4 ms·

The vast majority of the stars will pass through each other; what I'm curious about is what happens at the galactic cores, and how many solar systems will end u

The vast majority of the stars will pass through each other; what I'm curious about is what happens at the galactic cores, and how many solar systems will end up being swallowed by the super-massive black holes at the center of each galaxy. Also, what are the side effects of two black holes merging? Have we ever observed such a thing? https://www.youtube.com/watch?v=fMNlt2FnHDg https://www.youtube.com/watch?v=fMNlt2FnHDg

adameast1978 7y ago
Exactly what I was thinking. Also, the gravitational waves detected from LIGO were thought to be from a collision of two black holes.
ses1984 7y ago
>it’s extremely unlikely that any individual stars will collide What do you mean they'll pass through each other.
- lukifer 7y ago
  The distances are so vast, that essentially, yes. Think about two sailors in small boats, crossing an ocean in opposite directions, and how unlikely it would be that their boats collide. Now multiply that by several orders of magnitude. One of my favorite factoids: if you fire a laser from the surface of the earth (and the beam magically stayed perfectly straight), you have a small but significant chance of hitting the sun or the moon, but anything beyond that, the scale is so enormous, the odds that your laser would ever hit a celestial body anywhere in the universe are effectively zero.
  - cooljacob204 7y ago
    Anywhere in the observable universe. If it is truly infinite and we pretend the laser instantly reaches wherever we point it then the chance is pretty much 100%.
    - cdelsolar 7y ago
      From the photon's point of view, when it does reach something else it IS instant.
      - saagarjha 7y ago
        To itself.
  - Mirioron 7y ago
    Does that factoid assume that the laser beam's width doesn't increase?
    - saagarjha 7y ago
      Yes, otherwise even a laser beam that diverges slightly from being perfectly collimated would subtend a fraction of the night sky and thus contain a large number of galaxies.
  - ses1984 7y ago
    You're confusing me more. Stars passing through each other like you said before to me implies collision. Would you say these sailors are going to pass through each other?
    - ajna91 7y ago
      They stars won't pass through each other. The "solar systems" will pass through each other.
      - ses1984 7y ago
        I would have guessed that even that is highly unlikely.
    - AareyBaba 7y ago
      The stars will pass BY each other. The galaxies will pass THROUGH each other without collisions. There are many youtube videos that try to visualize the incomprehensible scale of space. Here is one just for our solar system https://www.youtube.com/watch?v=dCSIXLIzhzk https://www.youtube.com/watch?v=dCSIXLIzhzk
  - deleted 7y ago
    [deleted]
- nkrisc 7y ago
  Effectively they'll all miss each other. There is so much empty space in between stars that even when two galaxies collide the stars will interact gravitationally, but likely none will physically collide. Think of two clouds of gas colliding. The clouds as a whole collide, yes, but there's no molecular fusion happening. The molecules themselves don't collide. "Pass through each other" is not the clearest wording.
  - lukifer 7y ago
    > "Pass through each other" is not the clearest wording. It was not, mea culpa. :D
    - ncmncm 7y ago
      "Pass by each other". The Englese, she is the very hard language to speak.
- jcims 7y ago
  This provides some idea of how sparse the universe is: https://medium.com/@clay.c.edgar/if-earth-was-a-grain-of-sand-22ea58f43d5e https://medium.com/@clay.c.edgar/if-earth-was-a-grain-of-san...
hyperpape 7y ago
The first gravitational waves we observed are believed to have been caused by two massive black holes merging.
LASR 7y ago
There is no difference between a black hole of a certain mass and a star of the same mass, gravitationally. Stars can only get so massive, but black holes can get much much more massive. But even with supermassive black holes, it doesn’t change the odds of colliding solar systems that much. There is just way too much space in between to matter.
- saagarjha 7y ago
  The supermassive black holes at the center of the galaxy are thought to merge, however.
- raattgift 7y ago
  Your comment is accurate enough except for: > no difference ... gravitationally. "practically no difference ... gravitationally, provided one is at a very large distance compared to the outer surface of an astrophysical black hole." would be more accurate. Up close to a real black hole additional shear and tension becomes obvious, although you will generally be in the region that would be close to the surface of a star of comparable mass. Astrophysical black holes are more complicated objects than the theoretical Vaidya-Bonnor-Kerr (VBK) black holes, especially when they are not isolated. VBK black holes absorb radiation and emit Hawking radiation (V part), including self-interacting matter (B part, although that is mostly dealing with classical electromagnetism), and they also rotate (K part). VBK breaks several symmetries: the exterior spacetime is not static, not stationary, and spherical symmetry goes to axial symmetry, including all the incoming and outgoing radiation. Real rotating black holes would break axial symmetry because real matter tends to clump (in particular theoretical VBK exterior matter does not look like accretion discs). Small perturbations of the axial symmetry of VBK black holes tend to cause black holes to spin up as matter falls onto them. There is observational support to suggest that most real black holes are likely spinning extremely rapidly. The Kerr vacuum around a VBK is equipped with regions in which objects are dragged around in the direction of the VBK black hole's rotation. As the original angular momentum of the star may be increased by infalling material, a real black hole likely has a strong region extending well outside of where the surface of a star of comparable mass would be. While the dragging effect falls off quickly with distance outside the outermost surface of the outermost of these regions, if we replaced the sun with a real black hole which rotates at extremely high rates, all eight planets' orbits would precess compared to their orbits around the sun. Given enough observation time, this would be especially noticeable for objects on high-inclination, low-ellipticity orbits, even if they are near the outer edge of the Oort cloud. Reducing the angular momentum dramatically increases the observation time needed to distinguish gravitationally between a (growing!) solar-mass black hole and a (shrinking!) solar-mass star. Beyond the Oort cloud things aren't gravitationally bound to our solar system anyway, so by the time you're a couple hundred thousand astronomical units away, you are on safe territory with practical gravitational indistinguishability. In terms of light and radiation, though, swapping the sun for a solar-mass black hole would be pretty noticeable, for example https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_eff... or https://en.wikipedia.org/wiki/Yarkovsky_effect https://en.wikipedia.org/wiki/Yarkovsky_effect

