Dark Stars

The media is full of stories about the impending release of the first ever images of a black hole – images that not only represent cutting edge science, but are the culmination of 200 years of speculation and theory moving ever closer to observation.

This is huge for astrophysics, and a stunning example of how the mundane rules of our tangible, everyday world give physicists the ability to make intellectual leaps into the unknown – and the long-thought unknowable.

Dark Stars

The idea of a black hole can be traced to a calculation by English “natural philosopher” John Michell in 1783.

Along with his contemporaries, he imagined light as a stream of particles. Like balls thrown with superhuman force, these corpuscles emitted by the sun had no trouble reaching escape velocity and sailing away into space.

Michell pulled on a loose intellectual thread, asking how big a star would need to be before its own light could not escape its clutches.That is, when would the gravitational field of a star become so strong that, rather than setting off into the distance, light would fall back to the stellar surface?

Using no more than today’s high-school physics, Michell worked out that you could reach that point of no return by scaling up the sun by a factor of 500. A star 500 times greater in radius than the sun would be 125,000,000 (500x500x500) bigger by volume, and thus would “weigh” 125 million solar masses.

At that size, such a giant would become a dark star – vanishing from view.

From Newton to Einstein

Michell was working with the Newtonian model of gravity. In 1915, Einstein wove space, time and gravity into a single fabric with his General Theory of Relativity. Working in Berlin, at the heart of an empire locked in a global conflict, he hammered out a set of equations that explained gravity as the stretching of spacetime by massive objects. In the Einsteinian framework. the gravitational field of single, stationary object is described by the Schwarzschild metric, named for artillery officer Karl Schwarzschild, who did the maths while being treated for a painful (and eventually fatal) disease in a hospital near the Russian front, in the months after Einstein published his original equations.

To derive the Schwarzschild metric, you first imagine the object as a single idealised point. In the vicinity of this point, the math tells us that space and time turn inside out. This makes it impossible for light – or anything else – to escape past the event horizon into the world beyond. Beyond the event horizon normality is restored, and to the outside observer, it marks the external boundary of the black hole.

Mathematically, the black hole itself consists solely of empty space — it is a pucker in the fabric of spacetime. It is not so much that a black hole has mass; rather, the black hole is a memory of where mass once was.

The event horizons you would have calculated for all known astronomical objects in 1915 were far smaller than their actual size, in contrast to the vast dark stars extrapolated by Michell. In other words, after Einstein, black holes came to be understood as the endpoint of a collapse.

Relativistic Astrophysics

By the 1930s the pieces of what we now call “modern physics” – relativity, quantum mechanics, nuclear physics, along with a nascent understanding of particle physics – had been assembled. Armed with these tools, astrophysicists set about exploring the inner workings of stars. In the 1950s and 1960s we first observed pulsars and quasars (powered, respectively, by neutron stars and black holes) whose very existence is contingent upon these newly discovered laws of physics and these discoveries inspired that new field of “relativistic astrophysics”.

Texas became the epicentre of this emerging field – and two New Zealanders had ringside seats in the early 1960s. One of them, Beatrice Tinsley, went on to do fundamental work on the evolving universe; the other was mathematician, Roy Kerr, who took the step beyond Schwarzschild by solving Einstein’s equations for a spinning black hole, giving us what is now known as the Kerr metric.


The first black holes to be discovered were 10 or 20 times the mass of the sun; the husks of massive stars that burn fast and quickly die. But there’s another category of black hole in our universe — the supermassive black holes that live at the hearts of galaxies.

And it is these giants that we are about to see, as it were, in person.

The coming images have been captured by the Event Horizon Telescope, a network of radio telescopes stretching from the South Pole to Greenland that can operate as a single instrument. This composite telescope has been trained on the centre of our own Milky Way galaxy, and the much more distant heart of the giant elliptical galaxy M87.

The Milky Way’s black heart corresponds to an object weighing millions as much as our sun; and that of the M87 galaxy amounts to billions of times the weight of our sun. Perhaps poetically, they sit at either end of a similar scale to the dark stars originally imagined by Michell.

And, since astrophysical black holes are likely born spinning (since they inherit the rotation of whatever produced them), the images we are about to see may reveal not just a black hole, but a black hole for which the Kerr metric is a key part of our ability to understand it.

Of course, “seeing” a black hole is a paradox. What we see is the matter circling its event horizon, caught in the grip of the black hole’s gravitational field, a spinning vortex around a cosmic plughole. Detailed images of this accretion disk alone would be spectacular enough, but on top of that, we will it apparently twisted into fantastical shapes as the light is pulled and twisted along complex paths in the vicinity of the event horizon.

These images will test Einstein’s understanding of gravity, and provide unprecedented proof that black holes truly exist in our universe. Because seeing really is believing. Even for scientists.

Header image: Oliver James et al 2015 Class. Quantum Grav. 32 065001