The Sun

The Sun

Optically, we see a fairly bland surface, broken up by sunspots (and in this case, Mercury!)

Credit & Copyright: Rick Scott and Joe Orman

• What do we mean by taking the sun using the light of various atoms? This is what the solar spectrum looks like: dark lines correspond to various specific elements, and they have precsiely known wavelengths.

• Mass≈ 2 × 10³⁰ kg
  
• Radius ≈ 7 × 10⁸ m
  
• Density ≈ 1400 kg m^-3 (≈1.4 × H₂O)
  
• Surface temp ≈ 5785 K
  
• Luminosity = 3.9 × 10²⁶ W
  Elemental Abundances (by no of atoms)
  
  1. H, 10⁶ atoms, or approx 90%
     
  2. He, 10⁵ atoms, or 9%
     
  3. All "metals" 1%, including:
     
     1. C, 350 atoms
        
     2. N, 110 atoms
        
     3. O, 670 atoms
        
     4. Mg, 34 atoms
        
     5. Si, 35 atoms
        
     6. S, 16 atoms
        
     7. Fe, 25 atoms
        

SO (e.g.) we can pick out Hydrogen (H), which picks up the prominences very clearly

Credit & Copyright: Ralph Encarnacion

The Sun

A prominence erupts from the surface of the sun: note there is very little material actually in these

This is helium light: a "rosti" picture. Note the sunspots

This is in X-rays: note the hot X-rays come from the cool sunspots!

and this looks at the mag field: Dark is a N pole, light is a S pole

And this looks at the sun at 3 different temperatures: Red at 2 million, green at 1.5 million, and blue at 1 million degrees C.

Credit: SOHO - EIT Consortium, ESA, NASA

"Surface"

No surface in any normal sense, Photosphere is at a fairly uniform 5800 °C. Light coming out of interior of sun will scatter ∼ 1028 times (and take ∼106 years) : the surface is the position of last scatter, after which the light takes 7 minutes to reach earth. Chromosphere: ∼ solar atmosphere. Thickness ∼ 10⁴ km, Optically thin, so can only be seen directly during a solar eclipse. Hotter than the photosphere. Corona: very diffuse, very hot (1000000°°C) Core: all nuclear reactions go on in core of sun

Sunspots

Dark areas (umbra) surrounded by less dark (penumbra). Dark is relative: temperature in umbra ∼ 3800 K

Copyright: AURA/ NOAO

Strong magnetic fields: typical solar mag field ∼ .01 T = 100 gauss In sunspot, can be ∼ .4 T ∼ 4000 gauss (say about 1/4 that in MRI machine)

Can take magnetogram: look at sun in three (or more) nearby wavelengths. By subtracting these, can get "magnetic field" picture of the sun.

• Can get "magnetic field" picture of the sun. Usually in pairs (or larger groups) with opposite magnetic polarity.

• However, period varies from P ∼ 25 days at equator
  P ∼ 27 at 40⁰
  P ∼ 30 at poles
  
• Lifetime of large sunspots ∼ 2-3 months
  

• Show irregular 2 × 11 year cycle: few emerge near 400 (with,say N mag poles leading S) more emerge in lower latitudes, forming large groups number drops off, and they only appear close to equator back to 1) few emerge near 400 (with now S mag poles leading N)

Cycle is irregular, sometimes very few spots. Maunder minimum (long period of very few spots) coincided with "little ice age" in Europe (1550-1700)

From High Altitude Observatory

Granulation of solar surface: Hi-res pictures of sun (usually in H line) show "granules" of about 700 km radius surrounded by darker (i.e. cooler) "lanes" Lifetime ≈ few minutes.

