The structure of the Sun can be divided into three main parts: the interior, which consists of the core, radiative and convective regions; the photosphere, which is basically the surface of the Sun where light comes from; and the atmosphere, of which includes the corona. Since nuclear fusion occurs in the core of the Sun, it’s reasonable to assume that the core is the hottest part of the Sun. In fact we know the core is about 15 million K, and that as you move outward from the center the temperature drops, until you reach a temperature of about 5,800 K at the surface. One would expect that this cooling trend continues, so that the solar atmosphere is cooler than the surface. But what we observe is that the atmosphere, specifically the corona, gets hotter. Much hotter.
Blemish
Sunspots are dark regions that appear on the surface of the Sun. Despite their appearance, sunspots aren’t actually dark. They are cooler than the surrounding surface, which means they are less bright. When an image of the Sun is made dim enough to view directly, either by viewing the Sun through a filter or by projecting the Sun’s image onto a surface, the cooler, dimmer sunspots appear dark.
That’s So Random
Imagine you were standing in the center of a circle 100 meters in radius. How long do you think it would take to leave the circle? Usain Bolt could do it in 10 seconds, but most people could leave the circle in under a minute. After all, it’s just a casual 100 meter stroll and you are out of the circle. But suppose we added a rule that you couldn’t simply walk in a straight line. Suppose after each step you were required to change to a new random direction. Take a step, random direction. Another step, another random direction. You can’t control your direction, all you can do is take a step. Random turn, step, random turn, step, etc. Now how long would it take you to leave the circle? A minute? Ten?
Fire, Burn and Caldron Bubble
From Earth the Sun looks calm and never changing. It is a constant that rises and sets on schedule. Of course we now know that the Sun is quite active, with sunspots, prominences and solar flares. There are also more subtle changes on the Sun’s surface. One of these is an effect known as granules, seen in the image above.
X Factor
A solar flare is an intense burst of energy released at the surface of the Sun. It is caused by a process known as magnetic reconnection. The rotation of the Sun occurs at different rates depending on latitude. It has a rotational period of about 25 days at its equator, but about 34 days near the poles. This means the equatorial regions of the Sun rotate faster than the polar regions. Because of this differential rotation, the magnetic field of the Sun is twisted, so that over time the field lines of the magnetic field gradually wrap around the Sun.
Measuring the Sun
Last month there was an annular eclipse, but unless you happen to live in Antarctica, you probably didn’t get a chance to see it. You can, however make your own solar observation to measure the size of the Sun. This experiment uses the principle of parallax, and all you need is a sunny window, some cardboard, a pencil, and a tape measure.
Siblings
Stars don’t form alone. They form as part of a stellar nursery. We know this because we can see new stars forming in this way, such as in the Orion nebula. This means, of course, that our own Sun also formed in a stellar nursery with lots of other stars. There are stars out in the galaxy that are siblings to our Sun, and we think we know where a few of them are.
No Irish
It’s St. Patrick’s day, and that means many will celebrate their Irish heritage and wear the green. But stars never wear the green. You will never see a green star.
Song of the Sun
There’s a song of the Sun. It is produced by acoustic waves in the Sun’s interior, and the study of these waves is known as helioseismology.