# Announcements. - Homework #5 due today - Review on Monday 3:30 4:15pm in RH103 - Test #2 next Tuesday, Oct 11

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1 Announcements - Homework #5 due today - Review on Monday 3:30 4:15pm in RH103 - Test #2 next Tuesday, Oct 11

2 Review for Test #2 Oct 11 Topics: The Solar System and its Formation The Earth and our Moon The Terrestrial Planets The Jovian Planets Moons, Rings, Pluto, Comets, Asteroids, Dust, etc. The Sun Methods Conceptual Review and Practice Problems Chapters 5 8 and 10 Review lectures (on-line) and know answers to clicker questions Try practice quizzes on-line Come talk to me in office hours (Monday 9-11am) Bring: Two Number 2 pencils Simple calculator (no electronic notes) Reminder: There are NO make-up tests for this class

3 The Sun The Sun is a star: a shining ball of gas powered by nuclear fusion. Mass of Sun = 2 x g = 330,000 M Earth = 1 M Sun Radius of Sun = 7 x 10 5 km = 109 R Earth = 1 R Sun Luminosity of Sun = 4 x erg/s = 1 L Sun The Sun in X-rays over several years (amount of energy put out each second in form of radiation, = W light bulbs) We receive 1400 W/m 2

4 DEMO: Switch on the SUN! Temperature at surface = 5800 K => yellow (Wien s Law) Temperature at center = 15,000,000 K Average density = 1.4 g/cm 3 Density at center = 160 g/cm 3 Composition: 71% of mass is H 27% He 1% Oxygen 1% everything else Rotation period = 27 days at equator 31 days at poles Sun during solar eclipse Jan 2011

5 The Interior Structure of the Sun (not to scale) Let's focus on the core, where the Sun's energy is generated.

6 Core of the Sun Temperature : 15 million K (1.5 x 10 7 K) Density: 160 gm/cm 3, 160 times that of water, 10 times the density of lead

7 Review of Atoms and Nuclei Hydrogen atom: Helium atom: electron + proton + + The proton is the nucleus The nucleus is 2 protons + 2 neutrons _ What binds the nuclear particles? The strong nuclear force. Number of protons uniquely identifies element. Isotopes differ in number of neutrons.

8 Radiative ionization of H _ Review of Ionization + Energetic UV Photon "Collisional Ionization" of H + +

9 Clicker Question: In overall composition the Sun is most like what planet? A: Mercury B: Venus C: Earth D: Jupiter

10 Clicker Question: What is an ion? A: an atom (or molecule) with a net charge. B: Another name for a proton C: An anti-electron D: A charged neutron

11 Clicker Question: What happens when 4 H atoms (protons) are combined into a single He atom? A: Energy is absorbed, cooling the sun B: Energy is released, heating the sun C: No energy is produced, only neutrinos D: The sun becomes more negatively charged

12 What Powers the Sun Nuclear Fusion: An event where nuclei of two atoms join together. Need high temperatures. Energy is produced. nuc. 1 + nuc. 2 nuc. 3 + energy (radiation) Mass of nuc. 3 is slightly less than mass of (nuc. 1 + nuc. 2). The lost mass is converted to energy. Why? Einstein's conservation of mass and energy, E = mc 2. Sum of mass and energy always conserved in reactions. Fusion reactions power stars. Chain of nuclear reactions called "proton-proton chain" or p-p chain occurs in Sun's core, and powers the Sun.

13 In the Sun's Core... neutrino (weird particle) proton deuteron (proton + neutron bound together) proton positron (identical to electron but positively charged) photon 1) proton + proton proton+neutron + neutrino + positron { (deuteron) (heavy hydrogen) + energy (photon)

14 2) deuteron + proton 3 He + energy He nucleus, only 1 neutron 3) 3 He + 3 He 4 He + proton + proton + energy Net result: 4 protons 4 He + 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 4 He in Sun's core. Process sets lifetime of stars. Hydrostatic Equilibrium: pressure from fusion reactions balances gravity. Sun is stable.

15 Solar neutrino problem In 1960s Ray Davis and John Bahcall measured the neutrino flux from the Sun and found it to be lower than expected (by 30-50%) Confirmed in subsequent experiments Theory of p-p fusion well understood Solar interior well understood

16 Answer to the Solar neutrino problem Theoriticians like Bruno Pontecorvo realized There was more than one type of neutrino Neutrinos could change from one type to another Confirmed by Super-Kamiokande experiment in Japan in ,000 gallon tank Total number of neutrinos agrees with predictions

17 How does energy get from core to surface? core photon path "radiative zone": photons scatter off nuclei and electrons, slowly drift outwards: "diffusion". "convection zone" some electrons bound to nuclei => radiation can't get through => heats gas, hot gas rises, cool gas falls "surface" or photosphere: gas density low enough so photons can escape into space.

18 Can see rising and falling convection cells => granulation. Bright granules hotter and rising, dark ones cooler and falling. (Remember convection in Earth's atmosphere, interior and Jupiter). Granules about 1000 km across Why are cooler granules dark? Stefan's Law: brightness α T 4

19 Can see rising and falling convection cells => granulation. Bright granules hotter and rising, dark ones cooler and falling. (Remember convection in Earth's atmosphere, interior and Jupiter). Granules about 1000 km across Why are cooler granules dark? Stefan's Law: brightness α T 4

20 The (Visible) Solar Spectrum Spectrum of the Sun shows: 1) The Black-body radiation 2) Absorption lines (atoms absorbing photons at specific wavelengths). 10,000's of lines from 67 elements, in various excited or ionized states. Again, this radiation comes from photosphere, the visible surface of the Sun. Elements weren t made in Sun, but in previous stellar generations

21 'Atmosphere', atoms and ions absorb specific wavelengths of the blackbody spectrum Interior, hot and dense, fusion generates radiation with black-body spectrum Star

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

23

24

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

26 Filament Ejection Movie

27 Solar Storms!

28 Sunspot numbers vary on a 11 year cycle.

29 0.1% variation from maximum to minimum

30 0.1% variation from maximum to minimum

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

32 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

33 Clicker Question: What is a positron? A: A positively charged neutrino B: Another name for a proton C: An anti-electron D: A charged neutron

34 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

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

36 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.

37 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) tons/s lost. But Sun has lost only 0.1% of its mass from solar wind.

38 Space Weather Today s forecast: solar wind velocity = 434 km/s density = 4.3 protons/cm3 Sunspot number: 38 days without a sunspot since: 0 day For update see List of recent and upcoming Near-miss encounters and space related news.

39 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.

40 Space weather and solar science Coronal Mass Ejections: solar science and ultimately predicting space weather

41 Space weather and solar science Coronal Mass Ejections: Using a background pulsar to measure density and magnetic field of the sun and CME

42 Solar Probe Plus in 2018

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