Exploring the Sun: Our Closest Star

1. Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month- February 2, 2009 Dr. Uwe Trittmann

2. Today’s Topics • The closest Star: Our Sun • The Night Sky in February

3. The Sun – A typical Star • The only star in the solar system • Diameter: 100  that of Earth • Mass: 300,000  that of Earth • Density: 0.3  that of Earth (comparable to the Jovians) • Rotation period = 24.9 days (equator), 29.8 days (poles) • Temperature of visible surface = 5800 K (about 10,000º F) • Composition: Mostly hydrogen, 9% helium, traces of other elements

4. How do we know the Sun’s Diameter? • Trickier than you might think • We know only how big it appears • It appears as big as the Moon • Need to measure how far it is away • Kepler’s laws don’t help (only relative distances) • Use two observations of Venus transit in front of Sun • Modern way: bounce radio signal off of Venus

5. How do we know the Sun’s Mass? • Fairly easy calculation using Newton law of universal gravity • Again: need to know distance Earth-Sun • General idea: the faster the Earth goes around the Sun, the more gravitational pull  the more massive the Sun • Earth takes 1 year to travel 2π (93 million miles)  Sun’s Mass = 300,000  that of Earth

6. How do we know the Sun’s Density? • Divide the Sun’s mass by its Volume • Volume = 4π × (radius)3 • Conclusion: Since the Sun’s density is so low, it must consist of very light materials

7. How do we know the Sun’s Temperature? • Use the fact that the Sun is a “blackbody” radiator • It puts out its peak energy in visible light, hence it must be about 6000 K at its surface

8. Black Body Spectrum • Objects emit radiation of all frequencies, but with different intensities Ipeak Higher Temp. Ipeak Ipeak Lower Temp. fpeak<fpeak <fpeak

9. Why does the Sun appear yellow? • Tricky question. It is actually whitish-yellow. • The blackbody curve peaks at green. • Impression we get is a mix of all colors but with different intensities • Caveats: • Eye’s receptors are not equally sensitive to all colors • Atmosphere scatters away short wavelengths • When you can conveniently observe it (sunrise/set), it appears yellow

10. Color of a radiating blackbody as a function of temperature • Think of heating an iron bar in the fire: red glowing to white to bluish glowing

11. How do we know the Sun’s rotation period? • Crude method: observe sunspots as they travel around the Sun’s globe • More accurate: measure Doppler shift of spectral lines (blueshifted when coming towards us, redshifted when receding). • THE BIGGER THE SHIFT, THE HIGHER THE VELOCITY

12. How do we know the Sun’s composition? • Take a spectrum of the Sun, i.e. let sunlight fall unto a prism • Map out the dark (Fraunhofer) lines in the spectrum • Compare with known lines (“fingerprints”) of the chemical elements • The more pronounced the lines, the more abundant the element

13. Spectral Lines – Fingerprints of the Elements • Can use spectra to identify elements on distant objects! • Different elements yield different emission spectra

14. The energy of the electron depends on orbit • When an electron jumps from one orbital to another, it emits (emission line) or absorbs (absorption line) a photon of a certain energy • The frequency of emitted or absorbed photon is related to its energy E = h f (h is called Planck’s constant, f is frequency, another word for color )

15. Sun  • Compare Sun’s spectrum (above) to the fingerprints of the “usual suspects” (right) • Hydrogen: B,FHelium: CSodium: D

16. “Sun spectrum” is the sum of many elements – some Earth-based!

17. The Sun’s spectrum in some detail

18. The Sun’s Spectrum • The Balmer line is very thick  lots of Hydrogen on the Sun • How did Helium get its name?

19. How do we know how much energy the Sun produces each second? • The Sun’s energy spreads out in all directions • We can measure how much energy we receive on Earth • At a distance of 1 A.U., each square meter receives 1400 Watts of power (the solar constant) • Multiply by surface of sphere of radius 149.6 bill. meter (=1 A.U.) to obtain total power output of the Sun

20. Energy Output of the Sun • Total power output: 4  1026 Watts • The same as • 100 billion 1 megaton nuclear bombs per second • 4 trillion trillion 100 W light bulbs • $10 quintillion (10 billion billion) worth of energy per second @ 9¢/kWh • The source of virtually all our energy (fossil fuels, wind, waterfalls, …) • Exceptions: nuclear power, geothermal

21. Where does the Energy come from? • Anaxagoras (500-428 BC): Sun a large hot rock – No, it would cool down too fast • Combustion? • No, it could last a few thousand years • 19th Century – gravitational contraction? • No! Even though the lifetime of sun would be about 100 million years, geological evidence showed that Earth was much older than this

22. What process can produce so much power? • For the longest time we did not know • Only in the 1930’s had science advanced to the point where we could answer this question • Needed to develop very advanced physics: quantum mechanics and nuclear physics • Virtually the only process that can do it is nuclear fusion

