The Sun: A Glowing Ball of Gas in Our Solar System

1. The Sun Visible Image of the Sun • Our sole source of light and heat in the solar system • A very common star:a glowing ball of gas held together by its own gravity and powered by nuclear fusion at its center.

2. Pop Quiz #2 How do we know the deep interior structure of the Earth? What powers the Sun? Is the surface of the Sun burning?

3. Pressure (from heat caused by nuclear reactions) balances the gravitational pull toward the Sun’s center. This balance leads to a spherical ball of gas, called the Sun. What would happen if the nuclear reactions (“burning”) stopped?

4. Main Regions of the Sun

5. The Moon’s orbit around the Earth would easily fit within the Sun! Solar Properties Radius = 696,000 km (100 times Earth) Mass = 2 x 1030 kg (300,000 times Earth) Av. Density = 1410 kg/m3 Rotation Period = 24.9 days (equator) 29.8 days (poles) Surface temp = 5780 K

6. Luminosity of the Sun = LSUN (Total light energy emitted per second) ~ 4 x 1026W 100 billion one-megaton nuclear bombs every second! Solar constant: LSUN/ 4R2 (energy/second/area at the radius of Earth’s orbit)

7. The Solar Interior “Helioseismology” • In the 1960s, it was discovered that the surface of the Sun vibrates like a bell • Internal pressure waves reflect off the photosphere • Analysis of the surface patterns of these waves tell us about the inside of the Sun How do we know the interior structure of the Sun?

8. The Standard Solar Model

9. Energy Transport within the Sun • Extremely hot core - ionized gas • No electrons left on atoms to capture photons - core/interior is transparent to light (radiation zone) • Temperature falls further from core - more and more non-ionized atoms capture the photons - gas becomes opaque to light in the convection zone • The low density in the photosphere makes it transparent to light - radiation takes over again

10. Convection • Convection takes over when the gas is too opaque for radiative energy transport. • Hot gas is less dense and rises (or “floats,” like a hot air balloon or a beach ball in a pool). • Cool gas is more dense and sinks

11. Solar GranulationEvidence for Convection • Solar Granules are the tops of convection cells. • Bright regions are where hot material is upwelling (1000 km across). • Dark regions are where cooler material is sinking. • Material rises/sinks @ ~1 km/sec (2200 mph; Doppler).

12. The Solar Atmosphere • The solar spectrum has thousands of absorption lines • More than 67 different elements are present! • Hydrogen is the most abundant element followed by Helium (1st discovered in the Sun!) Spectral lines only tell us about the part of the Sun that forms them (photosphere and chromosphere) but these elements are also thought to be representative of the entire Sun.

13. Chromosphere

14. Chromosphere (seen during full Solar eclipse) • Chromosphere emits very little light because it is of low density • Reddish hue due to 32 (656.3 nm) line emission from Hydrogen

15. H light Chromospheric Spicules: warm jets of matter shooting out at ~100 km/s last only minutes Spicules are thought to the result of magnetic disturbances

16. Transition Zone and Corona

17. Transition Zone & Corona Very low density, T ~ 106 K We see emission lines from highly ionized elements (Fe+5 – Fe+13) which indicates that the temperature here is very HOT • Why does the Temperature rise further from the hot light source?  magnetic “activity” -spicules and other more energetic phenomena (more about this later…)

18. Corona (seen during full Solar eclipse) Hot coronal gas escapes the Sun  Solar wind

19. Solar Wind

20. Solar Wind • Coronal gas has enough heat (kinetic) energy to escape the Sun’s gravity. • The Sun is evaporating via this “wind”. • Solar wind travels at ~500 km/s, reaching Earth in ~3 days • The Sun loses about 1 million tons of matter each second! • However, over the Sun’s lifetime, it has lost only ~0.1% of its total mass.

21. Coronal holes are sources of the solar wind (lower density regions) Hot coronal gas (~1,000,000 K) emits mostly in X-rays. Coronal holes are related to the Sun’s magnetic field

22. The Active Sun UV light Most of theSolar luminosity is continuous photosphere emission. But, there is an irregular component (contributing little to the Sun’s total luminosity).

23. Sunspots Granulation around sunspot

24. Sunspots • Typically about 10000 km across • At any time, the sun may have hundreds or none • Dark color because they are cooler than photospheric gas (4500K in darkest parts) • Each spot can last from a few days to a few months • Galileo observed these spots and realized the sun is rotating differentially (faster at the poles, slower at the equator)

25. Sunspots & Magnetic Fields • The magnetic field in a sunspot is 1000x greater than the surrounding area • Sunspots are almost always in pairs at the same latitude with each member having opposite polarity • All sunspots in the same hemisphere have the same magnetic configuration

27. The Sun’s differential rotation distorts the magnetic field lines The twisted and tangled field lines occasionally get kinked, causing the field strength to increase “tube” of lines bursts through atmosphere creating sunspot pair

28. Sunspot Cycle Solar maximum is reached every ~11 years Solar Cycle is 22 years long – direction of magnetic field polarity flips every 11 years (back to original orientation every 22 years)

29. Heating of the Corona • Charged particles (mostly protons and electrons) are accelerated along magnetic field “lines” above sunspots. • This type of activity, not light energy, heats the corona.

30. Charged particles follow magnetic fields between sunspots: Solar Prominences Sunspots are cool, but the gas above them is hot!

31. Earth Solar Prominence Typical size is 100,000 km May persist for days or weeks

32. Very large solar prominence (1/2 million km across base, i.e. 39 Earth diameters) taken from Skylab in UV light.

33. Solar Flares – much more violent magnetic instabilities 5 hours Particles in the flare are so energetic, the magnetic field cannot bring them back to the Sun – they escape Sun’s gravity

34. Coronal activity increases with the number of sunspots.

35. Nuclear Fusion 4 H He The Proton-Proton Chain: What makes the Sun shine?

36. E = mc 2 (c = speed of light) But where does the Energy come from? • c2 is a very large number! • A little mass equals a LOT of energy. • Example: • 1 gram of matter  1014 Joules (J) of energy. • Enough to power a 100 Watt light bulb for ~32,000 years!

37. E = mc 2 (c = speed of light) But where does the Energy come from!? The total mass decreases during a fusion reaction. Mass “lost” is converted to Energy: Mass of 4 H Atoms = 6.693  10-27 kg Mass of 1 He Atom = 6.645  10-27 kg Difference = 0.048  10-27 kg (% m converted to E) = (0.7%) The sun has enough mass to fuel its current energy output for another 5 billion years

38. Nuclear fusion requires temperatures of at least 107 K – why? • Atomic nuclei are positively charged  they repel via the electromagnetic force. • Merging nuclei (protons in Hydrogen) require high speeds. • (Higher temperature – faster motion) • At very close range, the strong nuclear force takes over, binding protons and neutrons together (FUSION). • Neutrinos are one byproduct.

39. The energy output from the core of the sun is in the form of gammy rays. These are transformed into visible and IR light by the time they reach the surface (after interactions with particles in the Sun). Neutrinos are almost non-interacting with matter… So they stream out freely. Neutrinos provide important tests of nuclear energy generation.

40. Detecting Solar Neutrinos – these light detectors measure photons emitted by rare chlorine-neutrino reactions in the fluid. Solar Neutrino Problem: There are fewer observed neutrinos than theory predicts (!) A discrepancy between theory and experiments could mean we have the Sun’s core temperature wrong. But probably means we have more to learn about neutrinos! (Neutrinos might “oscillate” into something else, a little like radioactive decays…)