Prominences:

Prominences look very dramatic, but are actually quite benign: this shows a prominence erupting on the sun

This shows a huge "prominence" erupting from the sun's surface

Credit: Canadain Space Agency

Corona

Hard to see corona from earth, but magnetic field concentrates matter. Coronal loop is about 50000 km high

TRACE

The corona changes according to the stage of the sunspot cycle: this is at a minimum

• and this is close to a maximum: note the huge streamers of particles blowing out

  Image Credit: High Altitude Observatory, NCAR

and this is a "hole" in the corona

Aurora

This is a normal looking aurora

But we can see these from space as well

And we will always get N and S auroras

Auroras Over Both Earth Poles Credit: Polar VIS, JPL, NASA

Where do aurora come from?

This is a tube of magnetic flux on the sun: it confines particles. When it collapses, it squirts out the particles

• which travel through space

  Image credit: Goddard Space Flight Center .

and cause a magnetic storm/aurora here on earth

• The colours arise because of different interactions in the atmosphere

  Diagram from University of Alaska Geophysical Institute

And while we are about it, Jupiter also has aurora: this is on the night side of the planet as seen from the Galileo space probe

• and Saturn

  Saturn Aurora Credit: J. Trauger (JPL), NASA

How do stars work?

• \color{red}{p + p \Rightarrow ^2 He} can't occur, ²He doesn't exist
• \color{red}{p + p \Rightarrow d + e^ + + \nu } is the initial process
• and is very slow (the ν has only weak interactions, and so the whole process must go by that)

• \color{red}{ p + d \Rightarrow ^3 He + \gamma }
  followed by
  \color{red}{ d + ^3 He \Rightarrow ^4 He + p}
  Most important is the "executive summary"
• \color{red}{ 4p \Rightarrow ^4 He + 2e^ + + 2\nu }
• Every second the sun converts 4 million tons of matter into energy!
• And this is what keeps us warm! But is it true?

• Actually the sun produces 4x10²⁶ watts,
• which is 10³⁹ ν's/s ⇒ 9x10¹⁴ ν's m^-2s^-1
• (You didn't notice?)
• How do we know if it is true?
• Look for the ν's!
• Require a target which will give a suitable end-product when hit by a ν : e.g.
  \color{red}{ ^{37} Cl + \nu \Rightarrow ^{37} Ar + e^ - }
• ³⁷Ar is radioactive and decays after 35 days (back to Cl + e )
• Rates: 1 SNU (solar neutrino unit) = 10^-36 captures/atom/second
• Expectation in 1961 20 SNU's, now 5 SNU's

• Davis expt (133 tonnes) of Cl (in the form of C₂Cl₄: perchlorethylene).
• Find Ar atoms by flushing system with He, picks up Ar, then collect it and watch for decays

• How many? 5 SNU implies 1 atom of 37Cl converted/day (a needle in a haystack?). Run for 3 months, search for Ar37

• Now results show 7.9 SNU's expected 2.1 observed.
• Davis gets Nobel Prize 2003 (Bahcall, who did the calculations, should have done as well!)
• Where have all the ν's gone?
  • Bad experiment?
  • We don't understand the sun?
  • Maybe the sun has gone out!
  • Nuclear physics wrong?

• Need totally new experiment (and M$60)

  Sudbury Neutrino Observatory (SNO) How does this work? . Need to go deep to get away form cosmic rays

  SNO collaboration

in the Creighton mine (note the other bit of astronomy: the Sudbury basin is an old meteorite crater!)

SNO collaboration

• SNO uses 1000 tons of D₂ ν + d ⇒ e- + p + p allows ν's to be seen immediately.

• Must be pure (radioactivity gives false positives) < 10-15 parts Radium

  SNO collaboration

• Light (photons) are picked up by photo-tubes

  SNO collaboration

• 10000 look like this

  SNO collaboration

• Produces "rings" of light which allow incident particle to be reconstructed

• Answers in 2001
  • Bad experiment?
  • We don't understand the sun?
  • Maybe the sun has gone out!
  • Nuclear physics wrong?
• The ν's produced in the sun change to another kind of ν on the way over.

  Why? We don't know, but it means that ν's have mass.

• One consequence: the amount of mass in the universe due to ν's is more than all the mass of the "ordinary" matter combined.
• Now lets look at other stars