The shape of a black hole depends on whether the object is spinning. A black hole is a region of spacetime where gravity is so intense that nothing, not even light, can escape the boundary known as the event horizon. While often depicted as perfect spheres, this geometry is only accurate for a theoretical, non-rotating black hole. Most black holes in the universe are in motion, and their spin causes a slight but important distortion in their shape.
The Ideal Shape: Non-Rotating Black Holes
The simplest model for a black hole is one that possesses no angular momentum, meaning it does not spin. This theoretical object is described by the Schwarzschild solution to Einstein’s field equations, where the black hole’s geometry is defined solely by its mass. In this idealized scenario, the event horizon forms a boundary that is perfectly spherical.
Spherical symmetry arises because the gravitational field is uniform in all directions around the central mass. The size of this sphere is precisely set by the Schwarzschild radius, the distance from the center where the escape velocity equals the speed of light. If a non-rotating black hole were to form, its event horizon would be a mathematically precise sphere. This model serves as the foundational geometric baseline for understanding black hole physics.
The Shape in Reality: Rotating Black Holes
In astrophysics, objects rarely exist without some degree of rotation, and black holes are no exception. Most black holes are believed to be the remnants of stars that spun, conserving that angular momentum as they collapsed. The geometry of these spinning black holes is described by the Kerr solution, a far more complex mathematical model than the non-rotating one.
The angular momentum significantly changes the shape of the event horizon, causing it to flatten at the poles and bulge at the equator. This results in a shape known as an oblate spheroid, which is essentially a slightly squashed sphere, much like the shape of Earth. The degree of this equatorial bulge is directly proportional to the black hole’s spin rate; a faster spin results in a more pronounced flattening. This distortion occurs because the rotation effectively throws spacetime outward near the equator, increasing the circumference there relative to the distance between the poles.
The distinction from a perfect sphere is a direct consequence of the black hole’s spin, which introduces angular momentum in addition to its mass. This complex shape is a hallmark of all real-world black holes, from stellar-mass ones to supermassive black holes. While the non-rotating model is a useful simplification, the oblate spheroid is the shape of black holes in the universe.
Why the Shape Matters: The Ergosphere
The non-spherical shape of a rotating black hole gives rise to a surrounding region called the ergosphere, which does not exist around non-rotating black holes. The ergosphere is located just outside the event horizon and is a direct result of a phenomenon known as frame-dragging, or the Lense-Thirring effect.
Frame-dragging is the process by which a massive spinning object twists the fabric of spacetime around it in the direction of its rotation. In the ergosphere, this twisting is so intense that spacetime itself is forced to rotate with the black hole. Any object or particle within this region must co-rotate and cannot remain stationary relative to a distant observer.
The outer boundary of the ergosphere is called the static limit, where an object would need to travel at the speed of light just to appear stationary. Because the ergosphere is outside the event horizon, it is still possible for particles to enter and escape this region. This unique environment is theorized to be a mechanism for energy extraction, where the black hole’s rotational energy can be tapped.
Due to the black hole’s flattened shape, the ergosphere is also distorted, bulging outward at the equator and touching the event horizon at the poles. This non-spherical feature fundamentally alters the surrounding spacetime and influences the dynamics of matter near the cosmic object.