Black holes are regions in space where gravity is so strong that nothing, including light and other electromagnetic waves, has enough energy to escape it. They are some of the strangest and most fascinating objects in space, and they are extremely dense, with such strong gravitational attraction that not even light can escape their grasp. Key features of black holes include:

• Event Horizon: This is the boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon.

• Singularity: At the center of a black hole, general relativity predicts a point where spacetime curvature becomes infinite, known as the singularity.

• Types: There are four types of black holes: stellar, intermediate, supermassive, and miniature. Stellar black holes form when the center of a very massive, dying star collapses in upon itself. Intermediate black holes are believed to form from the merging of smaller black holes. Supermassive black holes are found at the centers of most galaxies, including our own Milky Way, and are thought to form from the merging of smaller black holes and the accretion of matter.

• Size: Black holes can be big or small. The smallest black holes are as small as just one atom, but have the mass of a large mountain. Stellar black holes can have a mass up to 20 times more than the mass of the sun, while supermassive black holes have masses that are more than 1 million suns together.

Black holes are invisible, and people can’t see them because no light can get out. However, space telescopes with special tools can help find black holes by detecting their effect on nearby matter. Although light can’t escape a black hole’s event horizon, the enormous tidal forces in its vicinity cause nearby matter to heat up to millions of degrees and emit radio waves and X-rays, which can be detected by telescopes. NASA’s Hubble, Chandra, Swift, NuSTAR, and NICER space telescopes, as well as other missions, continue to take the measure of black holes and their environments so we can learn more about these enigmatic objects and their role in the evolution of galaxies and the universe at large.