Stargazing Can Stars Be Spaghettified in a Matter of Days by Black Holes?

[Gear300]

TL;DR

on spaghettification of stars

I read that the gravitational tidal force of a nearby blackhole can twist and tear apart stars, sometimes even in a matter of days. The 'matter of days' part peaked my interest, and I figured I would ask here before looking it up. Have we ever observed any such event in a matter of days? Is it that unlike the human eye, the Hubble or James Webb have several focal points each with its own camera apparatus? Or is that a trait of astronomical interferometers and telescope arrays? (I guess the neat thing would be if we could zoom in on past recorded skies.)

 

[Drakkith]

Have we ever observed any such event in a matter of days?

Not sure. There couple of events I just looked up seemed to take place over a matter of months.

Is it that unlike the human eye, the Hubble or James Webb have several focal points each with its own camera apparatus?

While it may be possible with professional telescopes to split the incoming light into several beams that each go to their own instruments, I'm not quite sure what this has to do with the 'matter of days' part of your post. By and large even a professional telescope is just a larger and more complex version of your phone camera. That is, you point at a target and then take a picture. For astronomical images we just take many, many photos of the same target to combine together and each exposure is often 20+ minutes in length, compared to a fraction of a second for most photos you take with your phone.

 

[snorkack]

The timescale of the event depends on the size of the star being torn apart, as well as its response to having material removed. We have observed a merger of two neutron stars, that happens in a matter of microseconds, but seems the actual event was missed. Have we seen any mergers of a white dwarf with a white dwarf, neutron star or black hole?

 

[phyzguy]

A tidal disruption event (TDE) is a star being torn apart by a supermassive black hole (SMBH). Supermassive black holes are at the cores of galaxies, so these events are at cosmological distances. Note that we don't resolve the event with our telescopes. What we see is a sudden brightening of the SMBH followed by an exponential decrease in the light intensity, which matches our models of a tidal disruption event, so we believe that is what they are. These events typically take weeks to months. This paper describes one such event where the light curve decays with a time constant of 17 days, so I guess that is "a matter of days".
If you want to learn more, here is a catalog of these events which have been observed.

 

[phyzguy]

@snorkack , white dwarfs are quite common, so white dwarf mergers are also common. Most often, a white dwarf merger results in a Type-1A supernova. Many thousands of these events have been seen. However, if the combined mass of the two white dwarfs is small enough, they can merge to form a larger white dwarf, as in this paper. I'm not sure if white dwarf-neutron star or white dwarf-black hole mergers have been observed.

 

[snorkack]

A tidal disruption event (TDE) is a star being torn apart by a supermassive black hole (SMBH). Supermassive black holes are at the cores of galaxies, so these events are at cosmological distances.

Is that the strict definition? Because there are other compact objects which can "tidally disrupt stars" - stellar mass black holes, neutron stars and white dwarfs, which occur both in Milky Way and other galaxies.

Note that we don't resolve the event with our telescopes.

In the sense of not resolving the spatial structure.

What we see is a sudden brightening of the SMBH followed by an exponential decrease in the light intensity, which matches our models of a tidal disruption event, so we believe that is what they are. These events typically take weeks to months. This paper describes one such event where the light curve decays with a time constant of 17 days, so I guess that is "a matter of days".

But that´s the decrease, after the disruption is over.
It seems that we have tendency to miss the rise of light intensity from the pre-disruption object - the disruption itself.

 

[phyzguy]

Yes, I think what's called a tidal disruption event is caused by a supermassive black hole. The other events you describe would be considered collisions or mergers, and would look very different.

You have to be very lucky on these transient events to catch the increase. It's like supernovae. Unless you are staring right at the star when it explodes, you don't see the increase. All you notice is that a star has suddenly gotten brighter. We find these by taking images of a large number of stars and noticing that one of them has gotten brighter.

 

[timmdeeg]

Yes, I think what's called a tidal disruption event is caused by a supermassive black hole.

I wonder how this can happen. The tidal force goes with 1/M² with M the Mass of the supermassive black hole. Am I missing something?

 

[snorkack]

I wonder how this can happen. The tidal force goes with 1/M² with M the Mass of the supermassive black hole. Am I missing something?

