Every now and then, a strange thought will pop into my head, sitting there asking for an answer. Sometimes it’s trivial, and sometimes it seems silly, but then it leads to some fun insights.
This time, my brain decided to fixate on a simple question: What is 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 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 great things are round, and that’s no accident! Gravity is to blame. As a cosmic object grows, usually by accreting gas or through collisions with other bodies, its mass increases, and so does its gravitational field. At some point gravity becomes so strong that anything stuck too high will fall, a process that eventually causes the object to become spherical. You already know this; A mountain that gets too high will collapse, and you can only pile up so much sand on the beach before it falls. Each time this happens, the astronomical object becomes smoother, more spherical.
This property is created for objects when they grow about 400 kilometers, depending on what they are made of. So almost any discrete body larger than this is usually spherical: large asteroids, moons, planets, and even stars.
So which of these are the most geometrically perfect?
I looked for a while, thinking of all the astronomical objects I could, and finally the answer I got was a surprise: the sun – yes, our nearest star!
Stars are generally quite round, but even the most round deviate from being an ideal sphere. The largest source of this output is rotation because it creates centrifugal force.
Although you may have heard this is real strength within a rotating frame of reference, i.e. if you are on a curved path, it feels like something is pushing you outward. If you’re making a left turn in a car, for example, you feel like you’re being thrown to the right, to the outside of the turn.
For spinning spheres, the centrifugal force is maximized at the equator, where the speed of rotation is greatest. The amount of force depends on the size of the object and the speed at which it rotates; larger ones have more power and faster spins increase power.
The sun is undoubtedly large: its 1.4 million kilometer wide face could fit more than 100 Earths. But at the same time, our star rotates slowly, taking about a month to rotate once. It turns out that this relaxed spin may win the roundness contest here.
The gravity of the surface of the Sun is quite strong, it is 28 times greater than that of the Earth; if you were standing on its surface (and avoiding immediate evaporation), you would weigh 28 times more than on Earth. But at the solar equator the centrifugal force is much weaker; the outward force you would feel from the spin of our star is only 0.0015 percent of the force of gravity pulling down! No wonder the sun is so round.
Measuring exactly how round the sun is, however, is difficult. It is not be A surface similar to the earth; it is a gas, so the material inside it gets denser the further away from the center it is. However, near the “surface”, the density drops so rapidly that the edge of the sun appears sharp from Earth. The size is difficult to measure from the ground, because the Earth’s air is turbulent, because it blurs the view of that edge, to see the sphericity of the sun very well, astronomers turned to NASA’s Solar Dynamics Observatory, a space-based solar astronomical telescope. Taking very careful measurements, they found that flatness (how much the sun is level at the pole compared to the equator) is extremely small, a ratio of just 0.0008 percent! This means that the sun is 99.9992 percent spherical. The results have been published in the journal Science Express.
That’s round. Curiously, they found that this ratio does not change with the sun’s magnetic cycle. Right now we are at the peak of the power of the sun’s magnetismWhich waxes and wanes in an 11-year cycle. But this powerful force does not seem to disturb at all the unbearable roundness of the sun’s being.
I will notice that another body in the solar system is almost round – Venus –and for the same reason. Venus covers approx 243 days once to rotate, so the rotation is slow. This means that the centrifugal force at its equator is very small, and in fact observations indicate that the polar and equatorial width of the planet is equal to the measurement error. This arguably makes it rounder than the sun in principle, but in reality it has surface elevation changes of several kilometers, so to scale, it’s not as round as our star. (Earth’s obliquity is about 0.3 percent because our planet rotates much faster.) This is generally true for planets, so Venus is neither a sphere nor a sphere.
Other stars, however, can be incredibly aspherical. One reason is that some rotate so rapidly that the centrifugal force at their equator is enormous; The bright star Altair it is spinning so fast that its equatorial material is screeching along at nearly a million miles an hour! Therefore, the diameter at the equator is 20 percent wider than the diameter at the poles.
And some may be even rounder than our sun, even though they are so far away from our probes that we can’t accurately detect them. Some, however, we can analyze rather reliably from first principles – for example neutron starswhich, as a class, are the true heavyweight contenders of most spherical Objects. 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, essentially becoming a neutron ball barely two dozen kilometers across. Neutron stars are so dense that they can have surface gravity billions times of the Earth.
However, several forces can cause some neutron stars to spin very quickly; A star called PSR J1748-2446ad 716 makes a huge turn per second! This is a higher rate than the blades of a kitchen mixer. The centrifugal force at its equator, despite its Lilliputian size and Brobdingnag gravity, is almost enough to break the star apart.
Over time, however, a neutron star’s spin slows, and one that formed early in the universe may now be almost static. If true, intense gravity (I’d weigh over a billion tons standing on one!) would be enough to crush the neutron star into a very nearly perfect sphere, perhaps measured in atomic widths between its equator and poles. . Will astronomers ever find such a sphere? Maybe once you get the hang of it.
It’s more than a playful question, though. Understanding the internal structures of many cosmic objects is difficult because we cannot visit them, and the pressures and temperatures can be too great to replicate even in a laboratory. By measuring the exact shapes of things like the Sun and the planets, we learn more about what’s happening on their surface and what makes them tick.
Astronomers like to figure things out like this, even if it means asking seemingly stupid questions. That part is fun, of course, but finding the answer is when we really have a ball.
