What is a black hole?
A black hole is a region where so much mass is packed into so little space that gravity becomes overwhelming. Around it is a boundary called the event horizon: inside it, the speed needed to escape would be greater than the speed of light, so nothing, not even light, can get out. The size of the event horizon is set by the mass: for a black hole of the Sun’s mass it would be only about 3 km in radius.
Kinds of black holes
- Stellar-mass black holes (about 3 to 100 Suns) form when the cores of very massive stars collapse at the end of their lives.
- Supermassive black holes (millions to billions of Suns) sit at the centres of most large galaxies. Our galaxy’s, Sagittarius A*, is about 4 million Suns.
- Intermediate-mass black holes (hundreds to hundreds of thousands of Suns) are rarer and still being studied.
How gravity bends light
Einstein’s general theory of relativity describes gravity as the curving of space and time by mass. Light always travels along the straightest possible path, but near a massive object that path is curved, so light bends. This was first confirmed in 1919, when astronomers saw stars near the eclipsed Sun shifted by exactly the amount Einstein predicted. Because a black hole packs a lot of mass into a small space, light passing close to it can be bent dramatically. This bending is called gravitational lensing.
Einstein rings and arcs
A point-like lens forms two images of each background object: a bright one outside theEinstein radius and a fainter one inside it, on the opposite side. Images are stretched intoarcs around the lens. When the source lies exactly behind the lens, the light is bent equally all the way round and the images merge into an Einstein ring. Space telescopes have photographed many such rings and arcs produced by massive galaxies and galaxy clusters.
The equations
The simulator uses the lens equation for a point mass and the matching brightening formula:
β = θ − θE² / θ · A = (u² + 2) / (u √(u² + 4)), u = β / θE
Here θ is where we see light, β is where it really comes from, and θE is the Einstein radius, which grows as the square root of the mass. The event horizon’s size is the Schwarzschild radius, rs = 2GM/c², about 2.95 km for every solar mass.
Microlensing
For a stellar-mass black hole in our galaxy, the Einstein radius is only a few thousandths of an arcsecond: far too small to see the ring. But the lensing still makes the background star look brighter. As the black hole drifts past, the star brightens and fades in a smooth, symmetrical curve over days to months. Surveys watching hundreds of millions of stars use this microlensing to find objects that give out no light. In 2022 astronomers confirmed the first isolated black hole drifting through our galaxy this way.
How we observe black holes
Besides lensing, black holes reveal themselves through the stars and gas around them. Gas spiralling in heats up and glows brightly in X-rays; stars near the centre of our galaxy race around an invisible object of 4 million Suns. In 2019 the Event Horizon Telescope released the first image of a black hole’s shadow, in galaxy M87, and in 2022 the shadow of Sagittarius A*. Since 2015, detectors such as LIGO have also “heard” black holes merging through the gravitational waves they send out.
Key terms
- Event horizon
- The boundary of a black hole from inside which nothing can escape.
- Einstein radius
- The radius of the ring formed when a source is exactly behind a lens.
- Microlensing
- Temporary brightening of a star when a massive object passes in front of it.
Common misconceptions
- “Black holes are cosmic vacuum cleaners.” Far away, they pull like any other object of the same mass. If the Sun became a black hole, Earth’s orbit would not change.
- “Black holes are holes in space.” They are extremely dense concentrations of mass.
- “Black holes are invisible, so we cannot study them.” We detect them through lensing, orbits, glowing gas and gravitational waves.
Key takeaways
- A black hole’s gravity is so strong that light cannot escape from inside its event horizon.
- Mass bends light: a black hole acts as a gravitational lens, making arcs, double images and Einstein rings.
- The Einstein radius grows with the square root of the mass.
- Microlensing brightens background stars and lets astronomers find dark objects.
Model note: the view is enormously magnified, the black hole’s dark shadow is drawn much larger than it would really appear compared with the Einstein ring, and the lens is treated as a single point mass.