[Eternal Student (OP)]

Hi.

   I can't google my result because I just can't recall what it is called.
   Assume you have a small spherical thing which is held together by little more than just mutual gravitational attraction of it's particles.
   that small thing starts to get close to a bigger spherical thing.    Gravitational forces act on the small thing.  There is a certain point (a radius from the big object) where the  small sphere is pulled on the left side too much by the big sphere.   The force of attraction of that left side to the centre of the small sphere is insufficient to cancel that and the small object starts to be torn apart.

   There's a name for this process and I can't remember what it is,  and/or there's a scientist's name for the distance where this happens  -   the   Smith's radius  etc.
   If you remember, please let me know what the names are, thank you.

Best Wishes.

[Eternal Student (OP)]

Found it:       Roche Limit.
   I can't delete the original post,  I've tried.  A moderator should feel free to remove this thread.

[alancalverd]

On the contrary, it's good new information for some of us! Never thought about it before, but it has similarities with the liquid drop model of a nucleus and the assimilation of bubbles and droplets - all important stuff.

[Bored chemist]

If we are looking at the vocabulary in this field, I always think "Spaghettification" is a great word.

[evan_au]

Roche Limit.

There is a series of science fiction stories set on "Rocheworld" - a double planet which almost at the point of being torn apart, set against the very peculiar geography that might result...

https://en.wikipedia.org/wiki/Rocheworld

[Halc]

There is a series of science fiction stories set on "Rocheworld" - a double planet which almost at the point of being torn apart, set against the very peculiar geography that might result...

Phobos is currently like that, slowly spinning into Mars due to tidal drag. It already has cracks in it, beginning to break apart, but it isn't expected to actually reach the Roche limit for some tens of millions of years.
Once it does, it will break up and Mars will have nice rings of its own.

[Eternal Student (OP)]

Hi.

    Well I was recently reading about the Large (and also the Small) Magellanic Cloud in our Milky Way.
As you may know, this object is thought to be a dwarf galaxy in its own right that has started to merge with our galaxy.   Anyway, it's falling in toward the centre of our galaxy due to what is called "friction", although its more like gravitational braking rather than anything like air resistance.    Very roughly speaking - as the cloud moves through diffuse gas at the outer edge of our galaxy, the gravity of the Large Magellanic Cloud (LMC) deflects gas particles from our Galaxy toward itself.   This will end up causing some increased density in the wake of where the cloud has been.   This over-dense region behind the LMC is then a source of gravity acting on the LMC and will effectively create a net braking force on it.    Finally, since the LMC is experiencing this net torque applied to it, it's losing angular momentum, the orbit is decaying and the LMC is sinking down to the centre of our galaxy.

    Now, the bit that was relevant is that while the LMC is sinking, it's also being stripped of its own gas.   So there won't be much of the LMC left as one big blob by the time it's near the centre of galaxy anyway.   I was just suspicious that the stripping process might be much the same as hitting the Roche Limit  - but it doesn't seem to be.
    The gas stripping is happening at a distance from the centre of our galaxy which is much larger than the Roche limit.  I'm not sure why, except that the  Small and Large magellanic clouds are very gassy (i.e. not at all solid or rigid) and thus easily shredded.   They just do seem to be breaking up in a significantly different way.   Unlike the thing described by @Halc  just a moment ago,  the LMC and SMC aren't breaking up in such a sudden and dramatic way.  I suppose they are just being stretched and deformed progressively.   Anyway, that's what was troubling me a bit yesterday - why aren't they just doing what we know about the Roche limit, presumably it is just that the objects aren't at all rigid.   The Roche limit never gets mentioned when discussing the stripping of gas from these things but it just does seem that they should be related.
   

gas trails.jpg (86.3 kB . 957x503 - viewed 2403 times)

   Image based on this source:   https://esahubble.org/news/heic1314/

Best Wishes.

[Janus]

Hi.

    Well I was recently reading about the Large (and also the Small) Magellanic Cloud in our Milky Way.
As you may know, this object is thought to be a dwarf galaxy in its own right that has started to merge with our galaxy.   Anyway, it's falling in toward the centre of our galaxy due to what is called "friction", although its more like gravitational braking rather than anything like air resistance.    Very roughly speaking - as the cloud moves through diffuse gas at the outer edge of our galaxy, the gravity of the Large Magellanic Cloud (LMC) deflects gas particles from our Galaxy toward itself.   This will end up causing some increased density in the wake of where the cloud has been.   This over-dense region behind the LMC is then a source of gravity acting on the LMC and will effectively create a net braking force on it.    Finally, since the LMC is experiencing this net torque applied to it, it's losing angular momentum, the orbit is decaying and the LMC is sinking down to the centre of our galaxy.

