Exploring the Sun: Temperatures, Neutrinos, and Magnetic Fields

1. Units to cover: 52, 53, 26. 27

2. Observatories in Space

3. Images from the Hubble Space Telescope

4. Temperature and Pressure Are the Key • In the core of the Sun, the temperature exceeds 15 million K, and the pressure is very high • High temperatures imply that the nuclei in the core are moving very fast, and the high pressure is pushing them together • The high speeds of the nuclei allow them to collide and fuse via the proton-proton chain

5. The Proton-Proton Chain

6. Neutrinos • One product (aside from energy) of the proton-proton chain is a neutrino • Very low mass, very high energy particle • Passes through matter very easily, and so is hard to detect • Neutrino measurements on Earth confirm our models of fusion in the Sun’s core Davis and Koshiba 02

7. Sunspots • Sunspots are highly localized cool regions in the photosphere of the Sun • Discovered by Galileo • Can be many times larger than the Earth! • They contain intense magnetic fields, as evidenced by the Zeeman effect

8. A Sunspot’s Magnetic Field • The intense magnetic fields found in sunspots suppress particle motion • Solar ions cannot leave these regions of high magnetic field, and the field lines are “frozen” to the plasma • This trapped plasma keeps hot material from surfacing below the sunspot, keeping it cool.

9. Prominences • Fields have their “footpoints” in sunspots in the photosphere • These loops are relatively unstable, and can release vast quantities of plasma into space very quickly • Prominences are large loops of glowing solar plasma, trapped by magnetic fields • Coronal Mass Ejections

10. Solar Flares • Solar flares are huge eruptions of hot gas and radiation in the photosphere • Can damage satellites, spacecraft, and humans in space • The study of coronal mass ejections and solar flares is called “space weather”

11. A Coronal Mass Ejection

12. The Aurora • When CME material reaches the Earth, it interacts with the Earth’s magnetic field and collides with ionospheric particles • The collision excites ionospheric oxygen, which causes it to emit a photon • We see these emitted photons as the aurora, or Northern Lights

13. The number of sunspots seen increases and decreases periodically. Every 11 years or so, the sunspot number peaks. This is called Solar Maximum Around 5.5 years after Solar Maximum, the sunspot number is at its lowest level. This is called Solar Minimum Solar activity (CMEs, flares, etc.) peaks with the sunspot number The Solar Cycle

14. The Babcock Cycle

15. The Maunder Minimum • Very few sunspots were recorded between 1645 and 1725 • This is called the Maunder Minimum • Corresponds to relatively lower temperatures here on Earth, a “little ice age” • The reason for the Maunder Minimum and its effect on climate are still unknown

16. This does not work for Light! • If Galilean Relativity worked for light, we would expect to see light from a star in orbit around another star to arrive at different times, depending on the velocity of the star. • We do not see this – light always travels at the same speed.

17. The Michelson-Morley Experiment • Two scientists devised an experiment to detect the motion of the Earth through the “aether” • Light should move slower in the direction of the Earth’s motion through space • Detected no difference in speed! • No aether, and the speed of light seemed to be a constant!

18. The Lorentz Factor • It was proposed that perhaps matter contracted while it was moving, reducing its length in the direction of motion • The amount of contraction was described by the Lorentz factor • At slow speeds, the effect is very small • At speeds close to the speed of light, the effect would be very pronounced!

19. Einstein’s Insights • Albert Einstein started from the assumption that the speed of light was a constant, and worked out the consequences • Length does indeed contract in the direction of motion, by a fraction equal to the Lorentz factor • Time stretches as well, also by the Lorentz factor • Moving clocks run slow • Moving objects reduce their length in the direction of motion

20. Special Relativity • Time dilation and length contraction depend on the observer! • To an observer on Earth, the spacecraft’s clock appears to run slow, and the ship looks shorter • To an observer on the ship, the Earth appears to be moving in slow-motion, and its shape is distorted. • The passage of time and space are relative!

21. Possibilities for Space Travel • Example: A spacecraft leaves Earth, heading for a star 70 light-years away, traveling at .99c • To an observer on Earth, it takes the spacecraft 140 years to get to the star, and back again • To passengers on the ship, it only takes 20 years for the round-trip! • This means that high speed travel to the stars is possible, but comes at the cost of friends and family…

22. Differential Rotation • Different parts of the sun rotate at different speeds • Equator rotates faster than the poles • Solar magnetic fields get twisted as time goes on