How do we know that the sun rotates on its own axis?

13 June 2017

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Question

How do we know that the sun rotates on its own axis as it's a ball of fire?

Answer

Carolin - Yes, it does rotate. It rotates around an upright axis about once every four weeks and the first person to realise this was Galileo. How you know something rotates is you look at a fixed feature on it’s surface and you wait and you time how long it takes for that feature to come back into the field of view.

Normally when you look at the Sun, and this is not a recommended practice I hasten to add, there are not features. But he used his telescope to project images of the Sun and he saw sunspots for the first time and these were just like little black specks...well little - they’re bigger than the size of the Earth, they’re tens of thousands of kilometres across… they just look tiny on the surface of the Sun. These are slightly cooler, they’re slightly darker and they rotate with the Sun. So he tracked how those progressed across the disc of the Sun and worked out the Sun rotated about every four weeks.

However, it isn’t a solid body. So, even though the Sun rotates, it’s not like a planet where every bit on the surface rotates at the same rate and, in fact, the Sun shows something called differential rotation. Indeed, this is why we know it’s made of plasma rather than a solid thing. The Sun rotates unevenly and it changes with the latitude, and at the poles it takes ten days longer than it takes at the equator.

Chris - Does that mean it tangles itself up almost? It sort of screws itself up like winding up an elastic band because there is this differential effect?

Carolin - Yes. And it is really crucial for magnetic fields because a lot of the Sun’s activity is driven by magnetic activity. So not only do you have convection currents underneath the Sun, you’ve also got this differential rotation and you get this kinking, and this knotting of magnetic fields and that stores up magnetic energy. That will suddenly be released and that’s obviously to do with sunspots and how they are created, but also these enormous solar explosions that get generated. It’s from those magnetic fields reconnecting back after exactly that tangling and releasing this huge amount of energy.

Chris - Is that the Coronal Mass Ejection (CME) events. David Willits had a space weather forecasting system he set up in the UK to spot that?

Carolin - We should be very worried about them. This is one reason why we have so many satellites that are trying to understand the Sun. Trying to be able to predict these events because they do have, potentially, catastrophic implications.

Comments

How can the sun exert a gravitational pull on other objects if it is only a gaseous object?

Regardless of its composition, the Sun is massive - meaning it has a lot of mass. As such, according to Einstein's model of the Universe described in general relativity, it deforms spacetime creating a gravitational "well". The result is that the planets hang around in this well around the Sun.

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