    Now, the bit that was relevant is that while the LMC is sinking, it's also being stripped of its own gas.   So there won't be much of the LMC left as one big blob by the time it's near the centre of galaxy anyway.   I was just suspicious that the stripping process might be much the same as hitting the Roche Limit  - but it doesn't seem to be.
    The gas stripping is happening at a distance from the centre of our galaxy which is much larger than the Roche limit.  I'm not sure why, except that the  Small and Large magellanic clouds are very gassy (i.e. not at all solid or rigid) and thus easily shredded.   They just do seem to be breaking up in a significantly different way.   Unlike the thing described by @Halc  just a moment ago,  the LMC and SMC aren't breaking up in such a sudden and dramatic way.  I suppose they are just being stretched and deformed progressively.   Anyway, that's what was troubling me a bit yesterday - why aren't they just doing what we know about the Roche limit, presumably it is just that the objects aren't at all rigid.   The Roche limit never gets mentioned when discussing the stripping of gas from these things but it just does seem that they should be related.
   

gas trails.jpg (86.3 kB . 957x503 - viewed 2403 times)

   Image based on this source:   https://esahubble.org/news/heic1314/

Best Wishes.

This action seems to be a "Roche Lobe" effect : https://en.wikipedia.org/wiki/Roche_lobe

You get the same type of thing in some binary systems, where a white dwarf will siphon mass from its companion.  When enough mass accretes, you get a type Ia supernova.

[Halc]

Anyway, it's falling in toward the centre of our galaxy due to what is called "friction", although its more like gravitational braking rather than anything like air resistance.

Actually it's more like air resistance. The gas physically collides with MW stuff moving at a different velocity. Both MCs move more or less at the same speed as anything out there, but not the same velocity. If their speed was greater, they'd be moving faster than orbital velocity and would move away. But they're not orbiting at the same axis as the rotation axis of the MW, so they cross the plane of the MW once in a while, and that's where the friction takes place, each smacking gas away from its normal path. This accounts for most of the trails seen in the article.

Tidal forces (a gravity effect), if significant enough, would leave a trail of gas on both sides, just like moons torn apart when crossing the Roche limit lose material in both directions. The gravity wake thing of which you speak would slow all of the dwarf galaxy (and it does), not just tearing off the outer gas layers. But friction is an EM effect, and that does tear off the particles that physically collide with material moving at a different velocity.

Finally, since the LMC is experiencing this net torque applied to it, it's losing angular momentum, the orbit is decaying and the LMC is sinking down to the centre of our galaxy.
...
So there won't be much of the LMC left as one big blob by the time it's near the centre of galaxy anyway.

Yes, it's getting closer, but the effect mostly does what happens to all material near a galaxy: The forces tend to bend the orbit into the plane of the rest of the galaxy. Once that happens, the friction mostly goes away and further sinking ceases, so it isn't particularly expected that either MC core will find its way all the way to the center in any short term, not any more than will any other star. Earth is not currently sinking into the sun since friction is minimal at this point. But all the planets will eventually, given the continued stability of the solar system.

Of course the impending collision with Andromeda is really going to shake at that up and some stars will lose a lot of momentum and get closer in, while many will get flung away to great distances, possibly at escape velocity. It is unclear if either MC will be slowed, flung out, or simple be smashed up in the process.

I was just suspicious that the stripping process might be much the same as hitting the Roche Limit  - but it doesn't seem to be.

Friction and tidal forces are different things, so not the same.

The gas stripping is happening at a distance from the centre of our galaxy which is much larger than the Roche limit.

I'm not sure if a galaxy has a Roche limit. Sure, for really big things like galaxies, they tear each other up like the Andromeda thing will do. But stars orbit insanely close to the SMBH at the center (approaching about as close as Neptune to our sun) all without being torn apart, so there's not much tidal disruption to reasonably compact things. There are stars that are much closer to small black holes, and yes, you can see them getting material torn away.

I'm not sure why, except that the  Small and Large magellanic clouds are very gassy

Aren't we all...

The Roche limit never gets mentioned when discussing the stripping of gas from these things but it just does seem that they should be related.

Again, material would be stripped more or less equally in both directions if it was due to tidal forces, so I don't think the any Roche limit plays much of a role in this case.

I see Janus has posted about a Roche lobe which describes a deformation of both orbiting things (usually more comparable in mass) due to the non-circular equi-potential lines that form, which can result in a siphoning effect of material from one to the other.

[Eternal Student (OP)]

Hi.

    I've been reading a bit more and considering the above replies.

Actually it's more like air resistance. The gas physically collides with MW stuff moving at a different velocity......

