The sun? It's round. It's really, really round. Seriously round. Totally round. Inordinately, acutely, exorbitantly, violently round. It's so round, that it's the roundest thing that we've ever seen in nature, and that's round, man. Round.
Let me explain to you just how round the sun is. It's 1,400,000 kilometers in diameter, but the difference in width from measuring around the equator to measuring around the poles is only 10 kilometers. This means that if the sun were the size of a beach ball, it would be a perfect sphere within just 17 microns, which is less than the diameter of a human hair. It would also be a very, very dangerous beach ball.
Just to reiterate, this makes the sun rounder than any object known to exist short of a few very deliberately (and artificially) round things like the International Reference Kilogram and fused quartz Gravity Probe gyroscopes. It's a surprising fact — very surprising — because big round spinning things tend to go a bit blobby. Take Jupiter, for example, which is 7% wider at its equator than it is across its poles. Earth, too, is noticeably squished out at the middle. Until recently, we assumed that the sun was, to some extent, the same, but NASA's space-based Solar Dynamics Observatory has been able to lock down some long-term measurements of the sun's roundness, and it's both barely measurable and remarkably consistent over time.
The roundness of the sun is a hot topic right now because it's been thought that the sun's shape may effect the amount and variability of energy it puts out, which can mess with the climate here on Earth. Since the sun's shape apparently doesn't change even when it's more or less active, it looks like we can rule that out and start looking for other reasons why we get things like little ice ages every once in a while. And at the rate we're going, we could use a few more of those.