SOLAR FRIDAY
Added on 09 June 2023
It is always risky to set a date for anything to do with astronomy, kind of like buying a telescope, which as we all know comes with its own pre-packaged clouds, so midweek I consulted my garden weather station to see what the days leading up to 'Solar Friday' would bring. I was immediately encouraged by the pine-cone which was wide open and the small shred of bladderwrack which was bone dry! This was good news indeed and on the basis of this information I advertised the event for Friday at 6pm. The accuracy of these simple runes is quite remarkable compared to 'weather forecasts' on tv or other sources and so it was that Friday evening lived up to expectations and we had perfect conditions for the planned event.
Paul and I set up the solar telescope which sees the HA only and therefore prominences and solar flares etc., and his ED 102 refractor with white-light filter which sees the sunspots and those weird white patches called faculae. All done we made a cup of tea and waited for people to arrive. How pleasant it was sitting in the sunshine and taking in the beauty of the late afternoon surrounded by the hills and moors of Culloden battlefield. A buzzard soared lazily in a thermal until it exited stage left persued by a crow, unseen distant larks were singing their hearts out with joy, high in the azure sky, rabbits nibbled nervously at the short course grass by some gorse bushes and from somewhere overhead the strange and haunting sound of a snipe 'drumming'.
Our first visitor was a tourist from Tennessee who wandered over from his camper-van in the car-park just to see what we were up to. He asked if anything special was going on and we invited him to take a look at the Sun with the solar telescope as indeed there was something going on. A great arching prominence was towering above the limb of the Sun like an enormous bridge spanning thousands of miles and when he saw it first he was struck-dumb with surprise and amazement for what seemed a good minute or so until he spoke. "I have come all the way to Scaatland to see the Lock Ness monster but this is half-way as good as coming face to face with Nessie herself!!". "I had no idea that the Sun could do these things" he said. Indeed he, and most of the world's population have no idea that the Sun can do these things and I suggested that every school in the land should budget for one of these telescopes so that it should not be left to a once in a lifetime chance encounter to discover the secrets of the most important thing in our lives next to the air we breathe and wine. He agreed entirely and, thanking us profusely, went on his way. Now you could say that all you need is a book with pictures to enlighten the world to these treasures but I say no, you have to see these things for yourself and only then does the true nature become evident. You can look at pictures of let's say Sacsayhuaman or the boxes at Saqqara in Egypt and say "Ok so what" but to stand in front of these enigmas and see with your eyes will change your life forever. Same with astronomy, seeing is everything.
It was not long before our friends from the astronomy club arrived and our happy little band began our exploration of the Sun. Of course the solar telescope was the honey-pot for much of the time as it reveals the most profound images of the sun's 'surface' and lets you see the many twisted shapes embedded in the 'orange -peel ' of the disc as well as the awesome spires, tunnels and arching loops of the prominences and ejections from the limb but let's not forget Paul's white-light filter to see the sunspots! They were just amazing and so clear and detailed thanks to the precise nature of the optics on his telescope. That is Extra low dispersion glass with a coating of Lanthunum. As well as some pretty big spots there were so many tiny little ones , some in strings and others concentrated into wide fields of black specks and best of all you could clearly see the umbra and penumbra of the spots. They are so curious to look at and so much look like concavities or holes on the surface and the black of the centre is of such an unfathomable darkness that it inspires awe.
So we all spent ages taking turns to simply gaze in wonder at the sun with the two telescopes and needless to say there were many questions. Now I don't have a PHD in anything which gives me the unique ability to say "I don't know" to questions about the nature of things. I'm convinced that when students are handed their PHD certificates that some molecular change takes place in their brain that entirely removes the words "I don't know" or "I never thought of that" and forever after only invoke the answers that were handed on by their 'Prof'. So a cat may look at a king and we had promised that we would take a dive into the 'rabbit-hole' and 'all your questions answered' so here goes with the science.
THE HISTORY OF THE SOLAR MODEL
As part of an attempt to determine the surface temperature of the Sun, Lane (1869) wrote down for the first time the set of equations describing a gas sphere in hydrostatic equalibrium. We therefore credit Lane with constructing the first physical model of the solar interior. Interestingly, although his model predicted the central temp. of the Sun reasonably well, his predicted surface temp. of 30,000 degrees C. was well off the mark. This is because Lane's work was done without the aid of Stephan's radiation law which was published later in 1879. Instead, he relied on the earlier work of Dulong and Petit and of Hopkins on the rate of radiant energy from heated surfaces. Other solar models with convective interiors were constructed by Ritter (1878) and by Kelvin (1897). In 1861 Kelvin introduced the concept of 'convective equalibrium' in Geophysics which was later codified in the stellar context by Emden in his monograph 'Gaskugeln' (1907). Following the developments of atomic physics and the first calculations of absorption coefficients, Eddington (1926) improved Lane's stellar models by including a description of the transport of energy by radiation. Cecilia Payne then discovered that the Sun and stars are mainly composed of hydrogen. Later, Eddington showed that the only way he could produce a solar model with the correct luminosity was to assume that the Sun and presumably all the stars were composed of ionised hydrogen.
