APOD: Perseus Galaxy Cluster from Euclid (2023 Nov 08)

[Chris Peterson]

Anne said above:



I don’t know about that. I would think that planets and life in areas of lesser stellar density that “collide” will not experience much orbital disruption. I think stars would have to pass each other well within each others’ Oort clouds to cause any altering of cometary bodies that might one day wreak havoc on inner planets.

Although a bunch of comets thrown into an inner system could certainly be problematic, that's not the real problem. The real problem is tidal forces (and not "terrible tidal forces" at all, but trivially small ones) that perturb planetary orbits. No planetary system is stable, and it takes very little to tweak orbits. And it doesn't take much of an orbital change to massively disrupt the ability of a planet to sustain life. For solar mass bodies, and solar systems similar to our own, the magic distance is about a light year. Anything passing closer than that has a good chance of tweaking orbits enough to end life on planets. That distance is probably common over hundreds of millions of years even in low density parts of collisions. So yeah... as far as complex life is concerned, galactic mergers are probably bad news.

Ok, thanks for weighing in. I knew you would. :ssmile: And I stand corrected. But is there really enough of a gravitational effect from a star passing within 1 ly of the Sun to be able to - however minutely - perturb its planets' orbits? Let's see now: if the force of the Sun's gravity on Saturn is F at Saturn's distance of 10 AU, and a Sun-like star approaches within 1 ly (63000 AU), that would be 6300 times as far, and so its gravitational force would be 6300*6300 less. That's 40 million times smaller! But I guess even that tiny amount could perturb things significantly over a hundred million years?

The time scale is likely only a few thousand years for a passing star. But the issue isn't simple gravitational force, it's tidal forces. Differentials. You pull on Jupiter just a little differently than you pull on Saturn, you make tiny changes to the semimajor axes or eccentricities of each, and over millions of years those two tweak each other a little differently, with changes that cascade through the whole planetary system.

[Mischa Schirmer]

But is there really enough of a gravitational effect from a star passing within 1 ly of the Sun to be able to - however minutely - perturb its planets' orbits?

Planetary orbits in a star system are inherently chaotic in nature, and thus small changes could have large effects. Our solar system is no an exception to this.

https://en.wikipedia.org/wiki/Stability ... lar_System

[johnnydeep]

Although a bunch of comets thrown into an inner system could certainly be problematic, that's not the real problem. The real problem is tidal forces (and not "terrible tidal forces" at all, but trivially small ones) that perturb planetary orbits. No planetary system is stable, and it takes very little to tweak orbits. And it doesn't take much of an orbital change to massively disrupt the ability of a planet to sustain life. For solar mass bodies, and solar systems similar to our own, the magic distance is about a light year. Anything passing closer than that has a good chance of tweaking orbits enough to end life on planets. That distance is probably common over hundreds of millions of years even in low density parts of collisions. So yeah... as far as complex life is concerned, galactic mergers are probably bad news.

Ok, thanks for weighing in. I knew you would. And I stand corrected. But is there really enough of a gravitational effect from a star passing within 1 ly of the Sun to be able to - however minutely - perturb its planets' orbits? Let's see now: if the force of the Sun's gravity on Saturn is F at Saturn's distance of 10 AU, and a Sun-like star approaches within 1 ly (63000 AU), that would be 6300 times as far, and so its gravitational force would be 6300*6300 less. That's 40 million times smaller! But I guess even that tiny amount could perturb things significantly over a hundred million years?

The time scale is likely only a few thousand years for a passing star. But the issue isn't simple gravitational force, it's tidal forces. Differentials. You pull on Jupiter just a little differently than you pull on Saturn, you make tiny changes to the semimajor axes or eccentricities of each, and over millions of years those two tweak each other a little differently, with changes that cascade through the whole planetary system.

Ah. Yet, the planets of our Solar System have been stable enough over a few billion years to still allow life to arise and flourish on Earth, though the hapless dinosaurs might feel a bit differently.

