3 BLACK HOLES SLOW THE FLOW OF TIME
To an outside observer, an object falling into a black hole appears to slow down before stopping, caught in suspended animation at the boundary.
4 A BLACK HOLE REVEALS NO CLUES ABOUT WHAT IT’S SWALLOWED
As matter enters a black hole, it’s stretched, pulled and eventually shredded. Even if something were to leak out, it would bear no resemblance to what went in.
5 THEY HAVE NO SIZE LIMIT
In theory, black holes continue to grow in size indefinitely, but just how large they are able to get depends on their local environment.
6 AROUND THE SAME MASS AS THE SOLAR SYSTEM
Supermassive black holes contain the mass of at least 100,000 Suns compressed into a space that’s around the same size as our Solar System.
7 IT’S THE SIZE OF A BLACK HOLE THAT MATTERS, NOT ITS MASS
Just a few micrograms of matter would be enough to create a black hole if it were compressed into a small enough space.
8 SOME GALAXIES MIGHT HARBOUR ULTRAMASSIVE ONES
The galaxy OJ 287 has two black holes, one of which is thought to contain the mass of around 18 billion Suns.
9 BLACK HOLES FEED ON STARS, REVEALING THEIR LOCATIONS
Black holes cannot be seen directly, but the effect they have on their surroundings often reveals their presence. In the Cygnus constellation, a blue supergiant star is being pulled into a teardrop shape, causing its light to flicker as it spins. The star orbits once every 5.6 days, and as it turns, the outer layer of gas is stripped away from its surface at 1,500 kilometres (932 miles) per second as it’s funnelled to an invisible point.
The supergiant is part of a binary system and is locked in a fatal dance with a black hole known as Cygnus X-1. As the black hole spins, space and time spiral up with it, and dust and gas from the star accumulate in a vast swirling whirlpool known as the accretion disc. Particles spiral towards the event horizon, like water circling a drain, and as they tumble inwards the friction releases bright flashes and flares of X-ray light.
10 BLACK HOLES SPIN FASTER THAN THE STARS THAT MADE THEM
If a star is spinning when it dies, it will continue to spin if it becomes a black hole. However, it won’t spin at the same speed. Imagine the star is a twirling ice skater, holding their arms outstretched. As they spin, they pull their arms inwards and start to spin faster. This is down to the law of conservation of angular momentum. As the matter collapses in towards the centre of a dying star, its diameter decreases – like the ice skater, it spins faster.
11 THE CENTRE OF A BLACK HOLE COULD CONTAIN A SINGULARITY
The event horizon of a black hole can measure thousands of kilometres in diameter, but once matter crosses over the edge, it doesn’t stop moving. Exactly what happens on the inside is debated, but according to Einstein’s theory of general relativity, the curvature of space-time inside a black hole is extreme, and everything is directed towards a single point, known mathematically as a singularity. Every possible path leads back to the centre, and matter becomes so crushed into such a tiny space that it’s unrecognisable. The singularity is infinitely small and infinitely dense, creating an infinite curvature in space-time. Within a region of space known as the event horizon, anything that crosses over is compelled towards the centre with no hope of escape.
12 SOME OF THEM HAVE JETS
Some black holes spew impressive amounts of energy from their poles, marking their location like a beacon. As dust and gas race towards the event horizon of a spinning black hole, magnetic field lines direct some of the energy outwards, funnelling it into two energetic jets, like a particle accelerator.
NASA’s Wide-field Infrared Survey Explorer (WISE) identified a pair of black holes orbiting one another that together create gravitational and magnetic disturbances so intense that their jets are being warped and twisted into ribbon-like spirals.
13 THEY LEAK RADIATION
Stephen Hawking showed that black holes could actually radiate energy, known as Hawking radiation, releasing their scrambled contents back into the universe.
14 IT TAKES MILLIONS OF YEARS TO ORBIT OUR SUPERMASSIVE BLACK HOLE
Sagittarius A* lies around 26,000 light years from the Solar System. It takes 225 million years for us to complete a single orbit around the galactic centre of the Milky Way.
15 THEY WERE KNOWN AS DARK STARS
The idea of black holes has been around much longer than the science that predicts their existence, but in the 18th century they were known as ‘dark stars’.
