About 45 years ago, when I was a young physics undergraduate, I spent a summer at the Royal Greenwich Observatory’s then base in Herstmonceux. I actually got to stay in Herstmonceux castle for a couple of months. Which sounds very exciting, except my room was actually a small cell in the attic. It was a small price to pay to have the privilege of accessing the wonderful range of telescopes and other equipment that they had.
https://en.wikipedia.org/wiki/Herstmonceux_Castle
The problem I was given was to develop a computer model of the sun’s corona.
The surface temperature of the sun is about 6,000K. As it’s atmosphere extends out into space, you would expect it to become colder and colder. But the sun’s corona does something different. In fact it heats up to millions of degrees before eventually cooling off. This seems to be quite contrary to the laws of thermodynamics that say that heat should always flow from hot to cold. But in the case of the sun’s corona, the opposite seems to be happening. Even today, I don’t think this process is fully understood. The most popular theory 45 years ago was that it was something to do with the sun’s magnetic field.
I did develop some models, although I think it’s safe to say these were not my greatest academic success. I predicted that the corona was composed of 90% lead. Even then I doubted that this was entirely correct. Fortunately for the world I never went on to become a professional astronomer.
Almost all of the energy that we see here on earth comes from the sun’s “surface”, it’s “photosphere”. Because of the huge density and temperature at the core, it takes about half a million years for the energy generated at the core to perform a random walk to the surface, and then a further nine minutes to get from the surface to us. (The exceptions are solar neutrinos, which barely even notice that the sun exists.)
All the pictures of the sun that I’ve ever posted are of the photosphere. To take pictures of the photosphere you need a really good filter. One that blocks 99.99% of the sun’s radiation at all wavelengths. These are relatively cheap. You can buy a sheet of filter material for a few pounds.
With this filter in place, you can see the sun’s disk and any sunspots that might be present, but that’s about all. In order to explain how we can get better pictures, and more information, we need a little bit of physics.
The sun is mostly a big ball of hydrogen. Hydrogen atoms are as simple as an atom can be, with a single proton at the centre and a single electron in orbit. Left to itself, the electron will occupy the ground state in the atom. It can gain energy, either by absorbing a photon of light, or through thermal excitation – basically bumping into other atoms.
An H atom doesn’t stay excited for very long before the orbit decays back to the ground state. When it does so it emits a single photon of light at a very specific frequency that is called “Hydrogen Alpha”.
This picture is more of a classical representation of an atom. Both the proton and the electron should by fuzzy. And the scale is all wrong. If the proton was the size of a tennis ball, then the electron should be an infinitesimal spec of dust a couple of miles away. But just ignore all that.
The sun’s core generates electromagnetic energy with a very specific distribution of frequencies called “black body radiation”. It may seem odd to describe the brightest object in the sky as a black body, but the name comes from the study of the surface of furnaces that are perfectly black on the inside. The need to explain the measured intensity of frequencies in black body radiation was what led Max Planck to his famous hypothesis of quantisation of energy, but that’s a whole different story.
When this black body radiation from the core reaches the surface it encounters the hydrogen in the photosphere. Most of the energy passes straight through, but when the bits at the H Alpha frequency encounter an H atom, it gets absorbed and raises it’s electron to the first excited state. The atom then decays and emits a photon of energy, again at the H alpha frequency, but it does so in a random direction, some of which is back into the core. The photosphere therefore appears darker at the H alpha frequency compared to all nearby frequencies.
Of course there are other excited states of hydrogen and other trace elements in the photosphere. These all lead to dark bands in the sun’s spectrum. But H alpha is the most important example. The important thing to note is that the photosphere is very bright at a wide range of frequencies, except at H alpha.
In between the corona, at millions of degrees, and the photosphere, at 6000K, there is a thin layer of hydrogen called the Chromosphere. This is hotter than the surface but not as hot as the corona. It’s typical temperature is about 20,000K. This is hot enough to excite H atoms by thermal means which then decay via H alpha radiation. So the Chromosphere actually generates the very frequency that is absent in the photosphere. It is also close enough to the photosphere to share many of its characteristics.
All you have to do is isolate the H alpha frequency and you’ll get a detailed image of the chromosphere and many of the surface features that it replicates. The only catch is, these filters are extremely expensive – of the order of thousands of pounds. There are specialist telescopes with these filters built in, but they’re equally expensive. You can get cheaper, wide band, H alpha filters for nebula photography, but they just won’t do for solar imaging. (I’ve tried – you just get a red picture of the sun.)
If you want to use a reflector telescope, like mine, you also have to block out most of the other energy from the sun, particularly infrared, before it hits the telescope mirror and starts heating it up. Small aperture refractors are best.
It’s also best to use a mono camera. Most colour cameras are at their most sensitive in green light. H alpha is red.
There is another option. There’s a frequency called calcium K. This is in the ultraviolet and leads to spooky pictures like this.
https://www.sciencephoto.com/media/156348/view/calcium-k-sun
But, you guessed it, calcium K filters are even more expensive.
So if I wanted to take nice pictures of the chromosphere I’d have to buy a new telescope, a new camera and an expensive filter. All to take pictures of just the sun. It would have to be a very serious hobby for me even to consider it. So next time you see an amateur picture of solar flares and detailed surface features, just consider how much effort and expense has gone into it (and how much science has gone into making it possible).
Platitude of the Day 