The Sun Our Nearest Star

1. The SunOur Nearest Star

2. Our Sun: Background • By far the largest object in the solar system • Diameter 1,392,000 km = 109 times the Earth • Mass 333,000 x Earth • ~94% hydrogen, ~6% helium, 0.13% “metals” (mostly oxygen, carbon, nitrogen); this is by # of atoms, not mass The Sun and planets, shown to scale. Earth is the small blue dot in the middle of the box.

3. Solar Power • How much energy output every second? 4x1026 watts • Equal to 8x1016 of the largest power plants on Earth, which produce ~5000 Megawatts of power. • Every second the Sun puts out as much energy as 2 billion such power plants would put out every year

4. The Sun is Luminous • A luminous object creates its own light. • The Sun is luminous. • The Earth, Moon, and other planets shine by reflected light - they are NOT luminous.

5. Luminosity Defined • The luminosity of the Sun is defined as the total amount of energy emitted each second in all directions from its surface. • The Sun radiates about 4 x 1026 J/sec from its surface. When comparing the Sun to other stars we consider this value as 1 unit. • Stars range from 10-6 to 105 Sun units.

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7. Solar Constant The solar constant is the amount of energy passing through 1 cm2 at Earth’s distance from the sun The intensity of sunlight we receive is about 1400 W/m2 or 1/7 W/cm2 However, due to atmospheric reflection and to day/night and slanting angle of sunlight during the day, the average striking the earth is only about 250 W/m2.

8. Distance and Size The average distance between the Earth and the Sun is about 150 million km or 93 million miles. This is called 1 Astronomical Unit (AU). The Sun has a radius of about 696,000 km or 432,000 miles. It appears to be about the same size as the Moon, but it is 400 times farther away!

9. The Sun’s Makeup • Astronomers think that the Sun and its planets formed from a rotating cloud of interstellar gas and dust. • This is the nebular theory, proposed first by philosopher Immanuel Kant. • The Sun has more than 99% of the mass of the solar system and contains at least 70 chemical elements. • At birth the Sun was 71% H & 27% He.

10. Solar Structure

11. Layers of the Sun • Core: innermost 10% • Location of Sun’s energy generation by nuclear fusion • Temperature 16 million K • Density 100x water = 20x iron • Still gaseous all the way to center due to high temperatures: it’s plasma Core has 40% of Sun’s mass in 10% of its volume. Plasma: gas that is almost completely ionized..

12. Layers of the Sun • Radiative zone: where energy is transported from the superhot interior to the cooler outer layers by photons. Includes inner 85% of the Sun’s radius. • Convective zone: energy in the outer 15% of Sun’s radius is transported by the motion of gas. Here, ions are able to block the outward flow of photons more effectively, so convection is needed for energy transport from the very hot interior to the cooler outer layers. • Temperature here is a “cool” 4 million K

13. Layers of the Sun • Solar envelope puts pressure on core, thus maintaining core temperature • Less efficient for energy to move by radiation; heat energy starts to build up at edge of radiative zone • Energy begins to move by convection in huge cells of circulating gas several hundred km wide; cells nearer to surface are smaller than inner cells • Top of each cell called “granule” ~8 min lifetime

14. Photosphere • The Photosphere is the bright, thin, lowermost layer of the Sun’s atmosphere • Lies just above the convection zone and extends only a few hundred meters. • Zone where sunlight we see is emitted so we call it the Sun’s “surface” • Temperature 5840 K • Photosphere is cooler and less dense than deeper layers.

15. Chromosphere Solar filament in chromosphere, imaged by TRACE satellite • The chromosphere is a transition layer between the photosphere and the corona • Color is caused by H emission which is mostly in the red wavelength of about 656 nm or 6563 A • 2000-3000 km thick, low density • Temperature 7000K, hotter than photosphere • Visible during solar eclipse.

16. Corona • The corona is the faint outermost region of the solar atmosphere composed of ionized gas at very high temps • Visible during solar eclipses • Average temperature 1 million K, but can reach 3 million K • Most of corona is trapped close to Sun by its magnetic field

17. Sunspots Penumbra • Sunspots are dark regions of intense magnetic fields visible on the photosphere • Discovered by Galileo • 1000-1500 K cooler than the rest of the photosphere, thus emit less light, appear dark • Can last days to months • Galileo used sunspots to map Sun’s rotation • Equator rotates in 25 days, poles take up to 30 days Umbra

18. Under a Sunspot • Loops of the Sun’s magnetic field oppose convection & stop the energy flow to surface. • Immediately under the sunspot is a strong inflowing current of plasma • This undertow pulls near-surface material toward the spot & prevents the magnetic fields from flying apart (as in repelling poles of magnets) • This diverts the normal plasma flow bubbling up from the solar interior, creating a self-sustaining sunspot Simulation based on data from SOHO, using an instrument which explores the properties of the solar interior by detecting motions produced by sound waves as they interact at the solar surface – “Doppler imaging”.