But will we be significantly less likely to survive during a merger with Andromeda than without? Apparently, Gleise 710 has a 95% chance of approaching within 17000 AU of the Sun in 15 My. Thanks to Mischa Schirmer above for the link to the Wikipedia article about planetary system instability, and which referenced this study:

New stellar encounters discovered in the second Gaia data release
C.A.L. Bailer-Jones, J. Rybizki, R. Andrae, M. Fouesneau (Max Planck Institute for Astronomy, Heidelberg)

Passing stars may play an important role in the evolution of our solar system. We search for close stellar encounters to the Sun among all 7.2 million stars in Gaia-DR2 that have six-dimensional phase space data. We characterize encounters by integrating their orbits through a Galactic potential and propagating the correlated uncertainties via a Monte Carlo resampling. After filtering to remove spurious data, we find 694 stars that have median (over uncertainties) closest encounter distances within 5 pc, all occurring within 15 Myr from now. 26 of these have at least a 50% chance of coming closer than 1 pc (and 7 within 0.5 pc), all but one of which are newly discovered here. We further confirm some and refute several other previously-identified encounters, confirming suspicions about their data. The closest encounter in the sample is Gl 710, which has a 95% probability of coming closer than 0.08 pc (17 000 AU). Taking mass estimates from Gaia astrometry and multiband photometry for essentially all encounters, we find that Gl 710 also has the largest impulse on the Oort cloud.

[johnnydeep]

But is there really enough of a gravitational effect from a star passing within 1 ly of the Sun to be able to - however minutely - perturb its planets' orbits?

Planetary orbits in a star system are inherently chaotic in nature, and thus small changes could have large effects. Our solar system is no an exception to this.

https://en.wikipedia.org/wiki/Stability ... lar_System

Thanks! Great article. I referenced in my reply just above.

[Chris Peterson]

Ok, thanks for weighing in. I knew you would. :ssmile: And I stand corrected. But is there really enough of a gravitational effect from a star passing within 1 ly of the Sun to be able to - however minutely - perturb its planets' orbits? Let's see now: if the force of the Sun's gravity on Saturn is F at Saturn's distance of 10 AU, and a Sun-like star approaches within 1 ly (63000 AU), that would be 6300 times as far, and so its gravitational force would be 6300*6300 less. That's 40 million times smaller! But I guess even that tiny amount could perturb things significantly over a hundred million years?

The time scale is likely only a few thousand years for a passing star. But the issue isn't simple gravitational force, it's tidal forces. Differentials. You pull on Jupiter just a little differently than you pull on Saturn, you make tiny changes to the semimajor axes or eccentricities of each, and over millions of years those two tweak each other a little differently, with changes that cascade through the whole planetary system.

Ah. Yet, the planets of our Solar System have been stable enough over a few billion years to still allow life to arise and flourish on Earth, though the hapless dinosaurs might feel a bit differently. :ssmile:

But will we be significantly less likely to survive during a merger with Andromeda than without? Apparently, Gleise 710 has a 95% chance of approaching within 17000 AU of the Sun in 15 My. Thanks to Mischa Schirmer above for the link to the Wikipedia article about planetary system instability, and which referenced this study:

New stellar encounters discovered in the second Gaia data release
C.A.L. Bailer-Jones, J. Rybizki, R. Andrae, M. Fouesneau (Max Planck Institute for Astronomy, Heidelberg)

Passing stars may play an important role in the evolution of our solar system. We search for close stellar encounters to the Sun among all 7.2 million stars in Gaia-DR2 that have six-dimensional phase space data. We characterize encounters by integrating their orbits through a Galactic potential and propagating the correlated uncertainties via a Monte Carlo resampling. After filtering to remove spurious data, we find 694 stars that have median (over uncertainties) closest encounter distances within 5 pc, all occurring within 15 Myr from now. 26 of these have at least a 50% chance of coming closer than 1 pc (and 7 within 0.5 pc), all but one of which are newly discovered here. We further confirm some and refute several other previously-identified encounters, confirming suspicions about their data. The closest encounter in the sample is Gl 710, which has a 95% probability of coming closer than 0.08 pc (17 000 AU). Taking mass estimates from Gaia astrometry and multiband photometry for essentially all encounters, we find that Gl 710 also has the largest impulse on the Oort cloud.

We're lucky to be in a low density part of the galaxy. I doubt there's any complex life in the central bulge! And complex life on Earth is only a few hundred million years old, not billions. It's probably a lot easier for single cells floating around in oceans to deal with orbital shifts than it is land animals.

