1. The Sun Our sole source of light and heat in the solar system
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. Pressure (from heat caused by nuclear reactions) balances the gravitational pull toward the Sun’s center. Called “Hydrostatic Equilibrium.
This balance leads to a spherical ball of gas, called the Sun.
What would happen if the nuclear reactions (“burning”) stopped?

3. Main Regions of the Sun

4. 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
The Moon’s orbit around the Earth would easily fit within the Sun!

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

6. How do we know the interior structure of the Sun?
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?

7. The Standard Solar Model

8. 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

9. 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

10. Solar Granulation Evidence 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 ~1 km/sec (2200 mph; Doppler).

11. 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.

12. Chromosphere

13. 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

14. 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

15. Transition Zone and Corona

16. 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…)

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

18. Solar Wind

19. 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.

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

21. 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).

22. Sunspots
Granulation around sunspot

23. 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)

24. 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

26. 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

27. 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)

28. 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.

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

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

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

32. 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

33. Coronal activity increases with the number of sunspots.

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

35. E = m c 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  Joules (J) of energy.
Enough to power a 100 Watt light bulb for ~32,000 years!

36. E = m c 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 =  kg
Mass of 1 He Atom =  kg
Difference =  kg
(% m converted to E) = (0.7%)
The sun has enough mass to fuel its current energy output for another 5 billion years

37. 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.

38. 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.

39. 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…)