It is true that most stars are spherical, but there are also some that are “squashed”, and some that even look like a peanut or a funnel. How does this happen?
Stars, like our sun for example, are made of a hot gas called plasma. The gas is drawn into the center of the star due to gravity, and is supported by the internal pressure inside the star. So most stars are in a state of equilibrium, where their gas is not residing in the center of the star, and is not distributed into space. Gravity acts equally in all directions, and the gas is dispersed symmetrically around the center forming a shape of a perfect sphere… or at least almost perfect.
Most stars are rotating around their axes at different speeds. The surface of our sun, for example, completes a rotation in 25 days. The spinning gas tends to affect the rotation by being thrown away from the star, enough to deform its shape slightly. In fact, even planets that seem rigid and solid, tend to be flattened, i.e. shorter along the axis of rotation and wider at the equator.
Star shape is also affected by the gas flow within them. The core of the star is much hotter than the surface, and inside the star is a “convection zone”; gas close to the core is hot and rises up, while cold gas found near the surface sinks inside. This cycle is similar to boiling water where the water in contact with the bottom of the pot, rises to the surface.
The gas in an electrically charged planet and the strong currents of charged gas produce strong magnetic fields that affect the final shape of the star, as shown in a new study from the University of Hawaii.
However, the shape of a star is almost a perfect sphere. It is impossible to distinguish with the naked eye that they are flattened. Determining the difference between the length of the short and long axes can only be done with an accurate measuring instrument that is sensitive to a thousandth percent.
There are also some rare stars that interact with neighboring stars, resulting in a complete change of shape. Examples can be found in binary systems, which consist of double stars revolving around each other, sometimes at very short distances. When binary stars are close enough to each other, the gravity of one distorts the gas of the second creating an oval shape with the long axis directed toward the companion star.
Such a phenomenon also occurs on Earth, when the moon, and to some extent the sun, raise the sea level that’s closest to them. On Earth this is seen as a change of sea level by a few meters – we call these tides. In binary stars that are very close to one another, the influence may be even more significant. In fact, if binary stars are close enough they can even touch each other, so their surrounding gas clouds mix around the two cores. This shape reminds us of a rotating peanut.
In other binary systems, one of the neighbors can be a normal star, while the other is very small, but with a very strong gravity pull, such as a black hole or neutron star, that sucks gas from the first. In such a situation, the system takes the form of a funnel of gas, falling from the cloud that was once a sphere into its heavier neighbor.
As in most processes in astrophysics, the shape of a star is always determined by gravity, which tends to push everything towards its center. If gravity is symmetrical, like for a single star, it will form an almost perfect sphere, and is only slightly affected by the rotation of the planet and its magnetic field. If gravity is affected by a nearby star, the shape will be determined by the joint attraction of the two bodies.