun has lost only 0.1% of its mass from solar wind. Space Weather Today’s forecast: solar wind velocity = 331.5 km/s density = 4.3 protons/cm3 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. 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. Space weather and solar science ● Coronal Mass Ejections: solar science and ultimately predicting space weather Solar Probe in 2018 Clicker Question: What is the source of energy in the sun? A: fusion of protons into heavier nuclei B: burning of coal and other hydrocarbons C: the slow gravitational collapse of the sun D: nuclear fission of heavy nuclei into lighter elements Spectral Classes Strange lettering scheme is a historical accident. Spectral Class Surface Temperature O B A F G K M 30,000 K 20,000 K 10,000 K 7000 K 6000 K 4000 K 3000 K Examples Rigel Vega, Sirius Sun Betelgeuse Further subdivision: BO - B9, GO - G9, etc. GO hotter than G9. Sun is a G2. Classification of Stars Through Spectroscopy Ionized helium. Requires extreme UV photons. Only hottest stars produce many of these. 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 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 Stellar Sizes - Indirect Method Almost all stars too far away to measure their radii directly. Need indirect method. For blackbodies, use Stefan's Law: Energy radiated per cm2 of area on surface every second T 4 (T = temperature at surface) And: Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2) So: Luminosity (temperature) 4 x (surface area) 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 p R2) The Wide Range of Stellar Sizes Clicker Question: If the temperature of the Sun (at the photosphere) suddenly doubled from 6000 K to 12000 K, but the size stayed the same, the luminosity would: A: decrease by a factor of 4 B: increase by a factor of 2 C: increase by a factor of 4 D: increase by a factor of 16 Clicker Question: Suppose two stars (star A and star B) appeared equally bright but we knew that star A was 10 times further away, what do we know about the luminosity of star A? A: The two stars have equal luminosity. B: Star A is 10 times more luminous than star B. C: Star A is 100 times more luminous than star B. D: Star B is 10 times more luminous than star A. 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.