Bright and dark, near and far
We point out some astronomical ironies.
A recent paper describing the detection of a black hole in a nearby star system is interesting from several points of view. It even received notice in the mainstream (non-scientist) press, which is very unusual for a scientific paper not in the health or medical fields. It probably attracted their attention because it is now the closest known black hole, and “closest,” like “farthest” and “biggest” and other superlatives, seems to be important. But the casual reader could be forgiven by being a bit confused by titles linking a black hole (which, by definition, you cannot see) with a naked-eye star (which you can see). We think the paper is interesting because it highlights some of the ironies of astronomy, as well as showing something of how the science operates.
A black hole is an object predicted by General Relativity to be so dense that light cannot escape from it, and hence nothing can get out (except Hawking Radiation, which is not important here). Thus you cannot see it directly, and thus its name. But in our first irony, black holes power the brightest objects in the universe. If there is a stream of matter falling into the hole’s gravity, it heats up, trading off gravitational energy for thermal, and glows brightly (before, maybe, falling into the hole and disappearing from sight). A chunk of gas or plasma can convert an even greater fraction of its mass into energy this way than in nuclear fusion reactions. So a single black-hole-powered quasar can outshine whole galaxies of stars.
We can see quasars at enormous distances, billions of parsecs. But in our second astronomical irony, we can’t see this new black hole a mere 300 or so parsecs away. It is not accreting matter from anywhere, and so doesn’t glow at all. Distance is far less important for astronomical objects than intrinsic brightness. You can’t even see the nearest star, Proxima Centauri, without a telescope, at just over a parsec away; but on a clear, dark night you can make out by eye the Andromeda Galaxy at about 800,000 parsecs.
Our astronomer notes that this discovery shows several aspects of his science. First, serendipity: the authors weren’t looking for nearby black holes. They were studying a certain type of star, and gathered spectra of an example of this type. It turned out to be the mingled spectra of two of them, one of them showing periodic Doppler shifts indicating it was in orbit with something unseen. Next, dynamics, and a lower bound: from modeling the orbit they could determine something about the masses, but without knowing where we observe it from relative to the plane of the orbit, they could only set lower limits on them (they might be larger, but not smaller). Then, stellar models. We know enough about stars to say that, if the unseen object were a star of the mass they deduced, we would easily see its contribution to the spectra, distinguished from the other two. Finally, implications for other areas of the science: astronomers who model the formation of stars say there should be lots and lots of stellar-mass black holes, far more than we know of from those that are bright. This observation goes some way toward resolving that discrepancy.
We might also add the fact that astronomers have many names for the same object. This one is HR 6819, HIP 89605, HD 167128 and the more resonant QV Telescopii. They claim it’s not just to confuse non-astronomers.
