365 Project: Photo 204

I feel a science coming on....

The topic was Something Hot and I couldn't think of anything hotter than the sun, apart from other suns, and I decided I can't mention the sun without using it as an opportunity to talk about how cool it is, so to speak.

The Shell Building from Waterloo Bridge, a view I have used before on the blog

The sun is an unremarkable ball of plasma, neither particularly hot nor cold, not doing anything unusual, not overly large nor especially small, it's just hanging about approximately two thirds of the way along one of the arms of a bog standard, barred, spiral galaxy we call the Milkyway; the sun is also the single most awesome structure we're ever likely to encounter.

The sun, as you may have heard, it quite big. Jupiter is quite big too, as planets go. If you were to take all of the other seven planets (remember, Pluto is no longer a planet) they would all fit inside Jupiter; and if you took all the planets including Jupiter they would all fit, easily, inside the sun. Let's put it another way; if you were to tot up all of the matter in the solar system - the sun, all the planets, all the comets, everything in the asteroid belt and Kuiper belt, all the dwarf planets, everything - then the sun would make up about 99.86% of it, everything else would be less than 0.2%. So, pretty big.

I couldn't not put in this Hubble picture of a bog standard, and breathtakingly beautiful, barred, spiral galaxy (click to galactify)

The sun is also, you may have noticed, rather hot. How hot, though, depends on where you measure it. The sun has at least six distinct layers to it, the inner three of which can be thought of as the core, they are also the hottest; they weigh in at about 15 million degrees Celsius. This core behaves as a solid and rotates within the body of the sun. The layers outside the core; the convection zone, the photosphere and the chromosphere; behave like a viscous liquid something akin to magma on Earth. These layers rotate too but in different way to the core leading to a highly turbulent dividing layer called the tachocline. So, as we move out through the sun the temperature gets steadily lower until, at the surface, the temperature is only 5,500 Celsius. I know, surprisingly cold, right? The next time you hear something described as hotter than the surface of the sun you probably don't need to deploy your impressed face. That's not the end of the story though, something weird happens once you leave the sun. The area outside of the sun is known as the corona, it is this that you can make out during an eclipse when the moon is blocking out the main disc of the sun. The corona, oddly, actually gets much hotter, about 2 million degrees hotter. How it is that there is this vast temperature increase outside of the sun is not fully understood yet but the smart money appears to be on magnetic fields playing a role.

So, before the Government decides that the solar system would be better off privatising the sun because its function could be performed more efficiently and cheaply by private companies, let's establish what it is the sun actually does and why it's so good at it. Put simply, the sun is a fusion reactor, which is sort of the opposite of what happens in a nuclear reactor. It takes 4 protons (hydrogen nuclei) and fuses them together to produce an alpha particle (helium nucleus). This, normally, is an extremely difficult thing to do; protons are positively charged particles and, as every 12 year old knows, oppositely charged particles attract each other and particles with the same charge repel each other. This is one of the principles of electromagnetism which is, itself, one of the four fundamental forces of the universe (the others being gravitation, the strong nuclear force and the weak nuclear force). It is the strong nuclear force that holds the elementary particles within nuclei together and so nuclear fusion is the process by which the strong force manages to overcome electromagnetism. It is only in the insanely hot and dense conditions that exist in the sun's core where protons can be pushed near enough to each other such that the strong nuclear force can win out and, in the process, release a whole bunch of energy, generally in the form of gamma radiation. Each gamma ray is absorbed in just a few millimetres of solar plasma and released in a lower energy state having given off, over the course of its journey, several million photons - the source of all the marvels we are but one small part of here on Earth.

Image used under a CCL. Warning: image contains science

My favourite ever sun fact is one I only encountered a year or two ago but I absolutely love it. I particularly enjoy it when it is contrasted with this preliminary fact: the sun is approximately 8.5 light minutes away. What this means is that it takes a photon of light, once emitted from the sun, only 8.5 minutes to get to the surface of the Earth; the sun may be a very long way away but when you're travelling at just shy of 300 billion metres per second the distance is soon covered. Here is the fact I love: for a newly created photon in the sun's core to reach the surface of the sun takes about 170,000 years. That blows my mind. But why is this the case? From what I can tell it mostly seems to be a function of the sheer density of matter in the core. The heat and pressure is so great that hydrogen atoms are being fused together into helium atoms and the maelstrom is simply to difficult for the photons, massless and speedy as they are, to get through.

Okay, I've just checked and I'm at 1,000 words, which is unfortunate because that's probably somewhat more than you had bargained for when you clicked the link and I haven't even got to the specific topic I wanted to discuss yet. I think I'll quit while I'm ahead, thank all of you that have read this far and leave you with a promise of a follow up sun-based post later in the year where I'll ramble on about....
- How we are trying to solve the world's energy needs by creating the sun on the Earth
- How solar weather can affect us all
- And helioseismology, the study of sunquakes, the whole reason I started writing this post in the first place!!