4 ms·

My intuition is having trouble coming to grips with the notion of accelerating a fluid object to that velocity via an explosion.

iaw 8y ago
It's because your intuition is on a human scales. White dwarfs apparently have a density near 1 tonne/cubic centimeter and consist of plasma which only runs analogous to fluids so far before the analogy breaks down. One difference is that plasma will have mobile charge carriers likely imparting something similar to a London dispersion force if there's any charge directionality in the super nova blast wave. What likely has the most impact though is the density and gravity that holds the object together in a way fluid droplets on earth do not.
autocorr 8y ago
Since white dwarfs have around the mass of the sun in a size comparable to the Earth, the matter in them is quite exotic. Astronomer's have speculated that these hypervelocity stars (moving at a few tenths of a percent the speed of light in the referenced studies case[1]) are shot out like a slingshot. The original system is a binary, so they are already under extreme circular gravitational acceleration in their orbit. When the companion explodes in a core-collapse supernova, it loses that source of acceleration and moves in a straight line like a slingshot. The Gaia satellite is able to measure the velocity vector of the star very well, and show that it points back to an old supernova remnant. The scenario isn't controversial, but it does give quite a good example that this is how they come about.
- Steel_Phoenix 8y ago
  This makes me wonder about the nature of gravity. How much of the gravitational pull from the companion would disappear? How fast? It seems like a reduction in gravity would be immediately followed by a massive energy release. It's impressive that the remaining star is moving away fast enough to avoid being destroyed in the resulting blast.
  - whatshisface 8y ago
    The reduction in gravity isn't due to anything special, the matter in the "anchor" star is just leaving to go somewhere else. (In all directions.)
    - Steel_Phoenix 8y ago
      That makes it seem like gravitationally, the anchor star would just be expanding. My intuition thinks the remaining star gets slammed by debris before the direction and strength of the anchor dissipates enough to launch it away, though my confidence in my stellar intuition is low.
      - whatshisface 8y ago
        Your intuition contains some truth: according to Gauss's law, the gravitation of the expanding nova shell shouldn't change at all until it passes by its surviving companion. The matter expanding away from the companion will gravitate less, but that will be canceled out by the increased attraction to the matter expanding towards the companion. Really, we're just assuming the hot, dense, and small dwarf can sustain the licking it gets when the remnant hits it. The smallness helps it get hit by less material, moreso if it's far away. It's gravitationally bound so it won't shatter - the most you could do to disassemble it would be to boil off its matter by raising it all to escape velocity. It takes a lot of energy to do that.
- mirimir 8y ago
  Right, this white dwarf is moving at ~0.8% lightspeed. I gather that objects could be ejected from Sagittarius A* at even higher velocities.
- phyzome 8y ago
  Ahhh, this makes sense -- they're not pushed, but flung. Thank you.
aswanson 8y ago
That's slow on an interstellar scale for speed. A scarce fraction of the speed of light.
- lmilcin 8y ago
  That's one of the fastest macroscopic solid objects with relation to its original environment. It is an entire star traveling on hyperbolic trajectory out of Milky Way. Even at the distance where we are from the center of Milky Way the escape velocity is still over 500 km/s and if I remember the number of detected objects that are on trajectory to escape our galaxy can be counted on the fingers of your hands.
jtolmar 8y ago
If your intuition is saying it should have gotten splashed apart: so what? The portion traveling in the same direction as the main body of the star would fall back into one body. The rest would be somewhere else, no longer a star.
make3 8y ago
a white dwarf is very much not fluid, as it's by far denser than anything you've encountered in your lifetime (or you'd had died from the gravity)

iaw 8y ago

It's because your intuition is on a human scales. White dwarfs apparently have a density near 1 tonne/cubic centimeter and consist of plasma which only runs analogous to fluids so far before the analogy breaks down. One difference is that plasma will have mobile charge carriers likely imparting something similar to a London dispersion force if there's any charge directionality in the super nova blast wave. What likely has the most impact though is the density and gravity that holds the object together in a way fluid droplets on earth do not.

autocorr 8y ago

Since white dwarfs have around the mass of the sun in a size comparable to the Earth, the matter in them is quite exotic. Astronomer's have speculated that these hypervelocity stars (moving at a few tenths of a percent the speed of light in the referenced studies case[1]) are shot out like a slingshot. The original system is a binary, so they are already under extreme circular gravitational acceleration in their orbit. When the companion explodes in a core-collapse supernova, it loses that source of acceleration and moves in a straight line like a slingshot. The Gaia satellite is able to measure the velocity vector of the star very well, and show that it points back to an old supernova remnant. The scenario isn't controversial, but it does give quite a good example that this is how they come about.

Steel_Phoenix 8y ago
This makes me wonder about the nature of gravity. How much of the gravitational pull from the companion would disappear? How fast? It seems like a reduction in gravity would be immediately followed by a massive energy release. It's impressive that the remaining star is moving away fast enough to avoid being destroyed in the resulting blast.
- whatshisface 8y ago
  The reduction in gravity isn't due to anything special, the matter in the "anchor" star is just leaving to go somewhere else. (In all directions.)
  - Steel_Phoenix 8y ago
    That makes it seem like gravitationally, the anchor star would just be expanding. My intuition thinks the remaining star gets slammed by debris before the direction and strength of the anchor dissipates enough to launch it away, though my confidence in my stellar intuition is low.
    - whatshisface 8y ago
      Your intuition contains some truth: according to Gauss's law, the gravitation of the expanding nova shell shouldn't change at all until it passes by its surviving companion. The matter expanding away from the companion will gravitate less, but that will be canceled out by the increased attraction to the matter expanding towards the companion. Really, we're just assuming the hot, dense, and small dwarf can sustain the licking it gets when the remnant hits it. The smallness helps it get hit by less material, moreso if it's far away. It's gravitationally bound so it won't shatter - the most you could do to disassemble it would be to boil off its matter by raising it all to escape velocity. It takes a lot of energy to do that.
mirimir 8y ago
Right, this white dwarf is moving at ~0.8% lightspeed. I gather that objects could be ejected from Sagittarius A* at even higher velocities.
phyzome 8y ago
Ahhh, this makes sense -- they're not pushed, but flung. Thank you.

aswanson 8y ago

That's slow on an interstellar scale for speed. A scarce fraction of the speed of light.

lmilcin 8y ago
That's one of the fastest macroscopic solid objects with relation to its original environment. It is an entire star traveling on hyperbolic trajectory out of Milky Way. Even at the distance where we are from the center of Milky Way the escape velocity is still over 500 km/s and if I remember the number of detected objects that are on trajectory to escape our galaxy can be counted on the fingers of your hands.

jtolmar 8y ago

If your intuition is saying it should have gotten splashed apart: so what? The portion traveling in the same direction as the main body of the star would fall back into one body. The rest would be somewhere else, no longer a star.

make3 8y ago

a white dwarf is very much not fluid, as it's by far denser than anything you've encountered in your lifetime (or you'd had died from the gravity)