A black hole emits no light, which raises an obvious question: how does anyone know where one is? The answer is that black holes are never truly alone — they announce themselves by their effect on everything nearby.
Method one: stars behaving strangely
At the centre of our own galaxy, astronomers tracked individual stars for decades as they looped around an invisible point. One star completed a full orbit in fifteen years, reaching six percent of the speed of light at closest approach. Applying Kepler's laws to that orbit revealed roughly four million solar masses packed into a space smaller than our solar system.
Method two: gas and X-rays
Gas falling toward a black hole spirals into a disc and heats to millions of degrees, screaming out X-rays before it crosses the point of no return. X-ray telescopes can therefore map black holes in our galaxy and beyond, even when the hole itself stays hidden.
Method three: gravitational waves
When two black holes merge, they ripple the fabric of spacetime itself. Detectors on Earth measure those ripples as distortions thousands of times smaller than a proton — a direct observation of the objects themselves rather than their surroundings.
And the image that surprised everyone
A network of radio telescopes spanning the planet combined into one Earth-sized dish, capturing the shadow of the gas ring around a black hole fifty-five million light years away — the first direct picture of one.