Spaghettification sounds like a particularly unpleasant thing to happen to pretty much anything that isn’t a lump of pasta dough. Now imagine it happening to a star.
Thatâs precisely what astronomers around the world observed over a six-month period when a black hole stretched and ripped apart a star that was sucked into its intense gravitational grasp at a distance of 215 million light years from Earth, as detailed in an Oct. 12 study published through the Royal Astronomical Society. It was the closest observation of such an event to date, and a particularly illuminating one for looking at what happens to the parts of stars that arenât immediately consumed and instead ejected back out of black holes.
Matt Nicholl, lead author on the recent study, assistant professor at the University of Birmingham, and Royal Astronomical Society research fellow, said that these star-destroying “tidal disruption events” are quite rare, especially so close, and researchers got some really beautiful observations of this one.
“Enabled by the proximity and good data, we were able to get an amazing insight into the feeding process, and see how messy an eater this black hole was,” he explained. Â “A lot of the stellar debris was actually driven out of the system by the energy released as the black hole fed.”Â
The observation may have solved a long-standing question of why flares from these events are 100 times colder than predicted.
“The expectation being that the energy is produced in a really compact region around the BH, which must get really hot,” Nicholl said. “In this event, we found that the outflow of material allowed it to cool down, nicely explaining the observed lower temperatures.”
This animation created the European Southern Observatory depicts this kind of stellar feast and how some star material gets launched outward in the process.
Of course, a black hole doesn’t “eat” or “suck in” something like we traditionally picture eating food or sucking up a beverage through a straw, but Nicholl said those terms do give a pretty good sense of what’s happening.
“‘Falling’ is good, because this is gravity after all,” he said. “The scientific term would be to say that the disrupted star is ‘accreted’ be the black hole â the gravitational attraction brings them together and the black hole increases its mass by adding the mass of the star.”
The term spaghettification gives a more visceral description to what is going on with the star at the beginning of the tidal disruption event. It works pretty much what it sounds like.
“The gravitational pull of any object decreases the further away you are,” Nicholl explained. “In an extreme gravitational field like that around a black hole, this gradient becomes so pronounced that the side of the object (in this case a star, but it could be anything!) that faces the black hole can experience a much stronger pull than the side that is further away.”
“This difference in forces between the near and far side stretches the object out like a string of spaghetti, more and more the closer it gets to the black hole,” Nicholl said.
If you, as a person, were falling feet-first toward a black hole, you too would become spaghettified.
“Because your feet are slightly closer to the black hole than your head, they experience a much stronger pull, and that gets exacerbated the closer and closer you get,” Nicholl said. “You just get drawn out and out and out, and I think you would quite literally start to look like a string of spaghetti as you fall towards the black hole itself, so I think itâs quite an apt terminology really.”
The spaghettified star in this case was actually a similar distance away from its black hole as Earth is from the sun, Nicholl said, so if you imagine the sun in the sky being stretched like spaghetti toward us, that’s similar to what the black hole would have “seen.”
The distance that spaghettification occurs is relative to both the size of the black hole and the size of the object that’s falling toward it. That’s because disruption happens when a black hole’s force of gravity overwhelms the cohesion forces that keep an object together.Â
“The star is held together by gravity, and although itâs a whole star so thereâs a lot of gravity there, gravity is still really, really weak, whereas your body is held together by electromagnetic forces between atoms, which is way, way stronger than gravity,” Nicholl said, “So I think you wouldnât be disrupted at the same distance as the star would. I think you would have to get way closer, but I havenât calculated that distance yet so I donât know what the difference would be, but I think itâs safe to say that the star would get disrupted first as you fall in towards it.”
For a black hole like the one at the center of this recently observed tidal disruption event, which is about 1 million times the mass of the sun, the star was torn apart outside of the black hole, and as it stretched like spaghetti around the black hole it collided back with itself forming a hot accretion disk. As it whirls around, the disk heats up due to friction which produces the observable light, Nicholl said.
A hot accretion disk is what’s observed in the first-ever image of the Messier 87 black hole that was captured by the Event Horizon Telescope collaboration in April 2019. That particular accretion disk is a persistent one that’s always around that black hole, Nicholl said, while the one here is newly formed.
With larger black holes, things look a bit different. Or rather, we can’t look at them at all.
“For a black hole bigger than 100 million times the mass of the Sun, stars cross the event horizon, where no light can escape (you can think of this as the ‘surface’ of the black hole, but thereâs no actual barrier), before they get shredded,” Nicholl said.
In being able to observe what happens to objects around a black hole, Nicholl said we can better understand the effects of these supermassive regions and how affect the cosmos around them.
“Observing how matter behaves so close to a black hole is a great test of general relativity,” he said. “The energy released, and any outflows of material, help us to understand how massive black holes shape the galaxies they live in. A big puzzle is how black holes in the centers of galaxies got to be so massive very early in the life of the universe. Studying how they accrete stars might hold the key to this question.”
Black hole, Star, Spaghettification, Astronomy, Supermassive black hole, Galaxy, Tidal disruption event, Gravity
World news – CA – How black holes can spaghettify stars that fall too close