The nature of my degree program meant that I had very few elective courses in my curriculum, but one that I did have the opportunity to take was ‘space weather,’ although the class could just as accurately have been called ‘heliophysics,’ because that’s really what it was, and it was among the most interesting of the classes I took in that whole program. It might be obvious (unless you’re living in a cave or something), but the sun is pretty important, and its impact on us really cannot be overstated. You could try, but since the sun provides almost all energy to the solar system, and was arguably the creative force behind everything within it, you would be hard-pressed. Even distant Pluto and the Kuiper Belt are dominated by solar influence.
Understanding the Sun is the work of lifetimes, and we remain uncertain about many of its mysteries. The Parker solar probe, which perhaps we could call the opposite of New Horizons, will attempt to explore some of those mysteries, most significantly the origin of the solar wind, and the corona heating conundrum. In fact, it was a NASA article about the solar wind that reminded me that a post covering some elementary heliophysics might be a valuable addition to the site. We’ll get to that article and its jetlets, but first, we need to build ourselves a star.
Perhaps the most fundamental thing which you must understand about stars before anything else about them will make sense is that they are massive, naturally occurring nuclear fusion reactors. Irregularities in clouds of matter in the stellar medium cause gravitational instabilities, which cause that matter to begin to clump. When a critical mass of matter is reached, the resultant heat and pressure at the orb’s core is sufficient to start fusion, and thus a star is born. A star is not really a stable entity; instead, they exist in a state of dynamic equilibrium, in which the inward, crushing force of gravity is continuously balanced by the outward pressure resulting from successful nuclear fusion. When that balance can no longer be sustained, the star dies, whether that means collapsing under its own mass into a black hole or other post-stellar object, or exploding, or both.
Turning to the specific narrative of our Sun, which is a mid-range star, at some point about 4.568 billion years ago sufficient mass accumulated in this fashion that fusion occurred and the Sun was born. The material not included in the new stellar mass, or caught up in the new Sun’s gravitational field, was affected by the Sun’s gravity, and eventually coalesced into the planets and other solar system objects we know today (with the possible exception of Neptune, although that hypothesis is no longer considered likely). That’s the highly abbreviated version, and we’re not even going to begin to talk about how stars can begin to fuse heavier elements as their fuel supply diminishes, how they eventually die, and all of the different and dramatic options of stellar death. That will have to be another post.
Our Sun consists of layers. At the center is the core. It is the hottest part of the sun, exists in a peculiar state of matter that does not conform to the elementary level divisions of solid, liquid, gas, or plasma, and is where fusion actually occurs, hydrogen nuclei (protons) being forced so close together that they form helium (if you haven’t already, read our post on nuclear fusion for a more detailed explanation of the process). This region is so incredibly dense that photons, which are massless and travel at the speed of light in a vacuum, take upwards of six hundred thousand years to pass through the core to the surface of the Sun. For reference, it takes that same photon, once it reaches the surface, a mere seven minutes to reach Earth.
After the core comes the radiative layer, which is also highly dense. There are some fascinating dynamics that occur within the radiative layer, but at this overview level we can think of it as a semi-solid layer a little cooler than the core, because what is more relevant to us is how it interacts with the next layer, the convective layer. With the convective layer, things start to be slightly more familiar. It is a super-heated plasma, but it behaves somewhat like the Earth’s mantle, including convection currents that form when cooler parts of the convective layer fall through it towards the radiative layer, where they are heated again and therefore rise back up towards the surface. The interface between the relatively static radiative layer and the more dynamic convective layer is called the tachocline, and it is thought that many of the most interesting stellar surface dynamics actually arise from effects at the tachocline.
Next is the photosphere, which is the closest that we have to something we could call ‘the surface of the sun.’ It is here that phenomena like sun spots and granules occur, and from whence solar prominences and coronal mass ejections appear to originate. Note that I say ‘appear’, since these are really manifestations of dynamics that happen much deeper within the Sun. Magnetic reconnection, for instance, is really the result of the dynamo effect generated by the interactions at the tachocline, and sun spots are cooler regions resulting from the convective cycles occurring in the convective layer.
From the photosphere, we move into the solar atmosphere, which consists of the chromosphere, and the corona. As you might infer from the name, the chromosphere is response for the colors which we perceive when we (carefully) look at the sun. After it comes the corona, which like the Earth’s exosphere, some people will argue extends all the way to the edge of the solar system (and, in fact, people say that this is the definition of the edge of the solar system). However, the corona is typically conceived as a region of heated gas that eventually fades away and is characterized by its peculiar temperature behavior. The corona is hotter than the solar surface, and while theories abound, no consensus yet exists on why that would be.
After the corona (or perhaps part of it, depending on who you ask) comes the solar wind, which is the driving force of space weather and which prompted this entire post. It is the sleet of solar particles whizzing constantly and pervasively through the solar system. Earth is protected by its magnetic field, and it is the solar wind that creates auroras when charged particles from it are caught in the Earth’s magnetic field’s polar cusps. The edge of the solar system is sometimes defined as where the outward pressure of the solar wind is at equilibrium with the inward pressure of the interstellar medium.
Like I said at the beginning, I took an entire class on heliophysics, and people dedicate entire lifetimes to studying the subject, so this is just a very brief overview. However, it should at least give you an introduction and vocabulary, so that we can have more star-powered discussions in the future.