A Hole to Nothing
One of the most popular topics in astronomy is black holes, which occur when the gravity from an object is so strong it overwhelms all other forces like internal pressure, causing the source mass to contract to a point-like object from which nothing (even light) can escape. Black holes were mathematically implied in Einstein’s Theory of General Relativity, which describes how gravity distorts space and time, but early on the concept seemed to be an abstract curiosity when Einstein’s equations were taken to an extreme, rather than actual objects that occur in nature. These days, evidence for actual black holes has piled up, and barring some strange alternatives, it seems they really do exist. But how close to us is the nearest black hole? Well, back in May 2020, a team of European scientists reported evidence for one just 1,000 light years away or so, which sounds like a lot but is relatively close to the Sun from a Galactic perspective. This object, named HR 6819, seemed to be a triple system, where two objects, a star and a black hole, are in a close orbit and this pair is itself orbited by a second star. Even back then, however, the case wasn’t too clear cut, and an alternative interpretation of the data by another team suggested that a black hole was not needed to explain the observations.
In today’s news it seems that the case for this particular system having a black hole has indeed fallen apart, thanks to more recent higher resolution data. Using the European Southern Observatory’s MUSE spectrograph on one of its four Very Large Telescopes, as well as the GRAVITY instrument that combines the light from several of its telescopes to produce extraordinarily high resolution, a European team which included some of the astronomers who made the initial black hole claim, have determined that the orbits of the stars indeed do not require the existence of a black hole after all. Instead, what appears to be happening is that HR 6819 is “just” a binary system, albeit one in an orbit where one star closely passes by the other and strips off its outer layer in a dramatic act of “stellar vampirism.” Back in 2020, the first team may have simply spotted the pair just right after such a close passage. Oh well! Personally, I think black holes are scary enough that I’m OK with the closest one now being even farther than 1,000 light years away, but black hole enthusiasts may be disappointed today because it’s always easier to study examples of something that is closer than farther away (well, depending on its intrinsic size and brightness, of course). Anyway, good on this team for coming together to settle this debate! Here’s a link to the Astronomy & Astrophysics paper that describes the latest results, as well as a link to a ScienceAlert news article for those who’d like to learn more.