    Yes, seems sensible and well considered.  It's pretty much what I would have thought of when considering what "friction" usually is.  Although I would not have considered the LMC moving across the plane of the MW disc or half of what you've discussed.   Thank you for your thoughts and insight.
     I'm sure it happens and explains a portion of what is observed.  However, the text I was reading hardly bothered to discuss it.   I wasn't sure why but some of it is beginning to make sense.

    I should start by correcting something I said in an earlier post:   

Very roughly speaking - as the cloud moves through diffuse gas at the outer edge of our galaxy, the gravity of the Large Magellanic Cloud (LMC) deflects gas particles from our Galaxy toward itself.

    It's not any ordinary gas the LMC is moving through.   The important point is that there is Dark matter out there, the edge of the MW galaxy is the dark matter halo.  That was an oversight on my part and has somewhat changed my view of what sort of "friction" is possible.

But friction is an EM effect, and that does tear off the particles that physically collide with material moving at a different velocity.

     There isn't much electromagnetic interaction.   Gravitational interaction with DM could be all you will get.   Although we don't have detailed studies of how DM behaves, it is very likely that ordinary matter doesn't experience much conventional drag as it moves through DM.  There are few forces with which it can interact.
     Just to emphasize how little conventional drag there can be - an old view is that DM can pass through ordinary matter and neither one notices or experiences anything:   Dark matter ...... nearly always passes right through normal matter.  ( first paragraph of this article   https://www.scientificamerican.com/article/does-dark-matter-encircle-earth/ ).

Anyway, the online lecture I was looking at and most of the articles I've read over the last day all seem to be focusing on what I would call gravitational drag and is much as described in an earlier post.   This is called "friction" in most of those texts but at least half of them do put "friction" in quotation marks.   I suppose any decelerating force that one body experiences on passing another can be called friction, it's just that gravity is insignificant and not at all what I would usually think of as a friction force.   For a DM halo, it's a significant friction force because that's all there is.
 - - - - - - - - -

@Janus ,   thanks for your input.   I'm glad I'm not the only one who thinks the process is very much like some sort of Roche limit effect.   I have looked at the link you provided but this post is already long and I'm not going to say much more.   Overall, I do think the stripping of gas from the LMC (or SMC) is very much like gas being transferred from one thing (e.g. star) to another thing (e.g. another star).   However, it's messy, mainly because the LMC is already in or inside the other object (the outer edge or Halo of the MW).   We have a superficial similarity between the effects but no easy way to use the machinery or mathematics for one effect on the other.
    The mathematics, which is just a set of approximations as far as I can see, starts with a simple model similar to how you might consider a Roche limit disintegration but quickly goes it's own way.
   
Thank you to everyone for their input.

Best Wishes.

[Halc]

It's not any ordinary gas the LMC is moving through.   The important point is that there is Dark matter out there, the edge of the MW galaxy is the dark matter halo.  That was an oversight on my part and has somewhat changed my view of what sort of "friction" is possible.

Sorry, but the matter isn't very dark if going through a cloud of it produces enough physical friction to tear off huge clouds of material.

There isn't much electromagnetic interaction.   Gravitational interaction with DM could be all you will get.

That would, at best, slow the entire SMC, not just the gas parts. The SA article you link suggests a very high amount of EM interaction, making it ordinary matter that's only somewhat less interactive with baryonic matter.

The SA article suggests an insane amount of DM interaction, suggesting that much of geothermal energy of the planets is due to said friction. There seems to be no mathematics to back that up because the amount of DM they suggest hasn't enough kinetic energy to produce the sort of heat they're talking about. If the DM was interacting, it would all be stopped by now and residing inside Earth, not in a cloud around it. No mechanism is suggested explaining how DM came to be in a cloud around any planet in the first place. How was it captured? I mean, comets can be captured if they physically hit the planet, but gravity can't do it. A comet that passes through Earth isn't going to stick around. There's all sorts of problems with that pop article that are not explained. Maybe the paper on which it reports does better.

The article also posits regular DM/anit-DM annihilation, but without explaining where the energy would go. Would the dark mass be converted to dark light? It reminds me of this quote:

Every time you try and operate these weird black controls that are labeled in black on a black background, a little black light lights up in black to let you know you’ve done it.

-- The Restaurant At The End Of The Universe

Although we don't have detailed studies of how DM behaves, it is very likely that ordinary matter doesn't experience much conventional drag as it moves through DM.  There are few forces with which it can interact.

Considering that gravity isn't a force, that leaves no forces with which it interacts, except possibly very weakly. They're searching for evidence of such an interaction, a bit like the early efforts to make a neutrino detector which is another weakly interactive particle, but one that is known, measured, and predicted before they were found. Neutrinos also pass easily through planets and stars without slowing.

