Directions: Read the passage given below and answer the questions that follow by choosing the correct/most appropriate options:

Using data from ESA’s Gaia astrometry mission, astronomers have identified the closest known black hole, less than 1600 light-years away from Earth, and determined its mass. The black hole is orbiting a star similar to our Sun and was identified by tracking the star that the black hole is orbiting. It is expected to be the first one of many black holes to be discovered using the same method. At the same time, the properties of the binary star system are unexpected, indicating a serious gap in astronomers’ understanding of how such systems form in the first place.

Black holes are difficult to observe, by their definition: mass concentrated in a region with a diameter so small that the resulting extremely strong gravity allows nothing to escape, not even light. Still, these objects have long found their place in astrophysics. So-called stellar black holes, in particular, with a few solar masses, are the end state of very massive stars. Now, a group of astronomers led by Kareem El-Badry (Max Planck Institute for Astronomy [MPIA] and Harvard-Smithsonian Center for Astrophysics) has used a novel method to discover the closest known black hole. The discovery also shows up gaps in current astronomical knowledge, namely about the formation of binary star systems. There are an estimated hundred million stellar black holes in our home galaxy, the Milky Way, but only a small fraction has been detected so far. Some have been detected by gravitational wave detectors, which have measured almost a hundred mergers of stellar black holes, yielding additional data about black hole masses. Of those few dozen stellar black holes that have been detected using telescope observations, most orbit a companion star closely enough for the black hole’s gravity to pull hydrogen gas from the companion star into a so-called accretion disk that surrounds the black hole. The gas then becomes hot enough in the process to emit considerable amounts of X-rays. There are 20 known “X-ray binaries” of this kind, with an additional 50 candidate objects.

There have been several attempts to also find “quiescent” black holes in binary systems – black holes without an X-ray-emitting disk. The tool of choice: stellar spectra, the rainbow-like decomposition of starlight, which contains information about a star's motion. We know this, from everyday life, from the “Doppler effect” for sound: an ambulance with a blaring siren will sound higher-pitched when it is coming towards us and lower-pitched once it has passed us. Similarly, light in stellar spectra tells us about a star’s motion directly toward us or away from us. Over the past few years, there have been several claims of quiescent black hole discoveries that tried to deduce a binary’s orbit and the mass of an unseen companion exclusively from stellar spectra. However, all but one of them (the June 2022 discovery of the binary system VFTS 243, with El-Badry as co-author) have since been challenged or downright refuted by follow-up studies. The key problem: Spectra give only part of the information about stellar motion, and hence about the orbit and about the companion’s mass. The missing information is a fundamental source of uncertainty – and it’s also where ESA’s Gaia mission promises to help! For a few years now, there has been hope that ESA’s astrometry mission Gaia would open up a new way of detecting and characterizing black holes in binary star systems by _______. Gaia is designed for ultra-precise measurements of stellar position. This includes the ability to detect a visible star's motion in the sky, and from that to deduce the presence of an unseen companion.

Gaia BH1 is a spectacular find, but also a puzzling one. It is difficult to explain how a system like this could have formed in the first place. Specifically, the progenitor star that later turned into a black hole would be expected to have had a mass of at least 20 solar masses, which means its life would have been very short – on the order of a few million years. If both stars formed at the same time, this massive star would have turned into a supergiant, puffing up and engulfing space too far beyond the stars' common orbit, before the other star would have even had the time to become a proper, hydrogen-burning (“main sequence”) star. It is not at all clear how the solar-mass star could have survived that episode, ending up as apparently normal as the observations of the black hole binary indicate. Theoretical models that do allow for survival all predict that the solar-mass star should have ended up on a much tighter orbit than what is actually observed.

This leaves more unusual formation scenarios. For instance, the two original stars could have formed as part of a star cluster. Initially, they would have been considerably farther apart, so the massive star's supergiant phase would not have disturbed the solar-mass star’s evolution. Close encounters of the system with additional stars in the cluster could later have changed the orbit to its present much smaller size. Alternatively, the system could in fact have not two, but three components: Two massive stars instead of one, in close orbit with each other, and the one-solar-mass star orbiting the massive pair at a greater distance. The two massive stars would prevent each other from turning into supergiants. In that case, the 10-solar-mass object might not be a single black hole, but a pair of black holes in close orbit around each other. Since that orbiting pair would exert slightly different gravitational forces on the one-solar-mass star, precise future observations might confirm or rule out that possibility. All in all, Gaia BH1 is at least three things in one: It is an exciting discovery of the closest known black hole, less than half as far as any black hole detected previously. It is a promise of future similar discoveries to come within the next few years, but also a _______ about the formation of binary or, more generally, multiple star systems.