[johnnydeep]

The time scale is likely only a few thousand years for a passing star. But the issue isn't simple gravitational force, it's tidal forces. Differentials. You pull on Jupiter just a little differently than you pull on Saturn, you make tiny changes to the semimajor axes or eccentricities of each, and over millions of years those two tweak each other a little differently, with changes that cascade through the whole planetary system.

Ah. Yet, the planets of our Solar System have been stable enough over a few billion years to still allow life to arise and flourish on Earth, though the hapless dinosaurs might feel a bit differently.

But will we be significantly less likely to survive during a merger with Andromeda than without? Apparently, Gleise 710 has a 95% chance of approaching within 17000 AU of the Sun in 15 My. Thanks to Mischa Schirmer above for the link to the Wikipedia article about planetary system instability, and which referenced this study:

New stellar encounters discovered in the second Gaia data release
C.A.L. Bailer-Jones, J. Rybizki, R. Andrae, M. Fouesneau (Max Planck Institute for Astronomy, Heidelberg)

Passing stars may play an important role in the evolution of our solar system. We search for close stellar encounters to the Sun among all 7.2 million stars in Gaia-DR2 that have six-dimensional phase space data. We characterize encounters by integrating their orbits through a Galactic potential and propagating the correlated uncertainties via a Monte Carlo resampling. After filtering to remove spurious data, we find 694 stars that have median (over uncertainties) closest encounter distances within 5 pc, all occurring within 15 Myr from now. 26 of these have at least a 50% chance of coming closer than 1 pc (and 7 within 0.5 pc), all but one of which are newly discovered here. We further confirm some and refute several other previously-identified encounters, confirming suspicions about their data. The closest encounter in the sample is Gl 710, which has a 95% probability of coming closer than 0.08 pc (17 000 AU). Taking mass estimates from Gaia astrometry and multiband photometry for essentially all encounters, we find that Gl 710 also has the largest impulse on the Oort cloud.

We're lucky to be in a low density part of the galaxy. I doubt there's any complex life in the central bulge! And complex life on Earth is only a few hundred million years old, not billions. It's probably a lot easier for single cells floating around in oceans to deal with orbital shifts than it is land animals.

Yes, but "life, uh, finds a way" according to Jeff Goldblum:

Click to play embedded YouTube video.

[orin stepanek]

Kitty sees all; knows all!

Someone, Someday, will find the secret to dark matter and dark
energy!

[Ann]

Ok, thanks for weighing in. I knew you would. And I stand corrected. But is there really enough of a gravitational effect from a star passing within 1 ly of the Sun to be able to - however minutely - perturb its planets' orbits? Let's see now: if the force of the Sun's gravity on Saturn is F at Saturn's distance of 10 AU, and a Sun-like star approaches within 1 ly (63000 AU), that would be 6300 times as far, and so its gravitational force would be 6300*6300 less. That's 40 million times smaller! But I guess even that tiny amount could perturb things significantly over a hundred million years?

The time scale is likely only a few thousand years for a passing star. But the issue isn't simple gravitational force, it's tidal forces. Differentials. You pull on Jupiter just a little differently than you pull on Saturn, you make tiny changes to the semimajor axes or eccentricities of each, and over millions of years those two tweak each other a little differently, with changes that cascade through the whole planetary system.

Ah. Yet, the planets of our Solar System have been stable enough over a few billion years to still allow life to arise and flourish on Earth, though the hapless dinosaurs might feel a bit differently.

But will we be significantly less likely to survive during a merger with Andromeda than without? Apparently, Gleise 710 has a 95% chance of approaching within 17000 AU of the Sun in 15 My. Thanks to Mischa Schirmer above for the link to the Wikipedia article about planetary system instability, and which referenced this study:

New stellar encounters discovered in the second Gaia data release
C.A.L. Bailer-Jones, J. Rybizki, R. Andrae, M. Fouesneau (Max Planck Institute for Astronomy, Heidelberg)