23. Nuclear Fusion • Atoms:electrons orbiting nuclei • Chemistry deals only with electron orbits (electron exchange glues atoms together to from molecules) • Nuclear power comes from the nucleus • Nuclei are very small • If electrons would orbit the statehouse on I-270, the nucleus would be a soccer ball in Gov. Strickland’s office • Nuclei: made out of protons (el. positive) and neutrons (neutral)

24. Atom:Nucleus and Electrons The Structure of Matter Nucleus: Protons and Neutrons (Nucleons) Nucleon: 3 Quarks | 10-10m | | 10-14m | |10-15m|

25. Nuclear fusion reaction • In essence, 4 hydrogen nuclei combine (fuse) to form a helium nucleus, plus some byproducts (actually, a total of 6 nuclei are involved) • Mass of products is less than the original mass • The missing mass is emitted in the form of energy, according to Einstein’s famous formulas: E = mc2 (the speed of light is very large, so there is a lot of energy in even a tiny mass)

26. Hydrogen fuses to Helium Start: 4 + 2 protons End: Helium nucleus + neutrinos Hydrogen fuses to Helium

27. Could We Use This on Earth? • Requirements: • High temperature • High density • Very difficult to achieve on Earth!

28. Fusion is NOT fission! • In nuclear fission one splits a large nucleus into pieces to gain energy • Build up larger nuclei Fusion • Decompose into smaller nuclei Fission

29. Harvesting Binding Energy Most stable element in the universe Small harvest by decay Big harvest by fusion

30. The Standard Solar Model (SSM) • Sun is a gas ball of hydrogen & helium • Density and temperature increase towards center • Very hot & dense core produces all the energy by hydrogen nuclear fusion • Energy is released in the form of EM radiation and particles (neutrinos) • Energy transport well understood in physics

31. Standard Solar Model

32. How much energy does the Sun produce in theory? • Short answer: As much as it has to … • Longer answer: … to maintain hydrostatic equilibrium

33. Hydrostatic Equilibrium • Two forces compete: gravity (inward) and energy pressure due to heat generated (outward) • Stars neither shrink nor expand, they are in hydrostatic equilibrium, i.e. the forces are equally strong Heat Gravity Gravity

34. More Mass means more Energy • More mass means more gravitational pressure • More pressure means higher density, temperature • Higher density, temp. means faster reactions & more reactions per time • This means more energy is produced

35. Does too much Energy lead to Explosion? • No, there is regulative feedback: • More energy produced means more radiative pressure • This means the stars gets bigger • This means density, temperature falls off • This means less reactions per time • This means less energy produced

36. How do we know what happens in the Sun? • We can’t “look” into the Sun • But: come up with theory that explains all the features of the Sun and predicts new things • Do more experiments to test predictions • This lends plausibility to theory

37. Example: Solar Neutrino Crisis • We can detect the neutrinos coming from the fusion reaction at the core of the Sun • Way to few are seen! (1/3 to 1/2 of the predicted value!) • Possible explanations: • Models of the solar interior are incorrect • Our understanding of the physics of neutrinos is incorrect • Against all odds, #2 is the answer – neutrinos “oscillate”, they change their identity

38. Details • Radiation Zone and Convection Zone • Chromosphere • Photosphere • Corona • Sunspots • Solar Cycle • Flares & Prominences

39. Convection Zone • The core generates all energy, which propagates as particles of light, photons • By the end of radiation zone (500,000 km) all photons are absorbed by Sun’s gas • Energy is then carried to the solar surface by convection

40. Evidence: Granulation • Bright granules move up; dark one move down • About 1 km/sec • Granules are about the size of Earth’s continents

41. A Puzzle • Photons and Neutrinos move at light speed • Photons take about 700,000 years to leave the Sun • Neutrinos make it in a few seconds

42. Chromosphere • Above the photosphere • Gas too thin to glow brightly, but visible during a solar eclipse • Characteristic pinkish color is due to emmision line of hydrogen • Solar storms erupt in the chromosphere • Spicules – thin jets of matter thrown out from photosphere

43. Solar Corona • Thin, hot gas above the chromosphere • High temperature produces elements that have lost some electrons • Emission in X-ray portion of spectrum • Cause of high temperatures in the corona is unknown

44. Sunspots • Dark, cooler regions of photosphere first observed by Galileo • About the size of the Earth • Usually occur in pairs • Frequency of occurrence varies with time; maximum about every 11 years • Associated with the Sun’s magnetic field

45. From the Roof: Sunspots in October ‘03

46. Sun Spots • 1/180 second exposure • With Solar Filter!

47. Sunspots and Magnetism • Magnetic field lines are stretched by the Sun’s rotation • Pairs may be caused by kinks in the magnetic field

48. The Solar Cycle

49. Prominences • Loops or sheets of gas • May last for hours to weeks; can be much larger than Earth • Cause is unknown

50. Solar Flares • Like prominences, but so energetic that material is ejected from the Sun • Temperatures up to 100 million K • Flares and prominences are more common near sunspot maxima