“[The black hole] teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as ‘sacred,’ as immutable, are anything but.”
–John A. Wheeler
To an astronomer on any other world, the most important object in our Solar System wouldn’t be the Earth, but rather our Sun. Just one example of the hundreds of billions of stars in our galaxy, our Sun is a G-type star, burning at around 6,000 Kelvin and with a lifetime of around 10 billion years.
But stars come in a great variety of masses, sizes, temperatures and lifetimes.
Although they are the rarest type of stars, the bright blue O and B stars are perhaps the most spectacular in all of existence. With masses reaching, twenty, fifty, or even hundreds of times the mass of our Sun, these stars burn hotter and faster than our Sun ever could. Emitting energy often at rates in excess of 100,000 times our Sun’s, and frequently living for less than even one million years, these stars build up massive cores through nuclear fusion, and then have no choice but to collapse under the irresistible force of gravity.
Not, mind you, to contract into a white dwarf star, like our Sun will, but to destroy the individual atoms making up the star itself. In the most extreme cases, the stellar corpse will collapse into a black hole: an object so dense not even light itself can escape from it.
On its own, you’d probably never even know a black hole like this existed, given the fact that it’s likely going to be thousands of light years away. But every once in a while, we get lucky.
If a black hole happens to have a binary companion — particularly a large-sized binary companion — it can steal some of the mass from this much less dense star. When it does so, it not only forms an accretion disk around the black hole, but the matter can get accelerated by the black hole’s powerful magnetic fields, and shot out in a pair of jets, perpendicular to the disk, moving in opposite directions.
This acceleration, like all charged particles accelerated by magnetic fields, will cause the emission of light. Not visible light, mind you, but in the case of black holes, powerful X-ray light.
In our own galaxy and very nearby, we’ve detected a few black holes like this, where the mass of the black hole is only a few times the mass of our Sun. For example, GRO J0422+32, for which an artist’s impression is shown, above, has a black hole maybe 10 times the mass of the Sun, and is located about 8,000 light-years away.
One characteristic of these little black holes is that these powerful X-ray sources emit their energy in great bursts, which die down after a few years to become incredibly quiet. The other types of black hole — the supermassive ones at the centers of galaxies — do not do this at all!
Centaurus A, located about 11 million light years away, is a giant elliptical galaxy with an unusual dust lane in it. It’s also one of the closest active galaxies to us, with two radio jets extending for about a million light years in space, moving at speeds of about half the speed of light in the innermost regions. Needless to say, with this kind of power behind it and a supermassive black hole that’s many millions of times the mass of our Sun, these X-ray emissions aren’t going anywhere for quite some time.
But a much smaller black hole — even though it would be much less luminous to X-ray eyes — can have its intensity drop by a factor of hundreds or even thousands within just a single year, if you happen to be watching at the right time. So far, we’ve found many stellar-mass-scale black holes this way, but nearly all of them have been within our own galaxy, and none of them have been as far away as Centaurus A.
But let’s take a look at Centaurus A itself, in the X-ray.
Yes, the central, supermassive black hole and its jets are easily the most prominent feature in this image, but there are other bright X-ray sources, too. So you don’t just look at it once, you look at it many times, separated by long periods of time! What did we find when we did exactly that?
The team used the orbiting Chandra X-ray observatory to make six 100,000-second long exposures of Centaurus A, detecting an object with 50,000 times the X-ray brightness of our Sun. A month later, it had dimmed by more than a factor of 10 and then later by a factor of more than 100, so became undetectable.
That would be this object, above. It makes you wonder, of course, what all of these other bright X-ray sources are! Are they also small-ish black holes, ready to drop off in brightness as soon as their fuel is spent? Or are they more robust, long-lasting objects?
But make no mistake, this object just became the most distant, stellar-mass black hole ever discovered! Want some more proof? Take a look at this 2001 X-ray image of Centaurus A, and notice a very suspiciously missing point of light!
So, if you want to know where the first stellar-mass black hole located more than 10 million light-years from Earth is, it’s right here!
Now, that’s what I call a little black hole going a long way!