Passing stars may play an important role in the evolution of our solar system. We search for close stellar encounters to the Sun among all 7.2 million stars in Gaia-DR2 that have six-dimensional phase space data. We characterize encounters by integrating their orbits through a Galactic potential and propagating the correlated uncertainties via a Monte Carlo resampling. After filtering to remove spurious data, we find 694 stars that have median (over uncertainties) closest encounter distances within 5 pc, all occurring within 15 Myr from now. 26 of these have at least a 50% chance of coming closer than 1 pc (and 7 within 0.5 pc), all but one of which are newly discovered here. We further confirm some and refute several other previously-identified encounters, confirming suspicions about their data. The closest encounter in the sample is Gl 710, which has a 95% probability of coming closer than 0.08 pc (17 000 AU). Taking mass estimates from Gaia astrometry and multiband photometry for essentially all encounters, we find that Gl 710 also has the largest impulse on the Oort cloud.

Stars passing close to us are worrying, there's no question about that.

But what happens during a galactic collision? When two massive galaxies collide? The galaxies, particularly the least massive one, are torn apart and ripped open because of the terrible tidal forces ripping through the entire galaxies. No part of any of the galaxies, particularly the least massive ones, are spared.

Take a look at the simulation in the video below. It's a simulation of what will happen when the Milky Way and Andromeda collide and merge. Do you really think that any solar system in the Milky Way will be spared and stay undisturbed and orderly during such a collision?

Click to play embedded YouTube video.

Bear in mind that the Milky Way is believed to collide with the Large Magellanic Cloud before we ram Andromeda, and the Magellanic train wreck is going to be very significant, too. Can we really be sure that the the Earth's orbit around the Sun is going to stay unaffected when the LMC crashes into us?

Ann

[Mischa Schirmer]

The outer solar system is definitely rather chaotic in terms of orbital stability and history. There are indications that Jupier and saturn, or Uranus and neptune, must have switched orbits at some point to explain the large-scale distribution of asteroids and TNOs. Search for the "NICE model", "planet migration", you can find some long-term simulations. A short-term simulation of about 1 million years can be found here. It doesn't show what did happen , or would happen, to the solar system, but it shows that the gravitational interactions between the gas giants lend quite some instability to their orbits. While the inner-planet orbits are fairly isolated and more stable, they are not entirely immune.
I'm not an expert and I don't know how valid these specific computations are, but in general this appears to what scientists get when running this through a computer.
https://www.youtube.com/watch?v=kx4VNJZC61I

What I'm saying is that these effects happen on time scales much shorter than interactions with neighboring galaxies, big or small.

[Chris Peterson]

The time scale is likely only a few thousand years for a passing star. But the issue isn't simple gravitational force, it's tidal forces. Differentials. You pull on Jupiter just a little differently than you pull on Saturn, you make tiny changes to the semimajor axes or eccentricities of each, and over millions of years those two tweak each other a little differently, with changes that cascade through the whole planetary system.

Ah. Yet, the planets of our Solar System have been stable enough over a few billion years to still allow life to arise and flourish on Earth, though the hapless dinosaurs might feel a bit differently. :ssmile:

But will we be significantly less likely to survive during a merger with Andromeda than without? Apparently, Gleise 710 has a 95% chance of approaching within 17000 AU of the Sun in 15 My. Thanks to Mischa Schirmer above for the link to the Wikipedia article about planetary system instability, and which referenced this study:

New stellar encounters discovered in the second Gaia data release
C.A.L. Bailer-Jones, J. Rybizki, R. Andrae, M. Fouesneau (Max Planck Institute for Astronomy, Heidelberg)

Passing stars may play an important role in the evolution of our solar system. We search for close stellar encounters to the Sun among all 7.2 million stars in Gaia-DR2 that have six-dimensional phase space data. We characterize encounters by integrating their orbits through a Galactic potential and propagating the correlated uncertainties via a Monte Carlo resampling. After filtering to remove spurious data, we find 694 stars that have median (over uncertainties) closest encounter distances within 5 pc, all occurring within 15 Myr from now. 26 of these have at least a 50% chance of coming closer than 1 pc (and 7 within 0.5 pc), all but one of which are newly discovered here. We further confirm some and refute several other previously-identified encounters, confirming suspicions about their data. The closest encounter in the sample is Gl 710, which has a 95% probability of coming closer than 0.08 pc (17 000 AU). Taking mass estimates from Gaia astrometry and multiband photometry for essentially all encounters, we find that Gl 710 also has the largest impulse on the Oort cloud.

