Caoimhín's Content

Today we are going to take a look at the Sun. In some ways, wintertime may be one of the worst times to observe the Sun, because it is so low in the sky, it’s not up for very long, and it’s more likely to be blocked by trees or buildings. In another way, wintertime may be a good time to observe the Sun.

As I’ve said before, and I will no doubt say again, never ever look directly at the Sun, never observe the Sun directly. There needs to be some kind of protection involved. Pointing a normal telescope at the Sun is risky, not just for your eyes, but for the telescope due to the amount of heat that can be generated by focusing the Sun’s light. You need to use some sort of special equipment, whether that’s eclipse glasses or a special solar telescope that’s designed for looking at the Sun. Pinhole cameras that project the Sun onto a surface are the best indirect way to look at the Sun. We’re taking a look at the Sun in wintertime, and because it’s lower in the sky, it’s shining through more atmosphere. That’s going to cut out even more of the heat and the dangerous radiation coming from the Sun, it’s going to attenuate the light. On its own, this is not enough, you still need more help cutting down on the light from the Sun and making it safe to observe.

Observing the Sun at the moment, one of the first things you might notice are sunspots. Sunspots are pretty unpredictable, and if the Sun isn’t very active there may be none, but the Sun is pretty active at the moment. If we move through time, different sunspots will come into view due to the rotation of the Sun. What we’re seeing is the photosphere, or surface, it’s a little bit tricky with the Sun. Given that we’re able to see different parts of the Sun’s sphere indicates that the Sun’s rotation isn’t the same speed as ours. For every 24 hours that we turn around, the Sun has turned around a different amount, quite a bit less. As we move forward, every day each bit of the surface seems to be drifting westward. The information provided by Stellarium for planets does give a rotational period, but not for the Sun. Different parts of the Sun rotate at different speeds, the speed of rotation around the equator is faster than the poles, and this is because the Sun is a giant ball of plasma.

By getting rid of the atmosphere, we get a much better view of the Sun. This lets us see the Sun’s corona, usually only visible during a total solar eclipse. We can see different features visible around the edges of the corona, such as loops, but they’re fixed. For the corona, and the sunspots, shown in Stellarium, they aren’t reflective of current appearance of the Sun. The Sun is this giant ball of magnetized plasma, so all of its surface features shift and change around. Sunspots pop up and die away, and the lines visible in the corona shift over time as well. The brighter spots of the corona near the surface of the Sun, which may be solar flares or coronal mass ejections, are also going to shift and change over time. These features extend into space, but are tied to the Sun. Although we see the corona only around the sides of the Sun’s disc, the corona wraps around the entire sphere of the Sun. Some of it is between us and the bright surface of the Sun. Features of the corona move with the Sun’s rotation, disappearing on one limb and reappearing on another as the Sun turns around.

If we zoom in on the surface of the Sun, it has a kind of a dotted appearance, you might say it’s granular. The Sun is covered in what appear to be cells, bordered by a slightly darker boundary. This is called granular, even though, as I say, the Sun is a giant ball of plasma, and plasma behaves a little bit like a gas or a liquid, it’s fluid in how it moves around. As plasma is ionized, and in the case of the plasma in stars and the Sun in particular, it’s strongly affected by the magnetic field that’s going on around it. Plasma is ionized because the electrons are free, and these electrons are guided and moved around by the magnetic field. Deep inside the Sun, everything’s under so much pressure that the plasma is not as mobile, it’s a bit more compacted together. Light and heat radiate through it from the Sun’s core, where a lot of the fusion is actually happening. On the outside, the plasma is under a bit less pressure, it acts a bit more fluid. Thanks to the heat, it convects, just like the convection currents you’d find in boiling water and in the Earth’s core and mantle. These convection currents are when a hot central plume rises up and cools, and the cooler edge of the plume falls away.

These convection cells are what give the Sun this granular appearance, each of the bounded colored regions would be individual convection cells that are, like boiling water, moving around all the time. As they convect they twisting and slide past each other, dying away as new ones pop up. The surface of the Sun is a very dynamic system. The Sun’s magnetic field influences these convection areas. Magnetic fields tend to form lines, paths of magnetism that flow out of the poles of the Sun, around it and back in to the other pole, as magnetic fields usually do. Locally, the magnetic field varies in strength. The magnetic flux tubes, the columns of magnetism that rise up through the Sun, sometimes they may have stronger intensity or lesser intensity, and this can affect the local temperature of the Sun. That gives us the sunspots that we see. They start as solar pores, smaller areas where the magnetic flux has changed and causes a lower temperature.

