An artist’s illustration depicts what astronomers think is happening within the Cygnus X-1 system. (Illustration: NASA/CXC/M.Weiss)
When darkness falls, look east. The brightest star in that direction is blue Vega. Directly below it hovers the absolute number-one strangest thing in the universe.
First envisioned in 1798, black holes epitomize mystery and danger as no object ever known. The singularities at their cores are utterly unexplainable by science – prowlers in the inaccessible alleyways beyond our comprehension.
We can ease our way into this shaky realm by exploring the surest black hole in the heavens. That’s the one below Vega, in the lovely constellation Cygnus the Swan. The middle of the swan’s neck is defined by the medium star Eta. Nearby, to the lower left of Eta, lies a faint star just bright enough to appear in binoculars.
Something very fishy is going on here. First off, it whirls in a circle every 5.6 days, as if caught in the gravitational grasp of an immense object. Spectroscopic orbital analysis proves that its companion must weigh 8.7 times more than our Sun. Yet the celestial sumo wrestler that is twirling around this star like a puppet is strangely invisible. The most powerful telescopes reveal nothing there at all. Thus we have exhibit A: a heavy, under-luminous object.
Next, this spot of sky emits an intense beam of X-rays, which is always a sign of violence. Physics tells us that anything spiraling towards a black hole should be whipped to such frenzied speeds that X-rays are thrown off. Sure enough, this is the most brilliant hard X-ray source in the sky. It’s such an important clue that this entity is usually known by its name in X-ray catalogues: Cygnus X-1.
Tremendous changes in the X-ray intensity occur in a millisecond – a thousandth of a second, less than an eyeblink. Such near-instantaneous variations prove that it’s no larger than 1/20th the size of the Moon. Put all the evidence together and you’ve got a nearly ironclad case for a black hole.
The dimension into which black holes take us is as bewildering as downtown Guatemala City. Yet many of their features are simple. They can have no magnetic field, for example, nor a reachable surface; and we use these facts to eliminate other tempting candidates like neutron stars (which must always weigh less than three Suns, in any case). If in-falling material were heading towards a neutron star, it would release energy upon impact. But a black hole’s in-falling atoms only create X-rays while in orbit; they never terminate with any sort of decisive bang.
Black holes, undeniably, have had bad press. People distrust them, suspecting that they’ll gobble up the universe if given the chance. But while the phrase “black hole” suggests a poorly lit piece of emptiness, they’re not holes at all. They’re places where matter is intensely present and crushed. Any object could become a black hole if squeezed enough for gravity to get so strong that a speed greater than light’s is needed to escape. Mount Everest would become a black hole if every boulder and truckload of its material were crammed into the size of an atomic nucleus.
Black holes are scarce because matter normally does not voluntarily pack itself so firmly. The simplest mechanism involves obese stars – those more than 3 ½ times heavier than the Sun – going through a late-life crisis, when they cannot resist the gravitational urge to collapse. The smaller that one gets, the smaller it wants to be, until the escape velocity reaches 186,282 miles a second. Light then cannot leave, and the star effectively disappears from our universe.
In a way, nothing really changes at that instant, except that no one there returns your text messages. The star continues shrinking, unaware that the outside world is now calling it a black hole.
Cygnus X-1’s singularity – the collapsed star at its center – achieved black hole density when it became 3.7 miles wide. Yet the star shriveled still further, to the size of a beachball, then an appleseed. It continues to collapse until it occupies zero volume and achieves infinite density. Well, maybe. Our laws of science cannot deal with this, and many theorists think that some unknown process halts the collapse. No one knows.
If the star is rotating, certain angles of approach permit hypothetical paths into other places or times. Enter exactly the right way and you’re suddenly at the senior prom on the planet Maltese.
Surrounding the singularity is the event horizon: an invisible no-trespassing zone, which in Cygnus X-1’s case is 16 miles (26 kilometers) away. Step across it and you’re doomed. When we detect this black hole’s X-rays, we are hearing the final frantic yelps coming to us from the visible star’s stellar wind particles caught spiraling in the accretion disc…on their way to the unknown.