Stars passing close to us are worrying, there's no question about that.

But what happens during a galactic collision? When two massive galaxies collide? The galaxies, particularly the least massive one, are torn apart and ripped open because of the terrible tidal forces ripping through the entire galaxies. No part of any of the galaxies, particularly the least massive ones, are spared.

Take a look at the simulation in the video below. It's a simulation of what will happen when the Milky Way and Andromeda collide and merge. Do you really think that any solar system in the Milky Way will be spared and stay undisturbed and orderly during such a collision?

Click to play embedded YouTube video.

Bear in mind that the Milky Way is believed to collide with the Large Magellanic Cloud before we ram Andromeda, and the Magellanic train wreck is going to be very significant, too. Can we really be sure that the the Earth's orbit around the Sun is going to stay unaffected when the LMC crashes into us?

Ann

No. these forces are incredibly small when galaxies collide. A galaxy collision is one of the calmest, gentlest events in the universe. The forces that are involved would probably be far too small for us to be able to measure were we in the very middle of the collisions. What you see in these simulations, what we see in our images, is the consequence, over millions of years, of minuscule forces slightly altering the shapes of stellar orbits around galactic centers. These are not train wrecks, they are the lightest of breezes.

[Mischa Schirmer]

Hi Chris, major mergers (collisions of galaxies that have approximately equal mass) are some of the most profound events in the history of individual galaxies. Think of two rotating spiral galaxies, each with their considerable angular momentum falling toward each other from infinity. There is additional angular momentum if the trajectories have a non-zero impact parameter, that is the collision is not head-on. These angular momenta are entirely redistributed once the collision process is over. The end result is that virtually all stars in each galaxy have their orbit history completely erased or "thermalised", that is one cannot reconstruct the original orbits anymore from the post-collision state. A lot of stars are slingshot away, and the remainder forms an elliptical galaxy. This is a profound change.

Typical collision speeds are several hundred km/s within a galaxy cluster, and can exceed 1000 km/s in case of merging clusters. Indeed stars themselves hardly collide, but the gas in the galaxies does collide. The collision speed is much greater than the typical speed of the gas, that is the collision is super-sonic and forms a lot of shocks, resulting in violent star bursts for a - comparatively short - amount of time. It is not uncommon that after a merger a galaxy is unable to form stars anymore, as all the gas has been turned into stars, or was expelled and became part of the hot cluster halo gas.

On a galaxy scale, the only more violent events are active galactic nuclei and quasars.

In terms of planetary orbits, let me speculate a bit.
Looking at the global gravitational potential of the galaxies: The stars will for sure change their orbits entirely. However, this happens on time-scales much longer than the Lyapunov time of a planetary system. That is the time-scale on which a planetary system exhibits chaotic or unpredictable behaviour. In case of the inner solar system that is about 1-5 million years, but at least in our solar system the inner orbits have been remarkably stable globally https://journals.aps.org/prx/pdf/10.110 ... .13.021018. Going out on a limb, I'd say that an inner solar system might have a chance to survive a major galactic merger since it has ample of time to dissipate the gentle external disturbance among their orbits, looking at a big scale.

Looking at a small scale: a nearby encounter with another star certainly could have the potential to disturb a planetary system at a greater scale, as the interaction time-scale becomes more comparable with the planetary system's chaotic time scale. At least the orbits of the outer planets, and those in turn those of the inner planets should one of the outer ones decide to pay a visit to its inner siblings.

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

[Chris Peterson]

Hi Chris, major mergers (collisions of galaxies that have approximately equal mass) are some of the most profound events in the history of individual galaxies. Think of two rotating spiral galaxies, each with their considerable angular momentum falling toward each other from infinity. There is additional angular momentum if the trajectories have a non-zero impact parameter, that is the collision is not head-on. These angular momenta are entirely redistributed once the collision process is over. The end result is that virtually all stars in each galaxy have their orbit history completely erased or "thermalised", that is one cannot reconstruct the original orbits anymore from the post-collision state. A lot of stars are slingshot away, and the remainder forms an elliptical galaxy. This is a profound change.

