Black holes are not all freezing voids. The smaller they get, the hotter they burn — a mountain-sized one would glow like an X-ray star. Enter a mass and take its temperature.
Ask anyone what a black hole feels like and they'll say freezing. They'd be wrong, and the person to blame is Stephen Hawking, who in 1974 did the math and found that black holes glow. Every black hole radiates like a hot coal — it is called Hawking radiation — and the temperature of that glow depends on nothing but the hole's mass.
The rule is insultingly simple: temperature is inversely proportional to mass. Halve the mass, double the heat. A black hole the mass of the Sun sits at 6×10⁻⁸ kelvin — about 44 million times colder than the cosmic microwave background that fills all of space. It is not radiating much; it is actually absorbing the background's warmth and slowly growing, like a glacier sitting in a warm room.
Shrink it, though, and the tables turn fast. A black hole the mass of the Moon radiates at 1.7 kelvin — almost the temperature of the background itself. A black hole the mass of Mount Everest would glow at over ten million kelvin, blazing like the surface of a hot star. A black hole with a trillion kilograms of mass — a mountain compressed to a dot the size of a proton — would burn at 120 billion kelvin and dump gamma rays in every direction.
The same inverse law governs how long a black hole lives. Evaporation time scales with mass cubed, so the big ones are effectively immortal — a solar-mass black hole would take about 2×10⁶⁷ years to dissolve, longer than we can comfortably count. The small ones are in a hurry. A trillion-kilogram black hole evaporates in about three trillion years, which sounds slow until you remember the universe is only 13.8 billion years old. If any of these tiny furnaces exist — and some cosmologists think primordial black holes might be out there, maybe even making up dark matter — they are exploding right now like cosmic firecrackers.
Hawking radiation comes from quantum effects at the event horizon, and the horizon is smaller and sharper for a lighter black hole. Sharper geometry means stronger quantum effects, which means a higher temperature. It is the same reason a tiny black hole evaporates so much faster: temperature rises as you shrink, and evaporation rate climbs with the fourth power of temperature.
Not directly. No one has ever measured a black hole's glow — real black holes are too heavy and too cold to shine above their surroundings. But the mathematics is on very solid theoretical ground, and indirect astrophysical evidence keeps pointing the same way. It is one of those predictions physicists are nearly certain about and would still love to catch in the act.
Only if nothing feeds them. A black hole loses mass as it radiates, which makes it hotter, which makes it radiate faster — a runaway process called evaporation. But a black hole the size of the Sun is so cold that it absorbs more background radiation than it emits, so it grows instead. Only small black holes — or ones left alone for an astronomically long time — actually shrink away.
Possibly. Primordial black holes could have formed in the dense soup just after the Big Bang, and small ones would be sizzling today. Their final moments would release a burst of high-energy radiation, and astronomers have searched for those bursts. No confirmed detection yet — but the search is very much alive, and it doubles as a dark matter hunt.