Before the write-up proper, here is a sub-node
How to burn a hole in your retina
1. Point an astronomical telescope
(or other optical
instrument) towards the sun
2. Look through the eyepiece
3. There is no step 3
There is no pain, and no smell, but it is the easiest and surest way of making yourself partially blind.
Don't do it. Ever.
Damage to the retina caused by focussing the sun's rays on it is permanent. The damage cannot be repaired by time, surgery, transplants or spectacles.
Sun Spots—the intro
Basically, sunspots are small, dark regions on the "surface" of the sun. They appear dark compared with the surrounding areas, only because they are a little less hot (around 3700K
) than the surrounding stuff (around 5700K
There is a lot more to it than that, however. What, for example, does the "surface" of the sun mean? How are sunspots linked to magnetic fields? Why does the number of sun spots vary over an 11- (or 22-)year cycle?
To answer these questions, we need to learn a bit more about what the sun is and how it works.
Briefly, the sun is a star. It is a pretty average star, sitting somewhere near the middle of the main sequence, with average temperatures, half-way through its average lifetime of 9000 million years, average mass and average size (diameter 1.4 million km). It works by converting mass to energy in nuclear reactions. The conversion process releases lots of energy, and the outward pressure of this energy prevents the star from collapsing under its own weight.
Although nuclear fusion and gravity govern the overall size and energy balance of the sun, magnetism plays a very important role in its detailed behaviour.
Inside the sun, there are lots of different layers, which exist at different temperatures and different pressures. As we move radially out from the centre of sun, the pressures and temperatures fall, until we reach part of the star we can see: the photosphere.
Literally,the sphere which gives off light, the photosphere is at a temperature of around 6000 Kelvin. That is hot enough to melt most materials, but it is not mind-bogglingly hot. We can quite easily achieve that temperature here on earth. Inside the sun things really are mind-bogglingly hot, while the corona is outside the photosphere and reaches around 1 million K, which is also hot enough to boggle the mind. The bit of the sun we see when we look up at the noonday sky is (almost) the coolest part of the sun.
To answer my first question, the photosphere is taken to be the 'surface' of the sun.
The sun is a ball of very hot fluid stuff hanging together in a dynamic equilibrium between gravity and radiation pressure. It so happens that this ball of stuff spins around its axis. But the rate of spin at the equator is faster than the rate of spin at the poles. At the equator, the star stuff completes a revolution once every 10 earth days or so, but at the poles, the rotation rate is slower—perhaps once every 20 earth days. The sun is constantly twisting itself up like a rubber band
One of the effects of this is to tangle up the magnetic field lines within the sun. Although the sun is a large body with very powerful magnetic fields, this variable rotation rate makes the field lines even more complex and tangled. Another aspect of the sun is that it is made from plasma. This a very good electrical conductor. Thus, with physical motion, strong magnetic fields and electrically conducting fluids, we have all the elements required to generate very strong physical forces and violent motion (Think motors and generators).
Sunspots are themselves centres of high magnetic field density. This is cause-and-effect. If the magnetic field lines were nice and parallel, the local temperature in the photosphere would be similarly well-ordered. It takes energy to tangle up the field lines, and where the lines are especially mixed up, the energy is sucked from the local environment, giving rise to a local cooling in the photosphere at exactly the points where the field is most powerful.
So a sunspot is a slightly cooler area of the photosphere, but the reason for the cooling is that energy has been taken away from the local environment to generate very strong, complex magnetic fields. Darker (cooler) sunspots mean more energy for magnetic fields, and potentially more violent motion.
Most sunspots are around 1500km in diameter, though they can be bigger than the earth (13000km). As noted above, the sunspot temperature is typically around 2000 degrees (K) cooler than the surrounding photosphere. They last a few days, or perhaps a couple of weeks, flowing with the material of the photosphere as it rotates about the solar axis
Just as nature abhors a vacuum, it really dislikes tangled magnetic fields. They take up energy and are unstable So when the magnetic fields get too twisted up, the energy balances allow the lines to suddenly untangle themselves, with a huge release of energy. The resulting forces are capable of throwing large lumps of solar matter out into space. We can see the result of these re-alignments of the solar magnetic field in flares and prominences coming off the sun.
The amounts of energy involved in these coronal mass ejections are huge. Ten orders of magnitude greater than a terrestrial nuclear bomb.
One last question remains: Why the 22-year sunspot cycle?
You have probably guessed by now that the sunspot cycle is linked to magnetic activity. Scientists are still working on the exact mechanism for this cycle, but it is known that the polarity of the sun's magnetic field flips every 11 years, at the solar maximum. One flip (north-to-south) is half a full cycle, so the full north-south-north cycle takes 22 years. Astronomers know that a new half-cycle has begun only when they see the magnetic field flip over .
Sunspot numbers and locations have been recorded in Zurich, Switzerland since 1749. Astronomers can plot this data, showing number of spots, and their latitude on the sun against time. The resulting pattern (called a butterfly diagram) shows that at the start of the cycle, the spots (and hence areas of high magnetic field activity) are concentrated around 30 degrees from the equator (north and south). As the cycle progresses, the two populations move toward the equator. Soon after the two populations meet, the whole solar magnetic field flips over, and a new half-cycle begins, with the magnetic activity once more focussed on higher latitudes.
A note on observing sunspots
First read the micronode at the top of this write-up. Looking at the sun through any optical instrument is dangerous in the extreme.
Galileo Galilei was the first European to observe sunspots. In 1610, he turned his telescope on the sun, and used it to project a large image onto a white screen. This revealed for the first time, blemishes in the perfection of the solar disk.
The best way to observe sun spots is indirectly. Mount the telescope on a tripod, or preferably an equatorial mount, and point the objective toward the sun. It is probably best to leave a lens cap on until you are ready to start observing.
Arrange a large white sheet of paper or card (the projection screen) about 500mm away from the eyepiece, and then, making sure the telescope is pointed directly at the sun, remove the lens cap. The sun's image should form a large circle, perhaps 100mm in diameter on the projection screen, Move the screen back and forward, and adjust the focus to get a good image. Because of the rotation of the earth, the sun will quite quickly move out of the field of view, so you may have to keep adjusting your telescope position.