 If you look at the Sun, which you should only do with protection for your eyes or a special telescope, you will see a nearly perfect sphere roughly one and a half million kilometers or about a million miles across. It's round because this is the simple shape into which gravity squashes massive objects. The Sun would continue to contract were it not for nuclear reactions in its center which released just enough energy to fight against gravity. Many changes to either side of this very precise balancing act would in turn change the size of the Sun's sphere. Indeed, over billions of years the Sun has been and will continue to lose some of its substance, some of its mass, making the force of gravity slightly weaker and growing its size. Looking at the surface of the Sun and above, it is the force of magnetism which shapes our star. The invention of the telescope has allowed us to see darker so-called sunspots. These are regions on the Sun with a very strong magnetic field. The force of magnetism acts upon the stuff the Sun is made of, the ionized plasma of protons and electrons, and pins it in place. Under the tyranny of this force, hot material cannot rise to the surface as it does on the rest of the Sun. The whole sunspot region cools down and becomes darker than everything around it. It is a little bit like if you've ever seen a magnet make a dark spot on an old school CRT TV. Every eleven years, the face of the Sun undergoes a turbulent period and breaks out in spots over much of its surface. The same process happened to me when I too became a teenager. The Sun has overall north and south magnetic poles just like the Earth, and these two flip round during this time. Any material that breaks free, away from the surface near the sunspots, is deflected by the magnetic field and forced back down in large loops. The magnetic field extends far above the sunspots for millions of kilometers into a region known as the Sun's corona. In physics, a field is a region of space associated with a quantity, not necessarily of potatoes, and thus the magnetism around the Sun forms a very real force field. The actual magnetic field in the corona is sometimes difficult to see, but if it were instead like a force field you might see in films or video games, then this is how it might look. Any of the Sun's plasma that is able to escape the pull of gravity is kept confined inside this magnetic cage. Not very much of it does, so this region is much more faint than the rest of the Sun. To see the corona, the main disk of the Sun must be blocked out, such as during an eclipse or artificially on board a spacecraft. When you look at it this way, the Sun is much larger and more stereotypically star shaped than the ball we normally think of in the sky. Looking again at our visualization, there are holes where the force field doesn't cover some areas of the Sun. Indeed, there are phenomena known as coronal holes, which appear as large, dark patches in extreme ultraviolet images of the Sun. It's not that there is no magnetic field in these regions, in fact it can be quite strong, it's just that here it doesn't force the ionized plasma to loop back downwards. Instead, the plasma is free to stream outwards like an ordinary gas, so this flow is called the solar wind. As this wind escapes from parts of the corona, they cool down and appear as dark coronal holes. Unlike a force field you may have seen in science fiction, the magnetic field is not a perfect barrier, and some plasma does manage to escape and flow outwards, but it's thought to move slower than the solar wind from coronal holes. As it moves out past the Earth and the planets, the plasma carries the influence of the Sun with it. Particles from the Sun smack into planets like Earth and Jupiter, causing auroras at the poles. When activity at the Sun increases, it causes much brighter auroras. Geomagnetic storms soon follow in the faraway planets. The progress of the solar wind outwards under the dominance of its origin star continues for billions of kilometers. Eventually, at a boundary called the heliopause, the flowing particles thin out and our solar system gives way to the wider Milky Way galaxy. In the last decade, the two Voyager spacecraft were the only artificial objects to ever cross this threshold and send back any data. Just how big then is the Sun? In a sense, it is a giant, glowing, almost perfectly spherical ball. Alternatively, the ionized plasma inside a bumpy magnetic force field of the corona is also a part of it. In another sense, the Sun comprises a truly enormous bubble of charged particles, which envelops eight planets and assorted other rocks. Thank you for watching.