19. Multiwavelength Sunspots • Each wavelength records a different layer in the solar atmosphere • Visible: surface or “photosphere”, 5000K • UV: “chromosphere” or transition region, 10-100,000 K • X-rays: solar corona, millions of Kelvins Top to bottom: visible, extreme UV, and X-rays. Taken simultaneously of the solar active region AR 9393 when it was at its peak size: 10x Earth

20. The Solar Cycle • Have observed sunspots for hundreds of years • Thus have learned that the number of sunspots varies in an 11 year cycle • At the start of a cycle the number of sunspots is at a minimum and most are around 30-35 deg from the equator. When sunspot activity peaks ~5.5 years later, most are within 5 deg of equator. Historical record of the 11-year sunspot cycle, 1700-1995. Recent “solar max”: 2000. Upcoming “solar min”: 2007.

21. The Solar Cycle • Increase of activity at Solar Max includes large numbers of sunspots, large flares, huge prominences, and the flipping of the Sun’s north & south magnetic poles • Magnetic field is somehow responsible for the sunspot cycle • In one 11-year cycle the leading sunspot in a sunspot group will have a north magnetic pole, while the trailing sunspot in the group will have a south magnetic pole. In the next 11-year cycle the poles will switch so total cycle is 22 years. SOHO image at Helium wavelength showing large prominence & flaring.

22. Solar Prominences • Prominences are huge clouds of burning gas in the corona associated with sunspots & following magnetic field lines • Extend out 30x Earth’s dia • “Quiet” ones form 40,000 km above surface, lasting several days – several weeks. • “Surge” prominences last ~few hours, shoot gas up to 300,000 km above surface. Image taken in UV at wavelengths of ionized Helium. Hot areas appear white, while relatively cool areas appear red,

23. Coronal Loops • Some “quiet” prominences – loops of hydrogen gas following loops in the solar magnetic field – UV images. • Prominences throw electrons & ions out into the solar system.

24. Solar Flares • Solar flares are short-lived, explosive bursts of matter and energy shooting out from near sunspots • Last a few minutes – few hours • Lots of ionized material ejected • Humans travelling outside the protection of the Earth’s magnetic field need shielding from the powerful ions! Extreme heat of solar flares produces X-rays that create light when they hit the gas in the corona. There are different classes of flares, depending on strength. Some are followed by coronal mass ejections,. Image above taken in X-rays.

25. The Solar Wind • The solar wind is a steady stream of ions, mainly protons & electrons, flowing from the Sun • Velocities of ~500 km/s • When particles pass close to a planet they are deflected by its magnetic field • Wind hits magnetic field, compresses & creates shock wave • Van Allen radiation belts near Earth trap solar particles due to magnetic forces

26. The Solar Wind • Expanding coronal gas fills interplanetary space. • Solar magnetic fields embedded in this plasma form the interplanetary magnetic field (IMF). • Sun rotates; thus IMF takes the shape of a spiral from the anchoring of open field lines to the surface • Confines the magnetic field of Earth & other planets • Density decreases as inverse square of distance from Sun • At a large enough distance, solar wind can no longer “push back” the fields & particles of the local interstellar medium – solar wind slows to ~20 km/s: “heliopause” • Location of transition unknown but >50 AU; from Voyager 1 & 2, perhaps 130-170 AU. Extends beyond solar system?

27. Solar Spacecraft • SOHO – The SOlar & Heliospheric Observatory • Launched Dec. 1995, joint NASA/ESA project • 12 instruments designed to study internal structure of Sun, its outer atmosphere, & origin of solar wind • The Hubble for Sun studies! Nearly as expensive! • Solar orbit, 1 million miles from Earth (4x distance to moon): Sun never sets on SOHO, giving us an uninterrupted view of the Sun • TRACE – Transition Region and Coronal Explorer • Launched 1998, NASA mission • Purpose is to image solar corona and transition region at high spatial and time resolution • Explores solar magnetic field, coordinates with SOHO • Both still in flight.

28. Coronal Mass Ejections • Huge amounts of ionized material flung outward from solar surface • Last only few minutes to few hours • Temperatures recorded up to 11 million K • When these ions reach Earth, can interfere with radio communications, cause surges in power/phone lines, cause brownouts or blackouts, disable satellites. • “Space weather” Sun’s position indicated by white circle. Field of view extends >2 million km from the solar surface. Near solar min, frequency ~1 per week; near solar max, frequency ~2 per day!

