Supermassive black hole turns unlucky star into spaghetti Astronomers have watched a star be destroyed by the process of spaghettification, a rare event triggered when a sun strays too close to a black hole. AT2019qiz was ripped apart by a supermassive black hole in the constellation of Eridanus. Though it is 215 million light years from Earth, it is the closest star we've seen …
This article says that the supermassive black hole spaghettified an infalling star. The sidebar linked article " what would happen if Earth fell in" says that only normal black holes spaghettify and that with a supermassive BH, you fall inside the event horizon before the field gradient gets big enough.
Getting my head around the (truly mind boggling) forces involved here required some serious reading. Gravity can literally pull any object apart because it's operating at the sub-atomic level and everything has a structural limit while gravity just keeps on increasing as you add mass.
From the perspective of the singularity in a supermassive, the difference in structural integrity between a gas cloud and a neutron star is insignificant, both will arrive as a particle stream, the only difference is how far away it happens.
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In a supermassive the spaghettification happens after passing the event horizon, it's just that there's no way to view the process from outside. Theres quite a distance (many AU) between event horizon and the actual singularity itself.
Anything with less mass than a star wouldn't have the inbuilt gravity to hold itself together long enough to reach the event horizon.
Your original point was "In a supermassive the spaghettification happens after passing the event horizon, it's just that there's no way to view the process from outside."
I'm just pointing out that spaghettification starts before the event horizon, therefore it has a chance of being seen by astronomers. I assumed that is what is being seen in this case.
The answer to this is that what disrupts objects falling into a BH is not the strength of the gravitational field, it's the variation of that strength over the extent of the object falling in. This is normally called a 'tidal' effect because it's the same effect that causes the Moon to raise tides on the Earth.
As an object falls towards a BH, the difference between the field it feels on the side away from the BH and that it feels on the side closest to it increases, and it increases without bound (it becomes as large as you like) as you approach the singularity. At some point this difference in field overcomes whatever is holding the object together and it falls apart, becoming spaghettified.
Any large object like a planet or a star is held together entirely by gravity – in particular planets don't have significantly more 'structural integrity' than stars. So it is not that planets are somehow stronger than stars that means that planets will get spaghettified later.
Rather it is that stars are much larger than planets, so they experience hugely larger tidal forces as they approach the BH, and thus get disrupted much sooner.
A nice way of thinking about it is to realise that what matters, really, is how large the infalling object is compared to the radius of the BH's event horizon. The larger it is, the larger the tidal forces across it and the sooner it will get disrupted. If it is very dense, like a neutron star or something, it will tend to hold together for longer (neutron stars are also rather small, of course).
If the object is small enough compared to the radius of the BH's event horizon, it won't get disrupted until after it has passed the horizon (at least not classically). If it's much larger, it will.
In this case you can see that the star being disrupted is indeed pretty large compared to the BH: from the paper referenced in the article the BH has a mass of about a million solar masses while the star has a mass of about 1. This means that the radius of the BH's event horizon is only about 4 times the radius of the Sun. If we assume the star had a radius about the same as the Sun you can see it's really fairly large compared to the BH, which is why it will get disrupted.
For the technically-minded there is a rather easy expression for the distance at which an object will be disrupted: R_t = R_* (M_h / M_*)^(1/3) (approximately). Here R_t is the radius at which a star will be disrupted, the 'tidal radius', R_* is the radius of the star, M_h is the mass of the black hole and M_* is the mass of the star.
For a million-solar-mass BH, this means that the Sun would be disrupted about at about 23 times the radius of the event horizon, while Earth would be disrupted only about 14 times the radius. For a hundred-million solar-mass BH the Earth would only be disrupted inside the event horizon.
Note that there are other nasties that happen as you approach a BH even if you dont get disrupted. You probably don't want to be too close to an active accretion disk, for instance.
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Information loss is quite the quandary in black-hole physics. If the amount of information in the universe is constant, then what becomes of it once it reaches the event-horizon? Will a clear unshrouded event like this one help answer that question? And what form the storage may take? (or at least the information related to 1/2 the matter not ejected)?
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