Sunspots are thousands of degrees, a bit over 3,000 degrees Kelvin, whereas usually the surface of the Sun is almost 6,000 degrees Kelvin. Around 5 to 6,000 degrees Kelvin is the temperature of stars like our Sun, and the cooler sunspots drop down to 3 to 4,000 degrees Kelvin. This is still really hot, it’s glowing red hot, as hot as the surface of the many red stars. You can head back to the recent post about stellar classification if you’d like to know more about the colors and the temperatures of stars and how we talk about them. Our Sun is a G type star, and that’s partly to do with its color, the kind of yellow color, and its temperature of almost 6,000 K. The Sun is so bright that it makes the sunspots look dark, even though they would be glowing red, as bright as the Moon for large ones, if they weren’t surrounded by this much, much brighter Sun.

Sunspots do appear to be quite dark, and the dark central region is called the umbra just like the center of a shadow. The precursor to sunspots, solar pores, are really just umbra. These pores tend to grow or collect together, and as they grow they get surrounded by boundary region known as the penumbra. The bigger the sunspots, the more penumbra they tend to have and we often have sunspots where multiple spots are joined by one penumbra if the spots occur close together. In the umbra there are vertical magnetic field lines, whereas the penumbra has magnetic field lines that are a little bit tilted or skewed to the side. This disturbance in the magnetic field gives birth to a lot of solar weather. When magnetic field lines that are pointing in slightly different directions, but near each other, they may interfere and connect together. In the penumbral regions where the magnetic field line is twisting, there can be magnetic reconnection events. This is where two magnetic field lines that would be moving in different directions join up. Two loops of a magnetic field, one moving one way and one moving the other, may cut across each other and form two new loops perpendicular to the old ones. The old loops essentially split and join to each other to form two new lines. These changes at magnetic reconnection events generate, or let out, a lot of the energy that we see as solar flares and possibly coronal mass ejections.

Coronal mass ejections aren’t the best understood thing, but they are thought to be connected to magnetic reconnection. We can see the magnetic field in the corona of the Sun, which is high energy particles following the magnetic field, that’s part of what gives it its kind of a stripy shape. When magnetic field lines reconnect, energy that’s been stored up in these field lines gets released. Loops that have built up energy,can release this energy when the field line goes through reconnection and that release of energy is really gives us solar flares and possibly coronal mass ejections. The reconnection events take energy that the Sun’s been storing up for a while, minutes or hours, and it releases it almost in an instant or in seconds and this change in energy ejects material away from the Sun. The loops that we see in the corona is coronal material traveling along those field lines. The corona we see seems to rising out of the north and south poles and connect along the sides of the Sun, but some of it is curving down around in front of the Sun, stretching out just as much as the portion on the sides. We don’t see that part of the corona thanks to the light of the Sun coming from behind.

The more sunspots we see see, the more solar flares and coronal mass ejections that we’re going to see, and it’s when these sunspots are almost turned around facing us that the coronal mass ejection is going to come in our direction. If we see solar flares and ejections around the sides of the disk of the Sun, they’re pointing away from us. Even as the Sun turns and even as they come through space, they’re not very likely to hit us. It’s only when they’re almost facing us that they might strike us. As they travel through space so quickly, they hit us pretty soon after they erupt, making them hard to notice in advance. By looking at the sunspots and the magnetic field activity around these sunspots, that gives us a good idea of when flares are going to be happening. This ties in to all of the aurorae that people have been seeing lately. The Sun is quite active at the moment, it’s got a lot of sunspots, it’s got a lot of these magnetic reconnection events, and that’s creating the solar flares that are then coming through space, hitting our magnetic field and creating the aurorae that we see. Even though it’s very hard to predict solar flares, even though it’s very hard to predict sunspots, we are able to observe them. Thankfully, we know how they’re related, and we can use that to give us a better clue as to what’s going to happen in the near future.

I hope that you enjoyed this piece, I hope you enjoyed this look at the Sun and discussion of why it looks the way it looks and the different features of it. If you did enjoy this piece then please do like it. If you like this kind of content, then please subscribe to this website and my YouTube channel. Thank you very much for reading and hopefully I’ll see you back here next time.