29. Magnetic Fields & Aurorae • When charged particles from solar wind hit the atmosphere of a planet, the gas collisions excite molecules in upper atmosphere, causing emission. • On Earth, this is the Aurora Borealis & the Aurora Australis – the Northern & Southern Lights. • Red aurorae produced by hydrogren emission at the top of the atmosphere (>200 km above surface). Green is produced by oxygen emission & blue by ionized nitrogen 100-200km above surface. Magenta produced by nitrogen up to 100 km above surface. • Usually only seen above or below latitude 50 degrees – but during solar max energy of solar wind can make aurorae visible down to +/-30 degrees.

30. Aurorae

31. Other Views of Aurorae Aurora on both poles of Saturn, as seen by the Hubble Aurora on Earth, seen from the Space Shuttle Aurora on Jupiter, as seen by the Hubble

32. New Slide as Needed The part you need to watch is over. This is a place holder slide.

33. Helioseismology • Probe of Sun’s interior using pulsating motions of the Sun • Can be seen by measuring the Doppler shifts across the face of the Sun • Some parts expanding towards Earth while adjacent regions contract away – like modes in a ringing bell • Regions are 1000s of km across, pulsation periods ~few minutes long • Origin of pulsations was a mystery from 1962 discovery until 1975 observations Graphic of one pulsation mode. Blue is moving toward Earth, red away. These pulsations are thought to extend from surface far into the interior.

34. Surface oscillations due to sound waves generated and trapped inside Sun; produced by pressure fluctuations in the turbulent convective motions in the interior. • Trapped sound waves set the Sun vibrating in millions of different patterns or “modes”; by disentangling these modes, can use these waves to probe the solar interior. • How the waves propagate thru Sun & interact with each other depends on temperature, density, and composition of the material they pass through. • By observing effect of the waves on solar photosphere, can determine these quantities for different interior layers – just like seismology from earthquake waves.

35. Solar Neutrinos • Fusion reactions in core produce energy in the form of gamma-rays and neutrinos. • Gamma-rays are high energy, high frequency photons, which are absorbed & re-emitted by many atoms on their way from the envelope to the outside of the Sun. They lose energy in this million-year process, turning into many low-energy visible light photons. • Neutrinos are extremely nonreactive. To stop a neutrino you would have to send it through a light year of lead. Only takes 2 seconds to get from core to surface of Sun – 8.5 minutes to Earth. • Neutrinos are massless (nearly?!) particles which travel very close to (at?) the speed of light.

36. The Solar Neutrino Problem • Number of neutrinos produced by Sun is directly proportional to the number of nuclear reactions taking place at the core. • Thus we should be able to predict how many neutrinos are coming from the Sun.and how many we can ‘catch’ in specially designed experiments. • Problem: number of neutrinos detected is smaller than expected by at least 30% • Why the discrepancy between theory & observations? Super-Kamiokande Neutrino Detector

37. The Solar Neutrino Problem • Experiments not properly calibrated? Several different experiments give same results, so unlikely. • Nuclear reaction rate in Sun lower than predicted? Unlikely as there are strong constraints on this rate from our observations & lab experiments. • Neutrinos produced in core change into other types of neutrinos on their way out, a type our experiments aren’t designed to detect? Current best theory. • Can only do this if it has mass, which would have important consequences for the evolution of the universe. New fundamental physics?

38. Solar Eclipse

39. Nuclear Fusion • Fusion is a thermonuclear reaction in which small nuclei (H) are fused to make more massive nuclei (He) • To get positively charged nuclei to fuse, their electrical repulsion must be overcome. • To overcome the electrostatic repulsion requires high temperatures & high densities. At high temps, nuclei move fast enough to be driven close enough together to fuse. • High densities ensure that there are enough nuclei within a small volume for the collisions to take place at all. • The only place these conditions occur: stellar cores.

40. Nuclear Fusion in the Sun • Stellar core temperatures are greater than the ~8 million K required for hydrogen fusion • Amount of repulsion increases with more positive charge, so fusion of larger nuclei requires increasingly higher temperatures & densities (100 million K for helium fusion) • Thus stars fuse hydrogen first • Main reaction: 4 H fused to make 1 He + energy • E=mc2 4 H have less mass than 1 He – remainder is released as energy • Reaction efficiency ~0.008. Sun can last ~10 billion years on core hydrogen fusion

41. The Iron Sun • Iron is the last element produced by nuclear fusion • The ionized iron here has had 11 electrons stripped away by collisions with other atoms & electrons in the extreme temperature environment of the corona Image taken by SOHO at the wavelengths of UV light emitted by eleven-times-ionized iron at temperatures over 2 million degrees F.