Every now and again I’ll get a weird thought in my head that sits there demanding an answer. Sometimes it’s trivial, and sometimes it sounds silly but then leads into some fun insights.
This time, my brain decided to fixate on a simple question: What’s the roundest object in the universe?
By that I mean, what is the most spherical object we’ve ever found—not necessarily the smoothest, but one that is the most symmetric, where every point on its surface is the same distance from its center? (That’s the definition of a sphere, after all.)
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Lots of big things are round, and that’s no coincidence! Gravity is to blame. As a cosmic object grows, usually by accumulating gas or via collisions with other bodies, its mass increases—and therefore its gravitational field increases, too. At some point the gravity gets so strong that anything sticking up too high will collapse, a process that eventually drives the object to become spherical. You’re already familiar with this; a mountain that gets too tall will crumble, and you can only pile sand so high at the beach before it topples. Every time this happens, the astronomical object becomes more smooth, more spherical.
This property emerges for objects once they grow to roughly 400 kilometers across, depending on what they’re made of. So almost any discrete body larger than this will tend to be near spherical: big asteroids, moons, planets and even stars.
So which of these are the most geometrically perfect orbs?
I poked around quite a bit, thinking of every kind of astronomical object I could, and in the end the answer I got was a surprise: the sun—yes, our nearest star!
Stars in general are quite round, but even the roundest ones still deviate from being an ideal sphere. The biggest source of this departure is rotation because it creates centrifugal force.
Despite what you might have heard, this is indeed a real force within a rotating reference frame—that is, if you’re on a curving trajectory, it feels like something is pushing you outward. If you’re in a car making a left turn, for example, you feel like you’re being thrown to the right, to the outside of the turn.
For spinning spheres, centrifugal force is maximized at the equator, where the rotational speed is highest. The amount of the force depends on the size of the object and how fast it’s spinning—bigger ones experience more force, and faster spins increase the force as well.
The sun is big, no doubt: more than 100 Earths could fit across its 1.4-million-kilometer-wide face. But at the same time, our star spins slowly, taking roughly a month to rotate once. It turns out this sedate spin is what may win the roundness contest here.
The sun’s surface gravity is quite strong, about 28 times that of Earth’s—if you stood on its surface (and avoided being instantly vaporized), you’d weigh 28 times more than you do on Earth. But the centrifugal force at the solar equator is much weaker; the outward force you’d feel from our star’s spin is only 0.0015 percent the force of gravity pulling you down! No wonder the sun is so round.
Precisely measuring how round the sun is, though, turns out to be hard. It doesn’t have a surface quite like Earth does; it’s a gas, so the material inside it gets less and less dense the farther away it is from the center. Near the “surface,” however, the density drops so rapidly that from Earth, the sun’s edge appears sharp. Measuring the size from the ground is hard because Earth’s air is turbulent, smearing out the view of 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 sun telescope. Taking very careful measurements, they found that the oblateness—how much the sun is flattened at the pole versus the equator—is incredibly small, a ratio of just 0.0008 percent! That means the sun is 99.9992 percent spherical. They published their results in the journal Science Express.
That’s danged round. Weirdly, they also found that this ratio doesn’t seem to change with the sun’s magnetic cycle. Right now we’re at the peak of the strength of the sun’s magnetism, which waxes and wanes on an 11-year cycle. But this powerful force doesn’t seem to bother the sun’s unbearable roundness of being at all.
I’ll note that another solar system body is nearly this round—Venus—and for the same reason. Venus takes about 243 days to rotate once, so it’s a very slow spinner. That means the centrifugal force at its equator is very small indeed, and in fact, observations indicate the polar and equatorial widths of the planet are exactly the same to within measurement error. This makes it arguably rounder than the sun in principle, but in reality it has surface elevation variations of several kilometers, so to scale, it’s not as round as our star. (Earth’s oblateness is about 0.3 percent because our planet rotates much faster.) That’s true for planets in general, so Venus is neither sphere nor there.
Other stars, though, can be shockingly aspherical. One reason is that some rotate so rapidly that the centrifugal force at their equator is enormous; the bright star Altair is spinning so rapidly that material at its equator is screaming along at nearly a million kilometers per hour! Because of this, the equatorial diameter is 20 percent wider than the diameter through the poles.
And some may be even rounder than our sun, albeit so far removed from our probing instruments that we can’t precisely discern so. Some, however, we can somewhat reliably scrutinize from first principles—such as neutron stars, which, as a class, are true heavyweight contenders for Most Spherical Object. Each of these überdense orbs is the remnant of a star more massive than the sun that underwent a supernova; the core of the star collapses to essentially become a ball of neutrons a mere two dozen kilometers across. Neutron stars are so dense that their surface gravity can be billions of times Earth’s.
Various forces can cause some neutron stars to spin extremely rapidly, however; one star called PSR J1748-2446ad spins a whopping 716 times per second! That’s a higher rate than the blades on a kitchen blender. The centrifugal force at its equator, despite its Lilliputian size and Brobdingnagian gravity, is almost enough to rip the star apart.
Over time, though, a neutron star’s spin slows, and one that formed early in the universe could now be nearly static. If true, the intense gravity (I’d weigh upward of a billion tons standing on one!) would be enough to crush the neutron star to a very nearly perfect sphere, perhaps with the difference between its equator and poles measured in widths of atoms. Will astronomers ever find one this spherical? Maybe, once they get around to it.
This is more than just a playful question, though. It’s difficult to understand the internal structures of many cosmic objects because we can’t visit them, and the pressures and temperatures can be far too great even to replicate in a lab. By measuring the exact shapes of things like the sun and the planets, we learn more about what happens beneath their surface and discover what makes them tick.
Astronomers love to figure things like that out, even when it means asking what sound like silly questions. That part is fun, sure, but finding the answer is when we really have a ball.