It might happen when the density of the disrupted object is low enough.
But tidal disruption events should preferentially happen to degenerate objects - neutron stars, white and brown dwarfs. Because they have the property of shrinking on addition of mass - therefore when any mass spills over, the rump expands and spills over in turn in a rapid avalanche. Whereas stably fusing bodies such as main sequence stars and giants have the opposite property - they expand on addition of mass while the rump shrinks on removal of mass, so they would spill over over extended time, until forced over piecemeal by internal evolution or orbital inspiral.

 

[phyzguy]

I wonder how this can happen. The tidal force goes with 1/M² with M the Mass of the supermassive black hole. Am I missing something?

You're right that the tidal force drops off as the SMBH gets larger. But look at this Wikipedia page. If the tidal radius is less than the SMBH Schwarzschild radius, then the star just falls into the SMBH instead of getting tidally disrupted. But for a sun-like star, this crossover is at 1E8 solar masses, and there are many SMBH smaller than this where the tidal force is large enough to tear the star apart. Also, for a star with larger radius, like a red giant, then even the most massive known SMBH, with masses ~ 1E10 solar masses will tidally disrupt a red giant, because they are more loosely bound.

 

[phyzguy]

It might happen when the density of the disrupted object is low enough.
But tidal disruption events should preferentially happen to degenerate objects - neutron stars, white and brown dwarfs. Because they have the property of shrinking on addition of mass - therefore when any mass spills over, the rump expands and spills over in turn in a rapid avalanche. Whereas stably fusing bodies such as main sequence stars and giants have the opposite property - they expand on addition of mass while the rump shrinks on removal of mass, so they would spill over over extended time, until forced over piecemeal by internal evolution or orbital inspiral.

This is backwards. Compact objects like neutron stars and white dwarfs are too tightly bound to be tidally disrupted. It is main sequence stars and giant stars that are more loosely bound and thus more easily torn apart. Look at the Wiki page, you can use this to calculate whether the tidal force is sufficient to disrupt the star. For stars where R* is small, the tidal disruption radius is less than the Schwarzschild radius, and the star just falls in without being tidally torn apart.

 

[snorkack]

This is backwards. Compact objects like neutron stars and white dwarfs are too tightly bound to be tidally disrupted. It is main sequence stars and giant stars that are more loosely bound and thus more easily torn apart. Look at the Wiki page, you can use this to calculate whether the tidal force is sufficient to disrupt the star. For stars where R* is small, the tidal disruption radius is less than the Schwarzschild radius, and the star just falls in without being tidally torn apart.

When it is two normal matter objects, it is the rule of thumb that the less dense object spills over. Since black hole density decreases with mass, they don´t spill over (they´re black) and just swallow anything denser than themselves, such as white dwarfs and neutron stars, when the hole is big enough.
But while white dwarfs and neutron stars can be tidally disrupted only by small enough black holes (bigger ones just swallow), their disruption is event. It is easier for a main sequence star or a giant to spill over, but how does this make a tidal disruption "event"? Since the rump shrinks on spillover, it might continue stably orbiting the black hole and accreting mass over extended time?

 

[phyzguy]

When it is two normal matter objects, it is the rule of thumb that the less dense object spills over. Since black hole density decreases with mass, they don´t spill over (they´re black) and just swallow anything denser than themselves, such as white dwarfs and neutron stars, when the hole is big enough.
But while white dwarfs and neutron stars can be tidally disrupted only by small enough black holes (bigger ones just swallow), their disruption is event. It is easier for a main sequence star or a giant to spill over, but how does this make a tidal disruption "event"? Since the rump shrinks on spillover, it might continue stably orbiting the black hole and accreting mass over extended time?

Try reading this link and study the diagrams. The star gets torn apart. Some of the star's mass gets ejected, and some of the star's mass forms an accretion disk around the SMBH. The accretion disk is extremely hot, because the star's matter gains energy as it falls in toward the SMBH. Because the accretion disk is hot, it emits radiation that we can see. Over time, the matter in the accretion disk spirals into the SMBH, so the emitted radiation decreases with time.

 

[snorkack]

Try reading this link and study the diagrams. The star gets torn apart. Some of the star's mass gets ejected, and some of the star's mass forms an accretion disk around the SMBH. The accretion disk is extremely hot, because the star's matter gains energy as it falls in toward the SMBH. Because the accretion disk is hot, it emits radiation that we can see. Over time, the matter in the accretion disk spirals into the SMBH, so the emitted radiation decreases with time.

