1B11 Foundations of Astronomy The Sun. Silvia Zane, Liz Puchnarewicz [email protected] www.ucl.ac.uk/webct www.mssl.ucl.ac.uk/. 1B11 The Sun: a source of mystery to the human race. Newgrange, outside Dublin. Winter Solstice. 1B11 The Sun: a composite schematic illustration.
Newgrange, outside Dublin
Like all stars, the sun is in equilibrium between the force of gravity (collapse) and the expansion produced by the heat released through nuclear reactions (expansion).
In a convective zone:
These parameters can be used in input in a mathematical model (standard model) to determine, i.e., how density, pressure, temperature vary from the centre to the surface.
The Energy source is at the Sun’s core (T=15MK, ne=1.5x105kg/m3).
The Sun is a big Nuclear Reactor!
The dominant energy is produced from the proton-proton chain. 4 protons are fused together to produce one heavier He nucleus.
Very high Temperature (> 107 K) is required for p to overcome their mutual electric repulsion!
EFFICIENCY of this reaction = 0.7\%
3.86 x10 26 /(3x108)2= 4.3 x109 kg/s
4.3 x109 kg/0.007 = 6.14 x 10 11 kg/s
or ~ 600 million tonnes/s!
~ 0.1 x 1.99 x 10 30 kg/6.14 x 1011 kg/s
~ 3.2 x 10 17 s = 10 x 10 9 yrs
The Sun is the only star for which we can measure internal properties:
3He+4He 7Be + (Be = Beryllium)
p + 7Be 8B + (B = Boron, unstable!)
8B 8Be + e+ + e
However, the probability of absorption does increase with their energy!
37Cl + e 37Ar + e-
37Ar 37Cl + e+ +e
But 37Ar is unstable:
And this decay can be detected the mass of argon that is produced can be measured!
Other experiments (e.g. Kamiokande, GALLEX) cover a wide range of neutrino energies and give roughly the same results: Kamiokande 1/2 then predicted; Gallex: 60% than predicted
1) the standard solar model is uncorrect (so “physicists are right”)
2) something yet unknown happens to neutrinos (“so astronomers are right”)
Hundreds of proposals, no really satisfactory explanation!
The Sun oscillates in a complicated manner.
The solar oscillations are caused by the turbulent convection near the Sun surface.
Waves (somewhat like the seismic waves on the Earth) resonate in the solar interior and appear on the surface.
There are different type of waves: sounds or pressure waves (p-waves) and gravity waves (g-waves).
p-waves have periods between 2 minutes and hours; g-waves are predicted to have much longer period and have not yet been observed!
Measuring p-waves it is possible: need to measure Doppler shift relative to their width to an accuracy 1:106
Possible with good resolution spectrometers and long integration times (to average out noise)
Observational measures of p-waves allow us to probe the interior of the Sun, obtaining information about the temperature and the motion from the deeper regions to the surface.
The Sun acts as a resonant
cavity - bounded by a density drop near the surface and at the bottom by an increase in sound speed.
The speed of sound increases because the Sun is hotter at greater depths (VT1/2), hence the wave front is refracted (the deepest part is travelling at greater speed than the shallowest part).
Leighton, Noyes and Simon: first observation of solar oscillations (1960s) at the Mount Wilson Observatory.
With computer simulations it is possible to reconstruct resonant tones in the Sun interior (different colors are expanding and contracting regions).
Comparing with mathematical models, we can also mimic the rotational splitting of different waves induced by large scale flows.
Bad: the high helium model imply an even higher production rate for neutrinos, aggravating that problem!
Events of active Sun are localized, short-lived phenomena on or near the solar surface.
In general, the area of solar activity are named “active regions” and include
Photosphere, Chromosphere and an hot Corona
The photosphere has a bubbly look. Each bubble has an irregular shape, about 2000 km across and lasts for about 10 minutes.
Fraunhofer Spectrum: covers a complete range from the red to the violet
Spicules- we can tune a spectroscope to the H line, to see these small jets
They dileneate the
boundaries of the
Network cells are
~ 20,000 km in size
Jan 4 2001 -
image in H
- filaments are
The splendid coronal emission is also seen during eclipses!
WHY THE CORONA IS SO HOT?
Understanding why the corona is so much hotter than the surface of the Sun has been one of the main goals of solar physicists since the problem was discovered more than fiftly years ago!
Although the energy that is required to heat the corona is only 0.01 % of the Sun’s total luminosity, the actual mechanism is still unknown.
Biermann, Schearszchild & Schartz (1940): Sound waves?
No: the flux of acoustic waves has been measured and found to be a factor 100-1000 too low to heat the corona
Most probably: a magnetic dominated mechanism!
TODAY, 2 big classes of models:
1)Alternating current (AC) models, I.e. dissipation of waves
2)Direct currents (DC) models, I.e. dissipation of stressed magnetic fields
Yohkoh SXT Active Region Observations
Yohkoh X-ray images during Jan 92.
On the other hand, soft X-ray images of the Sun demonstrate the magnetic complexity of the coronal emission.
Notice the complex bright active regions.
Loops appear temporarily to connect active regions to other active regions, and there are also large diffuse loops which exists in their own right.
Sunspot appear as dark blotches on the solar disk.
With a temperature of about 4200K, a sunspot is cooler than the photosphere and so appears dark in contrast.
In fact a sunspot is almost 4 times fainter than the photosphere.
Sunspots have the tendency to form in groups.
When photospheric granules separate, a tiny spot appears betweem them as a dark pore. Such pores have B1T- enormous. Usually more pores soon become visible and coealesce over a period of several hours to form a sunspot!
What’s going on?
Magnetic Flux Tube Emerge from below Photosphere
The magnetic field has his own pressure, B2/(8).
This pressure push the plasma out the magnetic flux tube, until it reaches pressure balance with the gas outside: Pext = Pint + B2/(8)
Loss of mass inside the flux tube resulting magnetic buoyancy
causes flux tubes to rise!
This occurs when B reaches a critical value!
The “Magnetic Carpet”
Eventually, twist and kinks in the magnetic loops produce flares!
The 11 yrs cycle is not the only periodicity!
The strongest magnetic sunspots have B>0.4T, about 8000 times the average field at the Earth surface
If a spot group has the west spot with south polarity, in the next sunspot cycle the west spot will have a north polarity and in the following cycle, south polarity again
The complete cycle repeats every 22 yrs!
The field is more intense around latitudes of 30, due to a sin2 term in the Suns’ differential rotation, and this is the location of the first sunspots!
N1B11 The Sun: Solar cycle. Periodic Reversal of the field
The final stage of Babcock’s model describe the reversal of the poloidal field due to the fact that active regions tends to have the leading polarity at lower latitude of the following polarity.
Little historical evidence!
This corresponded to an unusual cold spell, sometimes called the Little Ice Age (the average T of the Earth dipped about 0.5 K), that extended from the sixteenth to the eighteenth century.