What makes stars spherical

But what about stars' characteristic pointy star shape? The science behind that is surprising and has less to do with the stars or the Earth or with space than it does with us. Stars are shaped like stars, says Henry Reich in the Minute Physics video above , because of imperfections in the back of our eyeballs.

Most intriguingly, says Reich, this biological explanation means that every one of us sees stars slightly differently. But how massive must an object be to achieve hydrostatic equilibrium? Meanwhile, an object made of of pure iron would need to be much more massive for its gravity to overcome the inherent strength of the iron.

In the Solar System, the threshold diameter required for an icy object to become spherical is at least kilometres — and for objects made primarily of stronger material, the threshold is even larger. But things get more complicated when you think about the fact that all objects tend to spin or tumble through space.

If an object is spinning, locations at its equator the point halfway between the two poles effectively feel a slightly reduced gravitational pull compared to locations near the pole. Read more: Even planets have their size limits. This is true for our spinning Earth, which has an equatorial diameter of 12,km and a pole-to-pole diameter of 12,km.

The faster an object in space spins, the more dramatic this effect is. As a result, it is much less spherical than Earth. Some stars are even more extreme. All of the planets are round because of gravity. When our Solar System was forming, gravity gathered billions of pieces of gas and dust into clumps which grew larger and larger to become the planets.

The force of the collision of these pieces caused the newly forming planets to become hot and molten. The force of gravity, pulled this molten material inwards towards the planet's center into the shape of a sphere.

Later, when the planets cooled, they stayed spherical. Planets are not perfectly spherical because they also spin. The spinning force acts against gravity and causes many planets to bulge out more around their equators.