In 1938, astronomers observed a burst of energy (nova), including visible light, from 7,800 light years away in the Cygnus constellation at a time when black holes were only considered theoretical. Although the outburst captivated observers, there wasn’t enough evidence to identify the emission’s source. However, a second nova in 1989 gave astronomers more promising information suggesting that the object, V404 Cygni, could be a black hole. Eventually, this led to it becoming one of the first objects in human history confirmed as such. The black hole rose to prominence to those within and outside of the astrophysics community, resulting in its activity being well documented and studied. Until recently, it was most famous for being a binary system in which a closely orbiting star is having its outer layers stripped by its gravitational force. However, an observation in 2024 shook the world yet again and made people rethink black hole formation itself.
A few months ago, a team of Massachusetts Institute of Technology (MIT) and California Institute of Technology (Caltech) physicists observed data from a variety of telescopes to determine if any new black holes had formed in the Milky Way recently. One of those physicists, Kevin Burdge, looked at an image of V404 Cygni purely out of curiosity and noticed two blobs of light near each other. He determined that one of them was the material from the previously mentioned nearby star being shed into the black hole and believed the second to be a distant star.
While this star had been documented before, many assumed it was just near the system by chance. Still, this didn’t stop the team from wondering whether it could be linked to the V404 Cygni system in any way. To do this, they observed existing data about both stars’ positions and velocities over time collected by Gaia, a satellite, and noticed that they appeared to move with each other around V404 Cygni. This implies that the further star, at a stupendous distance of 3,500 astronomical units (the distance between the Earth and the Sun), was also orbiting the black hole. This discovery made V404 Cygni and its satellites the first known triple black hole system and raised questions about its nature, particularly about how it influenced an object so far away.
To come up with an answer, the team would have to question physicists’ current understanding of black hole formation. It is generally accepted that black holes form through the collapse of a large, dying star’s core and a subsequent supernova explosion. Most of a star’s lifetime is a constant battle between energy from nuclear fusion (the process of fusing atoms’ nuclei to form new elements starting from hydrogen and ending at iron in stars) pushing outwards and gravity pulling everything in towards its core. Once a star fuses enough subatomic particles into iron, it is no longer able to fuse any heavier elements, resulting in its core and outer layers starting to collapse in on itself. During a core collapse, a massive amount of matter gets compressed rapidly into an extremely small, dense spherical region. If that space is massive enough, the region develops such a strong gravitational force that even light cannot escape, creating a black hole (otherwise, a neutron star is formed). Immediately afterwards, the energy of all the matter moving so quickly is also released in a supernova explosion, destroying close objects and repelling farther ones away.
If V404 Cygni was formed through a core-collapse supernova, the previously-discussed star would have been pushed too far from the black hole to be gravitationally influenced by it. However, this clearly was not the case for the star, implying that push was absent. This observation led astronomers to believe that V404 Cygni may have formed through a direct collapse, the formation of a black hole without energy release through a supernova explosion.
As of now, physicists believe that when a core collapses, atoms in it get so compacted that protons and electrons start to fuse into neutrons, converting the entire core into them while creating neutrinos, small and neutral subatomic particles. The neutrons stop the outer layers from completely collapsing into them for a brief period of time, causing the infalling material to bounce off of it and release a shockwave. While this shockwave itself doesn’t cause the supernova, it disrupts the neutrinos enough that they start to collide with and heat up the infalling matter, creating a force pushing outward from the core. If this force exceeds the strength of the other force that’s pushing in on the core, a supernova will occur, disrupting all objects near the newly formed black hole. However, evidence that doesn’t align with this theory, including the V404 Cygni system, has led to it being modified. Many astronomers now believe a scenario where the matter falls into the core creating a black hole without an explosion is possible if the outward force of the matter’s temperature rising does not exceed the inward one.
Until the V404 Cygni system was determined to be a triple, there was no strong evidence supporting that this scenario was even possible aside from mathematical calculations and theoretical physics. It was generally accepted by many physicists that all stars massive enough to collapse in on themselves eventually underwent a process involving a violent supernova explosion. The fact that such a faraway object could be gravitationally bound to the system may be the first true counter-evidence of the above statement and opens up the possibility that V404 Cygni’s formation is not a one-off occurrence. Because of this, astronomers are currently trying to look for other systems with distant objects gravitationally bound to black holes and other clear signs of one that possibly formed through direct collapse.
Even if the V404 Cygni system is determined not to have formed through direct collapse, it may give astronomers an idea of what to look for in other black holes to get a better idea of how they formed. It may also make them re-evaluate our understanding of well-studied black holes as V404 Cygni has proven that even the most documented objects still hold interesting unknowns for those looking for them (even by accident in the case of Kevin Burdge). It also goes to show how a process that was thought to be well-understood in many ways can still be questioned, and how astronomers’ understanding of the universe is and will always be constantly changing.
Works Cited:
Burdge, K.B., El-Badry, K., Kara, E. et al. The black hole low-mass X-ray binary V404 Cygni is part of a wide triple. Nature 635, 316–320 (2024). https://doi.org/10.1038/s41586-024-08120-6
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