In effect, the star is depicted as approaching the black hole on a highly eccentric orbit and being "torn apart" on a single periapse passage.
The emitted radiation will decrease on the timescale dictated by the viscosity of accretion disc. Whereas the risetime will be dictated by the timescale of periapse passage, and the requirement that it should be close enough to "tear apart" the star - that is, the freefall timescale of the star.
But my point is that it is the degenerate objects that are vulnerable to this type of "tearing apart". The reason is that main sequence stars and especially giants have mass strongly concentrated towards the core.
Just because periapse passage tears off some tenuous gas from the surface of the giant and forms an accretion disc around the black hole does not mean that the passage "tears apart" the dense core of the giant - it continues on its orbit and leaves periapse.
What next? Does the star continue to shed small amounts of surface gas on successive periapse passages? Or does the star brake to a low eccentricity orbit and shed gas continuously over a slow inspiral?

 

[Devin-M]

This is backwards. Compact objects like neutron stars and white dwarfs are too tightly bound to be tidally disrupted. It is main sequence stars and giant stars that are more loosely bound and thus more easily torn apart. Look at the Wiki page, you can use this to calculate whether the tidal force is sufficient to disrupt the star. For stars where R* is small, the tidal disruption radius is less than the Schwarzschild radius, and the star just falls in without being tidally torn apart.

In effect, the star is depicted as approaching the black hole on a highly eccentric orbit and being "torn apart" on a single periapse passage.
The emitted radiation will decrease on the timescale dictated by the viscosity of accretion disc. Whereas the risetime will be dictated by the timescale of periapse passage, and the requirement that it should be close enough to "tear apart" the star - that is, the freefall timescale of the star.
But my point is that it is the degenerate objects that are vulnerable to this type of "tearing apart". The reason is that main sequence stars and especially giants have mass strongly concentrated towards the core.
Just because periapse passage tears off some tenuous gas from the surface of the giant and forms an accretion disc around the black hole does not mean that the passage "tears apart" the dense core of the giant - it continues on its orbit and leaves periapse.
What next? Does the star continue to shed small amounts of surface gas on successive periapse passages? Or does the star brake to a low eccentricity orbit and shed gas continuously over a slow inspiral?

The gravity simulator Universe Sandbox can simulate tidal disruption events. I ran 2 simulations using a 100 solar mass black hole as the orbital parent. In the first video, it's orbited by the Sun, and the Sun gets tidally disrupted. In the second video it's orbited by a 2 solar mass neutron star, and no tidal disruption is seen. The 100 solar mass black hole has a Schwarzschild radius of 295km. The neutron star has a radius of 13.1km. The Sun has a radius of 696340km. The sun had an initial pericenter distance of 704000km. The neutron star's pericenter distance was initially set to 500km and then gradually decreased.

100 Solar Mass Black Hole vs Sun (1st Close Approach ~1:37):


100 Solar Mass Black Hole vs 2 Solar Mass Neutron Star (1st Close Approach ~11:45):

 

[phyzguy]

The gravity simulator Universe Sandbox can simulate tidal disruption events. I ran 2 simulations using a 100 solar mass black hole as the orbital parent. In the first video, it's orbited by the Sun, and the Sun gets tidally disrupted. In the second video it's orbited by a 2 solar mass neutron star, and no tidal disruption is seen. The 100 solar mass black hole has a Schwarzschild radius of 295km. The neutron star has a radius of 13.1km. The Sun has a radius of 696340km. The sun had an initial pericenter distance of 704000km. The neutron star's pericenter distance was initially set to 500km and then gradually decreased.

Thank you for running this. This backs up what I have been trying to say. The neutron star is simply too tightly bound to be tidally disrupted.

 

[snorkack]

Thank you for running this. This backs up what I have been trying to say.

But it backs up me, too. The core of Sun stays together even when mass is lost from outer envelope. On the first pass, Sun was trimed from 1 to 0,61 solar mass, on second pass from 0,61 to 0,48, on third pass no mass was lost any longer.

 

[Devin-M]

TL;DR Summary: on spaghettification of stars

I read that the gravitational tidal force of a nearby blackhole can twist and tear apart stars, sometimes even in a matter of days. The 'matter of days' part peaked my interest, and I figured I would ask here before looking it up.

In the simulation, it only took about 8 minutes for the Sun to lose ~40% of its mass (& 44% of its radius) on the 1st pass.