The power force for the Sun remained a mystery until the discovery of thermonuclear reactions. Kelvin in (1897) recognised the time scale problems associated with the gravitational contraction hypothesis proposed by Helmholtz (1854) --- the Sun would complete its contraction in significantly less time than the then current estimates of the age of the Sun. After the discovery of radioactivity, several authors, notably W.D. Harkins, J Perrin and A.S. Eddington (Chandrasekhar 1939) suggested that subatomic energy might provide enough energy to power the Sun over its lifetime. During the development of nuclear physics, the fusion of hydrogen into helium was discussed by several, in particular Atkinson (1931), Von Weisaker (1937) and Gamow, (1938). Bethe (1939) originally proposed that the Sun derived its power from the CN cycle. Later, with improved interior opacity calculations, it was recognised that the proton-proton chain is resposible for the luminosity of the Sun (Oke 1950).
Beirmann (1932) and Cowling (1935) were the first to use the mixing length theory to describe the transport of energy by convection in the outer layer of the solar model. Beirmann set the mixing length equal to the size of the granules observed on the surface of the Sun. Vitense (1953) (see also Bohm Vitense 1958) refined the mixing length theory by adopting a mixing length proportional to the pressure scale height which decreases as the surface is approached . In addition she accounted for radiative losses as the convective bubbles rise and sink. Demarque and Percy (1964) were the first to use the Bohm-Vitense mixing length theory in the construction of solar models.
In 1955 Hoyle and Schwarzschild used a sequence of static models to explain the evolution of a star. Shortly thereafter Haslegrove and Hoyle (1956) carried out the first evolutionary calculations which were produced on a digital computer. Schwarzschild, Howard and Harm (1957) then calculated the first evolutionary models of the Sun. The sequences followed the structural evolution of the models as the nuclear burning core converted hydrogen to helium. Calibrated solar models, equivalent to what we now call standard solar models (Demarque and Percy) 1964. In these models the mixing length parameter and the helium abundance of the solar model are adjusted to produce a model that has the Sun's observed radius and luminosity. These values were then used in model calculations of other stars (Demarque and Larson) 1964. Since then many authors have created solar models that are more realistic physically.
Today, the very precise observations of the Sun's five minute oscillations place severe demands on the standard solar model and most recently attempts have been made to replace the mixing length theory of convection with a model based on sophisticated numerical simulations of stellar convection, David Guenther and Pierre Demarque 2010.
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Well that left a gap in the conversation!
Science has invested a lot of time and money into coming up with answers to some profound questions and they have tools that we can only dream of so where is there room for incredulity or are we just too stupid to understand. Few of us will confess to being stupid so that just leaves credulity. So we discussed this and though were none the wiser at the end, we concluded thus.
So for thousands and thousands of years all we did was to fall on our knees and worship the Sun because we thought it was a God apparently and in the last 150 years we have gone from Hershel's 'planetary sun' with people and animals living contentedly under a luminous atmosphere to the current 'standard' model here described as a brief history. I don't think it is unreasonable for us 'amateurs' to look at the Sun and wonder about its mysteries and at the same time question what we are really seeing. After all that is only what the 'model' guys did anyway except that they could not bring themselves to say "I don't know". A mystery is always more alluring than a poor answer. Of all the people that have looked at the Sun with the solar telescope when I have explained the 'ball of gas' theory to them nine out of ten they have a hard time reconciling what they see with what you are telling them and you just can't discount that reaction. All we really see is something halfway between truth and phantasmagoria and where you set your foot is largely a matter of opinion though at the end of the day a 'fact' must only be the truth and you can only find that by yourself.
We continued to ponder these and other matters as the evening turned into dusk and the Sun began to set into a soft, damp mist away to the West and we concluded our evening together by looking at Venus with a polarizing filter which gave us a clear, sharp image of the planet, now showing a distinct phase just past the half with a hint of a curve toward a crescent. Venus is now past its western elongation and will head back toward the Sun as the weeks progress, finally showing us a thin crescent which I think is Venus at its most beautiful. The damp chill in the air was very noticeable now and our group dispersed for home. Everyone said that they had had a great time and had learned much which was new to them as well as food for thought on so many aspects of the universe around us . Thank you to 'Professor' Paul Moffett for his contributions to the discussions and to all who took the time to visit on Friday; it was great to see you all and I hope to see you again for more of these sessions throughout the summer.
And lastly, I do hope the man from Tennessee finds his Lock Ness monster and his journey of thousands of miles is not in vain.
Dave