Every now and then I get a strange thought in my head that demands an answer. Sometimes it’s trivial, and sometimes it seems silly, but then it leads to fun ideas.

This time, my brain decided to focus on a simple question: what is the roundest object in the universe?

By that I mean, what is the most spherical object we have ever found – not necessarily the smoothest, but the most symmetrical, where every point on its surface is the same distance from its center? (That is the definition of a sphere, after all.)

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Many big things are round, and that’s no coincidence! Gravity is to blame. As a cosmic object grows, usually by accumulation of gas or collision with other bodies, its mass increases and therefore its gravitational field also increases. At some point, gravity becomes so strong that anything sticking out too high will collapse, a process that eventually leads to the object becoming spherical. You already know this; a mountain that’s too high will collapse and you can only pile sand so high on the beach before it topples over. Each time this happens, the astronomical object becomes smoother, more spherical.

This property appears for objects once they reach about 400 kilometers in diameter, depending on their composition. So almost all discrete bodies larger than this will tend to be nearly spherical: large asteroids, moons, planets, and even stars.

So which of these orbs are the most geometrically perfect?

I searched quite a bit, thinking of all possible types of astronomical objects, and in the end, the answer I got was a surprise: the sun, yes, our closest star!

Stars in general are quite round, but even the roundest ones do not constitute an ideal sphere. The biggest source of this departure is rotation because it creates centrifugal force.

Despite what you may have heard, it is indeed a real strength in a rotating frame of reference, that is, if you are on a curved path, you feel like something is pushing you outward. If you’re in a car turning left, for example, you feel like you’re being thrown to the right, toward the outside of the turn.

For rotating spheres, centrifugal force is maximized at the equator, where the rotational speed is highest. The magnitude of the force depends on the size of the object and the speed at which it rotates: larger ones experience more force, and faster rotations also increase the force.

The Sun is big, there is no doubt about it: more than 100 Earths could fit on its 1.4 million kilometer wide face. But at the same time, our star rotates slowly, taking about a month to complete a single rotation. Turns out this calm version is what might win the plumpness contest here.

Gravity on the surface of the Sun is quite strong, about 28 times that of Earth. If you stood on its surface (and avoided being instantly vaporized), you would weigh 28 times more than on Earth. But the centrifugal force at the solar equator is much weaker; the external force you would feel from our star’s rotation is only 0.0015% of the force of gravity pulling you down! No wonder the sun is so round.

However, accurately measuring the roundness of the sun proves difficult. This is not the case to have a surface quite like the Earth; it’s a gas, so the material inside becomes less and less dense the further it gets from the center. However, near the “surface”, the density decreases so quickly that, when viewed from Earth, the edge of the sun appears sharp. Measuring the size from the ground is difficult because Earth’s air is turbulent, obscuring the view from that edge, so to get a really good look at the sun’s sphericity, astronomers turned to NASA’s Solar Dynamics Observatory, a space-based astronomical solar telescope. By taking very careful measurements, they discovered that the kurtosis (how flattened the sun is at the pole relative to the equator) is incredibly small, a ratio of only 0.0008%! This means that the Sun is 99.9992 percent spherical. They published their results in the journal Science Express.

That’s fucked up. Oddly, they also found that this ratio does not seem to change with the sun’s magnetic cycle. We are currently at the peak of the strength of solar magnetism.which increases and decreases in an 11-year cycle. But this powerful force does not seem to interfere at all with the unbearable roundness of the sun.

I will note that another body in the solar system is almost this round: Venus…and for the same reason. Venus takes approximately 243 days spin once, so it’s a very slow spinner. This means that the centrifugal force at its equator is very weak indeed, and, in fact, observations indicate that the planet’s polar and equatorial widths are exactly the same, within one measurement error. This arguably makes it rounder than the sun in principle, but in reality its surface has elevation variations of several kilometers, so on scale it is not as round as our star. (Earth’s flattening is about 0.3% because our planet spins much faster.) This is true for planets in general, so Venus is neither a sphere nor there.

However, other stars can be surprisingly aspherical. One reason is that some rotate so quickly that the centrifugal force at their equator is enormous; the bright star Altaïr rotates so quickly that matter at its equator moves at nearly a million kilometers per hour! For this reason, the equatorial diameter is 20% larger than the diameter passing through the poles.

And some may be even rounder than our sun, although so far from our survey instruments that we cannot discern them precisely. Some, however, can be examined quite reliably from first principles, such as neutron starswhich, as a class, are real heavyweight contenders for the title of most spherical object. Each of these überdense orbs is the remnant of a star more massive than the sun that underwent a supernova; the star’s core collapses to essentially become a ball of neutrons barely two dozen kilometers across. Neutron stars are so dense that their surface gravity can be billion times that of the Earth.

However, various forces can cause some neutron stars to spin extremely quickly; a star called PSR J1748-2446ad spins 716 times per second! This is a higher rate than the blades of a kitchen blender. The centrifugal force at its equator, despite its Lilliputian size and Brobdingnagian gravity, is almost enough to tear the star apart.

However, over time, a neutron star’s rotation slows, and the one that formed in the early universe could now be almost static. If true, the intense gravity (I’d weigh over a billion tons on one!) would be enough to crush the neutron star into a near-perfect sphere, perhaps with the difference between its equator and its poles measured in widths of atoms. . Will astronomers ever find one this round? Maybe, once they get started.

But it’s more than just a fun question. It is difficult to understand the internal structures of many cosmic objects because we cannot visit them, and the pressures and temperatures can be far too high even to reproduce in the laboratory. By measuring the exact shapes of objects like the sun and planets, we learn more about what’s going on beneath their surface and discover what drives them.

Astronomers love to understand this stuff, even if it means asking questions that seem silly. This part is fun, sure, but finding the answer is when the real fun is.