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chornedsnorkack
2010-Jan-25, 09:31 AM
Suppose that a black hole were moving through tenuous gas (typical interstellar environment, especially some distance away from Milky Way disc) at a substantial speed (in the region of 100 km/s, typical for old red dwarfs and other old stars of Population II - you would expect the remnants of massive Population II stars to be around, on similar orbits) and, significantly, the density of the tenuous gas around the black hole is constant or slowly changing, such that the dansity to each side of the path is nearly equal.

The gas that passes far away from the hole will follow hyperbolic orbits. As those hyperboles are parallel lines ahead of the hole, they will intersect some distance from the hole. The gas that converges on the trail of the hole will heat and slow down relative to the hole. But the gas that passed far from the hole will converge at small angles and lose little speed. It was well above the escape speed of the black hole before passing the hole, and therefore is still above the escape speed when it has lost speed - it remains on the trail of the hole.

Now, the closer the gas passes, the higher the collision angle and the more speed lost. At a certain distance, the gas loses enough speed to become bound to the hole.

But since it has no angular momentum - the densities to each side of the hole were equal and the angular momenta therefore cancelled on collision - it cannot form an accretion disc. Rather, it can only fall in direct line along the path into the hole.

It might or might not heat up as it is spaghettified near the hole; but any energy not radiated away in the very short time is swallowed by the event horizon.

Likewise, the gas close to the direct path of the hole is accelerated towards the hole, spaghettified and then swallowed up with any unemitted kinetic or heat energy.

Roughly how bright would an ordinary stellar mass black hole be sweeping through the tenuous gas of the edges of the Milky Way disc at typical speed of Population II objects?

And in which direction?

The hot and slightly emissive tail is in direct line in the trail of the hole. How visible would the hole be for an observer directly ahead on its path? On one hand, the radiation of the tail is partly absorbed by the hole; on the other hand, the rest could be gravitationally lensed.

WayneFrancis
2010-Jan-25, 10:53 AM
...Why do you think a black hole would be whizzing through the galaxy where it would interact with interstellar dust in this way? Any orbit that the black hole is on should be a similar orbit that intergalactic medium would also be on.

It would be very hard to get any matter to fall straight into a stellar mass black hole. The event horizon is only 3k in diameter. Hitting it with anything from just 1AU away would be like trying to hit a target 1cm in diameter from 500km away, for those that are metrically impaired that is like a 1/2 inch target from 350 miles away.

The only way I could see something like this happen would be in some galactic merger but then the whole system is in such a state...

This goes to the whole "can a black hole be a danger to an interstellar traveller" and my answer is that you probably won't find a situation where a traveller would be caught unaware.

Falling into a black hole should take some work by interstellar travellers.

chornedsnorkack
2010-Jan-25, 11:33 AM
...Why do you think a black hole would be whizzing through the galaxy where it would interact with interstellar dust in this way? Any orbit that the black hole is on should be a similar orbit that intergalactic medium would also be on.

Um, because Population II stars are not?

The stars of Population II, such as old giant Arcturus, or red dwarfs like Barnardīs Runner, observationally have high speeds of peculiar motion with respect to Sun, other disc stars and disc interstellar gas. Generally directed against Milky Way rotation as well as across the disc.

They can do so because, being massive compact objects, the experience little friction. Whereas gas is by friction forced to rotate as disc.


It would be very hard to get any matter to fall straight into a stellar mass black hole. The event horizon is only 3k in diameter. Hitting it with anything from just 1AU away would be like trying to hit a target 1cm in diameter from 500km away, for those that are metrically impaired that is like a 1/2 inch target from 350 miles away.



To get solid objects to fall straight, yes. They will be deflected and pass by the hole on hyperbolic orbits.

But gases have viscosity. Any gas passing by a black hole will be deflected onto a hyperbolic orbit in the plane of the path of the hole. Therefore the orbit shall intersect all other hyperbolic orbits that passed the hole at the same distance, and the gas will collide on the path.