\n\n
When a star meets a black hole <\/h2>\n\n
A growing black hole needs to eat, and makes a mess when it does <\/p>\n\n Everyone loves black holes, especially when they\u2019re doing classic black hole things like ripping stars apart and consuming them whole. The authors of this month\u2019s paper, led by Fulya Kiroglu from Northwestern University just outside Chicago, use powerful simulations to look at what happens when a Sun-like star, minding its own business, encounters a black hole which outweighs it by a factor of between a hundred and a thousand. <\/p>\n\n I should say at the start that it\u2019s not very clear that black holes of this intermediate size actually exist. We know smaller ones do, formed in supernovae at the end of a massive star\u2019s lifetime, and gravitational wave experiments have detected mergers of black holes that add up to about 100 solar masses. We also know that supermassive examples, weighing millions of times the mass of the Sun, live at the centres of galaxies. But in between \u2013 maybe \u2013 are black holes that form from the deaths of the most massive stars, or via merging with other black holes in the middle of dense star clusters. In either case, one way of detecting these elusive beasts would be to spot them as they grow by swallowing their stellar companions whole. Consuming a star should make a mess and it has been suggested that bright sources of X-rays found in the right sorts of star clusters might be caused in just this way, but that means we need better simulated models of what, exactly, happens when a star comes close to the black hole. <\/p>\n\n The answer, the team find, is that \u2018it varies\u2019. If the star keeps its distance, then it might get away with little lasting change. If it gets too close, though, it will be ripped apart. The simulations show that even a first passage close to the black hole, prior to being captured into orbit, will see the star lose a lot of mass. The disrupted star may even be ejected from the system after this initial encounter, or captured into an orbit where the black hole will cannibalise ever more of the star\u2019s mass on each subsequent passage. <\/p>\n\n \u201cThis is exciting. If we can detect X-ray flares, we should be able to work out how massive the black hole is from counting the flashes\u201d<\/p><\/blockquote>\n\n This seems to happen again and again. In one example looked at by the researchers, a star encountering a 10-solar-mass black hole swings by 16 times before what\u2019s left of it is ejected back into the cluster. For larger black holes, the number of orbits a star can survive is smaller, with a star orbiting around a 100-solar-mass black hole withstanding just five passages. <\/p>\n\n This is exciting; it means that if we can detect X-ray flares from the material being ripped from the stars, then we should see a repeating pattern of such events marking the position of a black hole and be able to work out how massive it is from counting the number of flashes. The simulations also show that each flare should be brighter than the last, creating a distinct signature that could be used to recognise these systems. Of course, more work \u2013 by which I mean creating more simulations, including stars of different sizes, masses and ages, as well as more details of the processes in play \u2013 is necessary, but in a year or two we could be hearing news that we\u2019ve finally filled in the missing link of black hole evolution, thanks to the flashing X-rays produced as examples consume stars. <\/p>\n\n![\"\"](\"https:\/\/dj9jqhxgw9833.cloudfront.net\/uploads\/sites\/77\/2022\/11\/d642964d-17ad-4b61-a36f-1d8da9e49cae.jpg\")
Taken captive <\/strong><\/h4>\n\n
\n\n
![\"\"](\"https:\/\/dj9jqhxgw9833.cloudfront.net\/uploads\/sites\/77\/2022\/09\/Chris-Lintott-PNG-1024x1024.png\")
<\/figure><\/div>\n\n