adameast1978 7y ago

Exactly what I was thinking. Also, the gravitational waves detected from LIGO were thought to be from a collision of two black holes.

ses1984 7y ago

>it’s extremely unlikely that any individual stars will collide What do you mean they'll pass through each other.

lukifer 7y ago
The distances are so vast, that essentially, yes. Think about two sailors in small boats, crossing an ocean in opposite directions, and how unlikely it would be that their boats collide. Now multiply that by several orders of magnitude. One of my favorite factoids: if you fire a laser from the surface of the earth (and the beam magically stayed perfectly straight), you have a small but significant chance of hitting the sun or the moon, but anything beyond that, the scale is so enormous, the odds that your laser would ever hit a celestial body anywhere in the universe are effectively zero.
- cooljacob204 7y ago
  Anywhere in the observable universe. If it is truly infinite and we pretend the laser instantly reaches wherever we point it then the chance is pretty much 100%.
  - cdelsolar 7y ago
    From the photon's point of view, when it does reach something else it IS instant.
    - saagarjha 7y ago
      To itself.
- Mirioron 7y ago
  Does that factoid assume that the laser beam's width doesn't increase?
  - saagarjha 7y ago
    Yes, otherwise even a laser beam that diverges slightly from being perfectly collimated would subtend a fraction of the night sky and thus contain a large number of galaxies.
- ses1984 7y ago
  You're confusing me more. Stars passing through each other like you said before to me implies collision. Would you say these sailors are going to pass through each other?
  - ajna91 7y ago
    They stars won't pass through each other. The "solar systems" will pass through each other.
    - ses1984 7y ago
      I would have guessed that even that is highly unlikely.
  - AareyBaba 7y ago
    The stars will pass BY each other. The galaxies will pass THROUGH each other without collisions. There are many youtube videos that try to visualize the incomprehensible scale of space. Here is one just for our solar system https://www.youtube.com/watch?v=dCSIXLIzhzk https://www.youtube.com/watch?v=dCSIXLIzhzk
- deleted 7y ago
  [deleted]
nkrisc 7y ago
Effectively they'll all miss each other. There is so much empty space in between stars that even when two galaxies collide the stars will interact gravitationally, but likely none will physically collide. Think of two clouds of gas colliding. The clouds as a whole collide, yes, but there's no molecular fusion happening. The molecules themselves don't collide. "Pass through each other" is not the clearest wording.
- lukifer 7y ago
  > "Pass through each other" is not the clearest wording. It was not, mea culpa. :D
  - ncmncm 7y ago
    "Pass by each other". The Englese, she is the very hard language to speak.
jcims 7y ago
This provides some idea of how sparse the universe is: https://medium.com/@clay.c.edgar/if-earth-was-a-grain-of-sand-22ea58f43d5e https://medium.com/@clay.c.edgar/if-earth-was-a-grain-of-san...