This is called "friction" in most of those texts but at least half of them do put "friction" in quotation marks.   I suppose any decelerating force that one body experiences on passing another can be called friction, it's just that gravity is insignificant and not at all what I would usually think of as a friction force.

Friction causes heat. Gravity drag might have a similar momentum-transfer effect, but would not produce any heat unless you put a lot of scare quotes around ""heat"". I mean, Jupiter's orbit has lost energy due to the gravity drag of all the spacecraft that have used it for a gravitational theft of momentum. Both Voyagers did this for instance. Nothing warmed up because of that. Few would call it 'friction'.

Overall, I do think the stripping of gas from the LMC (or SMC) is very much like gas being transferred from one thing (e.g. star) to another thing (e.g. another star).

Yes, the gas stream looks a lot like the pictures of a small BH siphoning material off one side (not both) of some companion star, but the star thing (Roche lobe) is a continuous thing, and the article you first quoted (the one with the Hubble picture) suggests most of the cloud was stripped away in a more brief event some 2 billion years back. That part is not consistent with a Roche lobe.

[Eternal Student (OP)]

Hi.

   Crumbs, you read the Scientific American article.   That was an old article 2009 and only about half of what was in it is still thought to be reasonable.   I only took a snippet out of it because I really did want to exhibit a very simple and old view of DM.   I really didn't mean to suggest you should spend the time reading it.  I am so very sorry for using your time (although all knowledge is usefull, which is what I'm going to tell myself so I don't feel too bad about the time spent - but I am very sorry).   
   Yes, there's a lot in that article that seems quite wrong or unlikely now.

Let's start by discussing the "fricition" with quotation marks.

Few would call it 'friction'.

    I'm on the same side as you and agree with a lot of what you've said BUT it's not my decision.   Astronomers and Astrophysicists have established some terminology like "ice", "metal" and "dynamical friction" that are slightly different to other areas of physics.   Everything other than Hydrogen is a metal, ice is not just solid water and "friction" is whatever they say it is.
   The term "dynamical friction" is in use, it's described here:  https://en.wikipedia.org/wiki/Dynamical_friction
of which the most salient part is:
In astrophysics, dynamical friction or Chandrasekhar friction, sometimes called gravitational drag, is loss of momentum and kinetic energy of moving bodies through gravitational interactions with surrounding matter in space. It was first discussed in detail by Subrahmanyan Chandrasekhar in 1943.
    It's a surprisingly old term but seems to have had a resurgence recently (let's say since about the time of that old Scientific American article) when things like the passage of the LMC through the DM halo were being considered.  As outlined in that Wikipedia article, there are many ways to think about it  (just to be clear:  you do not have to read that one but a quick glance over won't be time wasted).   A lot of modern articles are calling it "friction"  with or without the quotation marks.
   That's it - it's not my fault, it's what they call friction but I will probably continue to call gravitational drag.

Friction causes heat. Gravity drag might have a similar momentum-transfer effect, but would not produce any heat unless....

    Not my fault,   the "dynamical friction" used here does not need to cause heating.   Neither is this friction the direct cause of gas being stripped off the LMC, I may get to that later.   We could write a letter to the...  ?...Astronomical society? and ask them not to call this thing friction but life is too short.   We just have to be pragmatic about it all:   Stuff moves through stuff and is sometimes slowed down - that looks like friction and is a name they are happy to continue using, even if gravity was the only thing slowing it down.

Considering that gravity isn't a force, that leaves no forces with which it interacts,

   As I mentioned,  a lot of the mathematics I have seen is just an approximation.   Newtonian gravity is used in that mathematics.

-----------
   The post is getting long.   Other than putting the mathematics down here, I don't think there is much I can do that's useful.   I don't especially need to take up your time or forum space by doing that.  I'm grateful for the discussion that has already been provided.

Best Wishes.

[evan_au]

A month ago, I had the privilege of seeing 47 Tucanae (in the Toucan constellation) from a dark-sky park - it's a spectacular globular cluster.
- It's about 7% of the distance to the small Magellenic Cloud, which means that tidal effects from the Milky Way will be around 3,000 times stronger.
- However, this very dense concentration of stars has (so far) resisted tidal disruption, perhaps aided by the presence of a central black hole (not yet firmly established).
- It is possible that this is the core of a small galaxy, and that all of the outer stars have already been stripped off.
- The Gaia satellite has been able to spot stellar streams from other torn-apart globular clusters, but to date none have been detected from 47 Tucanae

This video from ESA starts with the Small Magellanic Cloud, and then zooms in on 47 Tucanae