@sonhousesaid @DeepThought So you are saying in the case of already existing binary systems, they got that way by ejecting planets and other mass from the system or they could not have assumed a binary stable orbit?
Wouldn't it be valid to say some binaries are made because the cloud that condensed to be stars and planets happen to have been large enough for two stars to independently ...[text shortened]... ass that made the two were already in an orbit around each other so they were BORN in stable orbits?
I'm specifically referring to bunnyknight's scenario of a neutron star entering the Solar System and forming a new binary with the Sun. Binary systems can form in more than one way, and can form with stable planetary systems.
@DeepThought When 2 masses interact by gravity only, shouldn't the total net energy remain the same? And if one mass gains momentum, the second should lose an equal amount, unless the tidal forces transfer some of the momentum into heat.
@bunnyknightsaid @DeepThought When 2 masses interact by gravity only, shouldn't the total net energy remain the same? And if one mass gains momentum, the second should lose an equal amount, unless the tidal forces transfer some of the momentum into heat.
That's correct. So if there were only two objects which drift into each other's vicinity then they could not become a bound system unless they actually collided, but see below. However, around a real star there is the solar wind that creates a little drag and, in large numbers of cases, planets and other smaller objects. So there is a mechanism for the two major objects to shed energy by ejecting the smaller objects from the system.
The caveat to the above statement is that this is only rigorously true in Newton's theory of Gravity. In Einstein's there is a correction term to the inverse square law which is microscopic, but if the objects are large enough then the extra 1/r^4 term causes them to spiral into each other. The excess energy is radiated away in the form of gravitational waves.
Another even stranger outcome would be if the neutron star swallowed our sun, in which case it would reach critical mass and turn into a black hole. Poof! .... no more sun!
Another even stranger outcome would be if the neutron star swallowed our sun, in which case it would reach critical mass and turn into a black hole. Poof! .... no more sun!
It depends on the mass of the neutron star. This is an extremely unlikely outcome in the short term, the neutron star would have to be headed exactly for the sun. A more likely scenario is that they'd form a tightish orbit and when the sun turned into a red giant the neutron star would accrete material.
If the neutron star's mass were at the low end of the mass range (1.1 solar masses) then the total mass available would not be enough to exceed the Tolman-Oppenheimer–Volkoff limit (2.16 solar masses). If the neutron star were at the top of the mass range then you are probably right. So what happens in between?
In the event of a head on collision the first thing to happen would be an almighty explosion as hydrogen would rapidly be fused into heavier elements on the neutron stars surface. This would cause matter to be ejected in large quantities. So it is not at all obvious to me that a remnant would be anything other than a larger neutron star.
Since every neutron star is constantly gaining mass by sucking up dust and gas in its vicinity, then logically every neutron star should eventually become a black hole. Furthermore, what if a black hole is nothing more than a neutron star with enough gravity to hold back light?
Since every neutron star is constantly gaining mass by sucking up dust and gas in its vicinity, then logically every neutron star should eventually become a black hole. Furthermore, what if a black hole is nothing more than a neutron star with enough gravity to hold back light?
The accretion rate from interstellar space is to all intents and purposes zero. In the end yes, but they're in a race with the expansion of the universe.
We expect neutron degeneracy pressure to be overcome at the Tolman-Oppenheimer–Volkoff. After that there may be quark degeneracy pressure, but Gravity always wins. The escape velocity at the event horizon of a black hole is the speed of light and inside that the classical theory predicts that all the matter will collapse into an infinitely dense singularity. The catch of course is that we expect quantum gravity effects to be significant as the radius of the collapsing star gets down to the Planck scale. So our theories say that the neutron star stops being a neutron star and at the very least you're looking at a quark star, but once an event horizon's formed further inward collapse is inevitable. The shape space is just forces it.
neutron star
01 Nov '19 05:07
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