Typical collision speeds are several hundred km/s within a galaxy cluster, and can exceed 1000 km/s in case of merging clusters. Indeed stars themselves hardly collide, but the gas in the galaxies does collide. The collision speed is much greater than the typical speed of the gas, that is the collision is super-sonic and forms a lot of shocks, resulting in violent star bursts for a - comparatively short - amount of time. It is not uncommon that after a merger a galaxy is unable to form stars anymore, as all the gas has been turned into stars, or was expelled and became part of the hot cluster halo gas.

On a galaxy scale, the only more violent events are active galactic nuclei and quasars.

In terms of planetary orbits, let me speculate a bit.
Looking at the global gravitational potential of the galaxies: The stars will for sure change their orbits entirely. However, this happens on time-scales much longer than the Lyapunov time of a planetary system. That is the time-scale on which a planetary system exhibits chaotic or unpredictable behaviour. In case of the inner solar system that is about 1-5 million years, but at least in our solar system the inner orbits have been remarkably stable globally https://journals.aps.org/prx/pdf/10.110 ... .13.021018. Going out on a limb, I'd say that an inner solar system might have a chance to survive a major galactic merger since it has ample of time to dissipate the gentle external disturbance among their orbits, looking at a big scale.

Looking at a small scale: a nearby encounter with another star certainly could have the potential to disturb a planetary system at a greater scale, as the interaction time-scale becomes more comparable with the planetary system's chaotic time scale. At least the orbits of the outer planets, and those in turn those of the inner planets should one of the outer ones decide to pay a visit to its inner siblings.

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

I disagree. (But this may come down to semantics in some ways.) The forces involved are tiny. The speeds involved are tiny. The time scales are long.

I would equate the collision of two galaxies to a large crowd wandering through a park. The entire dynamic of how the people move is affected. Play the interaction at high speed and it would look violent. But individual people are scarcely aware of anything happening. Galaxy collisions are incredibly gentle things. Some of the gentlest interactions in the Universe.

[Mischa Schirmer]

Here is a simulation of a major merger resulting in the formation of a large elliptical galaxy. Stars are shown in the left, gas temperature at the right.

https://www.illustris-project.org/movie ... _1pMpc.mp4

Certainly from the perspective of an individual star and on time-scales shorter than a million years or so not much is happening, but the bigger picture is quite transformational. Using your (central) park analogy, if two crowds would walk or run toward each other from oposite ends veryone with a bottle of beer in their hands, once everything is over, you would have a large happy spherical party in the middle, a considerable fraction would be scattered across NY or the entire state of NY wondering what just happened. And some might find themselves in a low-earth orbits, or even left Earth (hypervelocity stars). Probably not too unrealistic after all

[Chris Peterson]

Here is a simulation of a major merger resulting in the formation of a large elliptical galaxy. Stars are shown in the left, gas temperature at the right.

https://www.illustris-project.org/movie ... _1pMpc.mp4

Certainly from the perspective of an individual star and on time-scales shorter than a million years or so not much is happening, but the bigger picture is quite transformational. Using your (central) park analogy, if two crowds would walk or run toward each other from oposite ends veryone with a bottle of beer in their hands, once everything is over, you would have a large happy spherical party in the middle, a considerable fraction would be scattered across NY or the entire state of NY wondering what just happened. And some might find themselves in a low-earth orbits, or even left Earth (hypervelocity stars). Probably not too unrealistic after all :lol2:

A lot happening isn't how I would define "violent", however. Put a drop of food coloring in a container of water and watch it at 1000X normal speed and you might be tempted to see a violent reaction. But is it?

[VictorBorun]

Here is a simulation of a major merger resulting in the formation of a large elliptical galaxy. Stars are shown in the left, gas temperature at the right.

https://www.illustris-project.org/movie ... _1pMpc.mp4

Certainly from the perspective of an individual star and on time-scales shorter than a million years or so not much is happening, but the bigger picture is quite transformational. Using your (central) park analogy, if two crowds would walk or run toward each other from oposite ends veryone with a bottle of beer in their hands, once everything is over, you would have a large happy spherical party in the middle, a considerable fraction would be scattered across NY or the entire state of NY wondering what just happened. And some might find themselves in a low-earth orbits, or even left Earth (hypervelocity stars). Probably not too unrealistic after all

A lot happening isn't how I would define "violent", however. Put a drop of food coloring in a container of water and watch it at 1000X normal speed and you might be tempted to see a violent reaction. But is it?