hyperpape 7y ago

The first gravitational waves we observed are believed to have been caused by two massive black holes merging.

LASR 7y ago

There is no difference between a black hole of a certain mass and a star of the same mass, gravitationally. Stars can only get so massive, but black holes can get much much more massive. But even with supermassive black holes, it doesn’t change the odds of colliding solar systems that much. There is just way too much space in between to matter.

saagarjha 7y ago
The supermassive black holes at the center of the galaxy are thought to merge, however.
raattgift 7y ago
Your comment is accurate enough except for: > no difference ... gravitationally. "practically no difference ... gravitationally, provided one is at a very large distance compared to the outer surface of an astrophysical black hole." would be more accurate. Up close to a real black hole additional shear and tension becomes obvious, although you will generally be in the region that would be close to the surface of a star of comparable mass. Astrophysical black holes are more complicated objects than the theoretical Vaidya-Bonnor-Kerr (VBK) black holes, especially when they are not isolated. VBK black holes absorb radiation and emit Hawking radiation (V part), including self-interacting matter (B part, although that is mostly dealing with classical electromagnetism), and they also rotate (K part). VBK breaks several symmetries: the exterior spacetime is not static, not stationary, and spherical symmetry goes to axial symmetry, including all the incoming and outgoing radiation. Real rotating black holes would break axial symmetry because real matter tends to clump (in particular theoretical VBK exterior matter does not look like accretion discs). Small perturbations of the axial symmetry of VBK black holes tend to cause black holes to spin up as matter falls onto them. There is observational support to suggest that most real black holes are likely spinning extremely rapidly. The Kerr vacuum around a VBK is equipped with regions in which objects are dragged around in the direction of the VBK black hole's rotation. As the original angular momentum of the star may be increased by infalling material, a real black hole likely has a strong region extending well outside of where the surface of a star of comparable mass would be. While the dragging effect falls off quickly with distance outside the outermost surface of the outermost of these regions, if we replaced the sun with a real black hole which rotates at extremely high rates, all eight planets' orbits would precess compared to their orbits around the sun. Given enough observation time, this would be especially noticeable for objects on high-inclination, low-ellipticity orbits, even if they are near the outer edge of the Oort cloud. Reducing the angular momentum dramatically increases the observation time needed to distinguish gravitationally between a (growing!) solar-mass black hole and a (shrinking!) solar-mass star. Beyond the Oort cloud things aren't gravitationally bound to our solar system anyway, so by the time you're a couple hundred thousand astronomical units away, you are on safe territory with practical gravitational indistinguishability. In terms of light and radiation, though, swapping the sun for a solar-mass black hole would be pretty noticeable, for example https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_eff... or https://en.wikipedia.org/wiki/Yarkovsky_effect https://en.wikipedia.org/wiki/Yarkovsky_effect