16 CYGNUS X-1 WAS THE VERY FIRST BLACK HOLE TO BE IDENTIFIED
Cygnus X-1 is one of the brightest radio sources in the sky and is currently in the process of devouring a blue supergiant.
17 THEY CREATE WAVES
Einstein predicted that as massive objects like black holes move through space, they create gravitational waves that ripple through spacetime, now confirmed.
18 THE UNIVERSE IS SHAPED BY THEM
Supermassive black holes are found at the hearts of almost all large galaxies and act as the linchpins of the universe around which stars and planets turn.
19 STELLAR BLACK HOLES CONTAIN THE MASSES OF FIVE OR MORE SUNS
Black holes formed during the death of a star usually contain at least as much mass as five Sun-sized stars, compressed into an area measuring just a few kilometres across.
20 BLACK HOLES BEND SPACE-TIME
Einstein showed that the universe is made from a fabric known as space-time, and just like a piece of cloth, it can be bent, twisted and stretched. Massive objects, including planets and stars, make dips in the fabric of space-time, like bowling balls sitting on top of a trampoline. The more mass that’s collected in one area, the more of an impression it makes in the fabric and the more energy is required to escape its gravitational field. One object in orbit around another can be thought of as being similar to a cyclist in a velodrome. The cyclist is trying to travel in a straight line. However, the curved floor forces them to move around in circles. If they pedal faster, they might be able to gather up enough speed to climb out of the top of the dome, and if they slow down they will start to drift back in towards the centre.
21 BLACK HOLES ARE SPHERICAL
They’re often depicted as being funnel-shaped, but these diagrams are simply used to explain the idea that massive objects cause space-time to bend. In reality, space has at least three dimensions, and the impression that a black hole makes in space-time is much more complicated.
The black hole itself, like most massive objects, is actually spherical. Gravity acts equally in all directions, and the event horizon represents the point beyond which gravity becomes so intense that it’s inescapable.
We speak to Douglas Richstone of the University of Michigan about the origins of supermassive black holes
22 ALMOST EVERY GOODSIZED GALAXY HAS A SUPERMASSIVE BLACK HOLE
“For every galaxy that’s reasonably good-sized and regular – that is, a galaxy with a disc and a bulge, and possibly spiral arms, or a so-called elliptical galaxy that looks round – there’s a black hole. Moreover, the black hole’s mass tracks the mass of the host galaxy and is about a thousandth of the galaxy’s mass. These black holes range from 1 million to nearly 10 billion solar masses. However, for galaxies that are very small, irregular or possibly only have a disc and no round component, or bulge, the situation is much more complicated. Some of these galaxies appear to have black holes, while others don’t.”
23 QUIET SUPERMASSIVE BLACK HOLES USED TO BE QUASARS
“We don’t know for certain how the biggest black holes form, but there is a clue. The amount of mass in galaxies at present tied up in black holes is almost exactly the amount of mass needed to power quasars [very bright objects thought to be black holes accreting matter] when the universe was about a fifth of its present age. It’s reasonable to identify the black holes in galaxies now as the relics of quasars.”
24 IT’S IMPOSSIBLE TO SEE THEM DIRECTLY
Black holes do not emit or reflect any electromagnetic radiation except Hawking radiation, but their gravitational effects are detectable.
25 SOME BLACK HOLES SPIN AT HALF THE SPEED OF LIGHT
By looking at the pattern of X-rays in the area surrounding a black hole, the speed at which it’s spinning can be determined.
26 THERE ARE TWO DIFFERENT TYPES OF BLACK HOLE
Schwarzschild black holes are the simplest and are made up of just an event horizon and a singularity. Kerr black holes rotate and have a third component known as the ergosphere.
27 BLACK HOLES ARE NOISY
In 2003, NASA’s Chandra X-ray Observatory revealed that a black hole in the Perseus Cluster makes a sound in the pitch of B flat.
28 WE’LL NEVER KNOW WHAT’S REALLY INSIDE A BLACK HOLE
Light cannot escape across the event horizon of a black hole, preventing us from ever seeing inside one. There’s also no definitive answer about what really happens inside a black hole.
29 ONE DAY, BLACK HOLES WILL DOMINATE THE UNIVERSE
Black holes evaporate so slowly that they will exist long after the last of the stars fade and die, leading scientists to predict that one day they will be all that’s left in the universe.