Suppose a strange star's Oort cloud passes through our one. Would Earth be in danger like in the time of early bombardment?

[Chris Peterson]

Here is a simulation of a major merger resulting in the formation of a large elliptical galaxy. Stars are shown in the left, gas temperature at the right.

https://www.illustris-project.org/movie ... _1pMpc.mp4

Certainly from the perspective of an individual star and on time-scales shorter than a million years or so not much is happening, but the bigger picture is quite transformational. Using your (central) park analogy, if two crowds would walk or run toward each other from oposite ends veryone with a bottle of beer in their hands, once everything is over, you would have a large happy spherical party in the middle, a considerable fraction would be scattered across NY or the entire state of NY wondering what just happened. And some might find themselves in a low-earth orbits, or even left Earth (hypervelocity stars). Probably not too unrealistic after all :lol2:

A lot happening isn't how I would define "violent", however. Put a drop of food coloring in a container of water and watch it at 1000X normal speed and you might be tempted to see a violent reaction. But is it?

Suppose a strange star's Oort cloud passes through our one. Would Earth be in danger like in the time of early bombardment?

One Oort cloud almost certainly can't alter the dynamics of another one. The degree of risk would depend on how far away the passing star was. Whether it has its own Oort cloud or not probably doesn't matter.

[johnnydeep]

A lot happening isn't how I would define "violent", however. Put a drop of food coloring in a container of water and watch it at 1000X normal speed and you might be tempted to see a violent reaction. But is it?

Suppose a strange star's Oort cloud passes through our one. Would Earth be in danger like in the time of early bombardment?

One Oort cloud almost certainly can't alter the dynamics of another one. The degree of risk would depend on how far away the passing star was. Whether it has its own Oort cloud or not probably doesn't matter.

Wikipedia says our own Oort cloud extends to "distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years)". If another star with a similar Oort cloud passes with, say, 1 ly, the two Oort clouds would pass through each other. So, you're saying that although the gravity of the passing stars may well perturb each others' Oort clouds, the bodies in the clouds themselves likely wouldn't because the sparseness of their populations means that the chance of a close encounter is almost nil?

[Chris Peterson]

Suppose a strange star's Oort cloud passes through our one. Would Earth be in danger like in the time of early bombardment?

One Oort cloud almost certainly can't alter the dynamics of another one. The degree of risk would depend on how far away the passing star was. Whether it has its own Oort cloud or not probably doesn't matter.

Wikipedia says our own Oort cloud extends to "distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years)". If another star with a similar Oort cloud passes with, say, 1 ly, the two Oort clouds would pass through each other. So, you're saying that although the gravity of the passing stars may well perturb each others' Oort clouds, the bodies in the clouds themselves likely wouldn't because the sparseness of their populations means that the chance of a close encounter is almost nil?

Exactly. The total mass of the Oort cloud is probably on the order of a few Earths, which is distributed over a massive volume. We could probably sail a probe around in the Oort cloud for thousands of years and never come close enough to a body to detect it. Two Oort clouds passing through each other are not going to change much. The tidal effects of the stars, though... that's another matter. Oort cloud bodies are so far from the Sun that their orbits can be significantly altered by very tiny forces.

[johnnydeep]

One Oort cloud almost certainly can't alter the dynamics of another one. The degree of risk would depend on how far away the passing star was. Whether it has its own Oort cloud or not probably doesn't matter.

Wikipedia says our own Oort cloud extends to "distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years)". If another star with a similar Oort cloud passes with, say, 1 ly, the two Oort clouds would pass through each other. So, you're saying that although the gravity of the passing stars may well perturb each others' Oort clouds, the bodies in the clouds themselves likely wouldn't because the sparseness of their populations means that the chance of a close encounter is almost nil?

