Sometimes a strange thought pops up in your head that just sits there begging for an answer. Sometimes it’s trivial, sometimes it sounds silly, but then it leads to some fun insights.
This time, my brain decided to focus on a simple question: “What is the roundest object in the universe?”
In other words, what is the most spherical object ever discovered? What object is the most symmetrical, if not necessarily the smoothest, with all points on its surface the same distance from the center? (That’s the definition of a sphere, after all.)
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Many of the larger ones are round, and that’s no coincidence. It’s because of gravity. As a space object grows, its mass increases, usually through the accumulation of gas or collisions with other objects, and thus its gravitational field. At some point, the force of gravity becomes so strong that anything that sticks out high collapses, a process that eventually causes the object to become spherical. You probably already know about this. If the mountain gets too high, it will collapse, and the only option is to pile the sand higher on the beach before it collapses. Each time this happens, the object becomes smoother and more spherical.
This property appears when an object grows to about 400 kilometers in diameter, depending on what it’s made of. Therefore, almost all discrete objects larger than this tend to be nearly spherical, such as large asteroids, moons, planets, and even stars.
So which orb is the most geometrically perfect?
I researched quite a bit, thinking about every type of astronomical object I could. And the answer I finally got was surprising. The sun is the closest star to us.
Stars are generally fairly round, but even the roundest stars deviate from an ideal sphere. The biggest cause of this deviation is rotation, which creates centrifugal force.
As you may have heard, this is an actual force within a rotating frame of reference. So if you’re on a curved trajectory, it feels like something is pushing you outwards. For example, if you are in a car making a left turn, you will feel like you are being thrown to the right, to the outside of the turn.
For a rotating sphere, the centrifugal force is greatest near the equator, where the rotational speed is highest. The magnitude of the force depends on the size of the object and its rotational speed. The larger it is, the more force it receives, and the faster it rotates, the more force it receives.
There is no doubt that the sun is big. More than 100 Earths could fit on the Sun’s 1.4 million-kilometer-wide surface. But at the same time, our star rotates slowly, taking about a month to complete one revolution. It turns out this calm spin might win the roundness contest here.
The surface gravity of the Sun is extremely strong, about 28 times that of Earth. If you were to stand on the surface of the Sun (and avoid instant evaporation), your weight would be 28 times greater than gravity on Earth. However, the centrifugal force at the solar equator is much weaker. The outward force you feel from the rotation of our star is only 0.0015 percent of the force of gravity pulling you down. No wonder the sun is so round.
However, measuring exactly how round the Sun is has proven difficult. it’s not have The same surface as Earth. Since this is a gas, the further away you are from the center, the less dense the material inside it. However, because the density drops rapidly near the “surface,” the edge of the Sun appears sharp from Earth. Measuring its size from the ground is difficult because Earth’s atmosphere is so turbulent that it obscures the view of its edges. So to get a really good look at the sun’s spherical shape, astronomers turned to NASA’s Solar Dynamics Observatory, a space-based astronomical solar telescope. After very careful measurements, we found that the oblateness (how flat the Sun is at its poles relative to the equator) is incredibly small, at just 0.0008 percent. This means that the Sun is 99.9992% spherical. They published their results in the journal science express.
That’s dangerous. Curiously, they also found that this ratio does not seem to change with the Sun’s magnetic cycle. Right now, we are at the peak of the Sun’s magnetic strength, which waxes and wanes on an 11-year cycle. But this powerful force doesn’t seem to mind the Sun’s unbearably round presence at all.
Note that another solar system object, Venus, has a similar shape and for the same reason. Venus is approx. 243 days It is a very slow spinner as it only rotates once. This means that the centrifugal force at its equator is actually very small, and in fact, observations have shown that the planet’s polar and equator widths are exactly the same, within measurement error. Masu. Therefore, although in principle it could be considered rounder than the Sun, in reality it has a surface height variation of several kilometers, so on scale it is not as round as our star. (Because our planet rotates much faster, Earth’s oblateness is about 0.3 percent.) This is true for planets in general. Therefore, Venus is neither a sphere nor located there.
However, other stars can be surprisingly aspheric. One reason for this is that some rotate so fast that the centrifugal force at the equator is huge. The bright star Altair is spinning so fast that matter at its equator is screaming at about 1 million kilometers per hour. Therefore, the diameter at the equator is 20% wider than the diameter through the poles.
Others may be even rounder than the Sun, although they are too far from the spacecraft to accurately identify them. However, there are some things, such as neutron stars, that can be scrutinized with some degree of certainty from first principles, and these as a class are real candidates for the most spherical objects. Each of these super-dense spheres is the remains of a star more massive than the Sun that went supernova. The star’s core collapses into what is essentially a ball of neutrons just 20 kilometers in diameter. Neutron stars are so dense that their surface gravity is billions of Sometimes earthly.
However, various forces can cause some neutron stars to rotate very quickly. The star, called PSR J1748-2446ad, rotates a whopping 716 times every second! This is faster than a kitchen blender blade. The centrifugal force at the equator, despite its small size and Brobdingnagian gravity, is almost sufficient to tear the star apart.
However, over time, the rotation of neutron stars slows down, and the rotation of neutron stars formed in the early universe may become almost stationary. If true, the intense gravity (standing on a star would weigh more than a billion tons!) would be enough to break a neutron star into a nearly perfect sphere. Perhaps the difference between its equator and pole is measured in atomic widths. . Will astronomers ever discover objects this spherical? Maybe once they get used to it.
However, this is not just a playful question. Understanding the internal structure of many space objects is difficult. Because we can’t visit them, and the pressures and temperatures are too high to even reproduce them in the lab. By measuring the precise shapes of things like the sun and planets, we learn more about what’s going on beneath their surfaces and discover what makes them tick.
Astronomers love figuring out things like that, even if it’s a stupid-sounding question. Sure, that part is fun, but it’s when you really have the ball that you find the answer.
