The Sun is a fascinating object. A fusion generator almost 1.4 million kilometres across, it's so big that you could pack 1.3 million Earths inside it, and it's so massive that even with giant planets like Jupiter and Saturn on our side, it still tips the scale with 99.98% of the mass of our entire solar system.
Sunspots regularly dot its surface, and it frequently throws off large flares, some of which disrupt our satellites and communications. It has a magnetic field so powerful that it extends far beyond the orbit of Pluto, where even at that distance it diverts potentially harmful cosmic particles around our solar system, similar to how the Earth's magnetic field protects us from the Sun's solar wind.
So, with all of that, the last thing I thought I'd hear that was baffling scientists about the Sun was its shape.
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However, a team of four astronomers from the U.S. and Brazil has used the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory to take incredibly accurate measurements of the shape of the Sun, and they found that it is almost the roundest object ever measured.
Normally, with a rotating object that has fluid properties, like the molten mantle under the Earth's surface or the liquid hydrogen inside a gas-giant planet like Jupiter, there will be a bulging at the equator. The Earth bulges so much that it's 42 kilometers wider at the equator than it is from pole to pole. This is called an object's "flattening", for which Earth has a value of 0.0033528.
By comparison, the Sun's flattening value is 9×10−6 or 0.000009. That means that if you shrunk the Sun to the size of a beach ball, the difference between the diameter at its equator and the diameter between its poles would be less than the width of a human hair.
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With the Sun rotating every 28 days, and with its gaseous surface, we should see more flattening than that. This indicates that something else is going on under the surface of the sun as a counter to what would be happening from rotation. Also, since this flattening value is constant with time, this counter would also need to be constant. That perhaps points towards subsurface turbulence, or possibly the Sun's magnetic field as the reason.
Out of curiosity, I looked up the roundest object ever measured. It could be the electron. Could there be a relation?