Exactly. The total mass of the Oort cloud is probably on the order of a few Earths, which is distributed over a massive volume. We could probably sail a probe around in the Oort cloud for thousands of years and never come close enough to a body to detect it. Two Oort clouds passing through each other are not going to change much. The tidal effects of the stars, though... that's another matter. Oort cloud bodies are so far from the Sun that their orbits can be significantly altered by very tiny forces.

[VictorBorun]

Wikipedia says our own Oort cloud extends to "distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years)". If another star with a similar Oort cloud passes with, say, 1 ly, the two Oort clouds would pass through each other. So, you're saying that although the gravity of the passing stars may well perturb each others' Oort clouds, the bodies in the clouds themselves likely wouldn't because the sparseness of their populations means that the chance of a close encounter is almost nil?

Exactly. The total mass of the Oort cloud is probably on the order of a few Earths, which is distributed over a massive volume. We could probably sail a probe around in the Oort cloud for thousands of years and never come close enough to a body to detect it. Two Oort clouds passing through each other are not going to change much. The tidal effects of the stars, though... that's another matter. Oort cloud bodies are so far from the Sun that their orbits can be significantly altered by very tiny forces.

that is a good news.
Of course it depends on how correct we model the forming of rogue asteroids and Oort clouds. We can't see or radar them now and they can be more numerous for all we know.
Or can we see them after all, not in their star's light, but in the stellar wind of a Nova or Supernova?

[VictorBorun]

Exactly. The total mass of the Oort cloud is probably on the order of a few Earths, which is distributed over a massive volume. We could probably sail a probe around in the Oort cloud for thousands of years and never come close enough to a body to detect it. Two Oort clouds passing through each other are not going to change much. The tidal effects of the stars, though... that's another matter. Oort cloud bodies are so far from the Sun that their orbits can be significantly altered by very tiny forces.

that is a good news.
Of course it depends on how correct we model the forming of rogue asteroids and Oort clouds. We can't see or radar them now and they can be more numerous for all we know.
Or can we see them after all, not in their star's light, but in the stellar wind of a Nova or Supernova?

here I did not mean the Solar System's Oort cloud. I meant just an Oort cloud that happens to have its star blow up and change every object into a comet-like tailed thing or a shadow-throwing thing

[Chris Peterson]

that is a good news.
Of course it depends on how correct we model the forming of rogue asteroids and Oort clouds. We can't see or radar them now and they can be more numerous for all we know.
Or can we see them after all, not in their star's light, but in the stellar wind of a Nova or Supernova?

here I did not mean the Solar System's Oort cloud. I meant just an Oort cloud that happens to have its star blow up and change every object into a comet-like tailed thing or a shadow-throwing thing

Anything that has a comet tail has an extremely short lifetime. Nothing is large enough to be seen by its shadow, other than a rare occultation of a distant star.

[VictorBorun]

that is a good news.
Of course it depends on how correct we model the forming of rogue asteroids and Oort clouds. We can't see or radar them now and they can be more numerous for all we know.
Or can we see them after all, not in their star's light, but in the stellar wind of a Nova or Supernova?

here I did not mean the Solar System's Oort cloud. I meant just an Oort cloud that happens to have its star blow up and change every object into a comet-like tailed thing or a shadow-throwing thing

Anything that has a comet tail has an extremely short lifetime. Nothing is large enough to be seen by its shadow, other than a rare occultation of a distant star.

(sigh) So even Betelgeuse blowing up any time soon would not let us a glimpse on its Oort cloud. OK

[Chris Peterson]

here I did not mean the Solar System's Oort cloud. I meant just an Oort cloud that happens to have its star blow up and change every object into a comet-like tailed thing or a shadow-throwing thing

Anything that has a comet tail has an extremely short lifetime. Nothing is large enough to be seen by its shadow, other than a rare occultation of a distant star.

(sigh) So even Betelgeuse blowing up any time soon would not let us a glimpse on its Oort cloud. OK

I think that the event would be so bright that anything happening in the stellar system would be overwhelmed.

[VictorBorun]

I hope when they make the 3d map of all the Euclids's galaxies up to redshift of 2,
they present it in an online 3d model, able to wobble slightly at user's command.

I don't mean a side view of a galaxy made up by some AI. I mean a little wobble to let the user feel the depth