Astronomers See Another Star Torn Apart By a Black Hole 127
The Bad Astronomer writes "A star in a galaxy 2.7 billion light years away wandered too close to a supermassive black hole and suffered the ultimate fate: it was literally torn apart by the black hole's gravity. The event was seen as a flash of ultraviolet light flaring 350 times brighter than the galaxy itself, slowly fading over time. Astronomers were able to determine that some of the star's material was eaten by the black hole, and some flung off into space. Although rare, this is the second time such a thing has been seen; the other was just last year."
Will black hole devour dark matter, anti-matter? (Score:5, Interesting)
We know that black holes can suck in matter - the gamma ray radiation emitted when matters are flatten to a disc before it's being sucked in are indication of black hole devouring matter.
But how about dark matter, or anti-matter?
Will black hole's gravity pull pulls in dark matter and/or anti-matter?
What effect would that have?
Re:Will black hole devour dark matter, anti-matter (Score:3, Interesting)
Note exactly rare (Score:4, Interesting)
This scenario was observed twice in two years. Not exactly rare when you realize how little of the sky we watch.
Re:Unbelievable Gravity (Score:3, Interesting)
Hey, I think I can maths this one out.
Say that the black hole has mass M, and the star has mass m. The radius of the star is r, and the distance between the black hole and the centre of the star is R. So the edge of the star closest to the black hole is at a distance of (R-r), and experiences a gravitational field from the black hole of:
G M / (R-r)^2
where G is the gravitational constant. (Don't worry - this will drop out eventually.) The edge of the star farthest from the black hole is at a distance of (R+r), so it experiences a gravitational field of:
G M / (R+r)^2
The difference between these two values tells us how fast these different bits of the star would accelerate away from each other, if they weren't being held together. Doing a bit of algebra (and assuming that r is a lot less than R, to make a few approximations), I get:
G M / (R-r)^2 - G M / (R+r)^2
= ( (R+r)^2 - (R-r)^2 ) * G M / (R+r)^2 / (R-r)^2
~= 4 R r G M / R^4
= 4 r G M / R^3
But both edges of the star are also being pulled together by the star's gravity, with a strength of
G m / r^2
So, from the gravitational force of the star alone, they'd be accelerating together at twice this value (each with that much acceleration towards the centre of the star). The point at which the star starts getting stripped of material is when these two effects (star getting pulled apart; star holding itself together) exactly balance, i.e.
4 r G M / R^3 = 2 G m / r^2
hence:
2 M / m = (R/r)^3
So, let's say that the star has the same mass as the sun, and the black hole has 3,000,000x that, so (M/m) = 3,000,000. Let's also say that the star is 0.2 billion miles across (r = 0.1 billion miles). Then we get:
R = (0.1 billion miles) * (6,000,000)^(1/3) = 18 billion miles
So the black hole would start stripping off the star's atmosphere when it's about five times as far away as Pluto is from the sun.
Re:Unbelievable Gravity (Score:4, Interesting)
Good maths and all but there's one thing you need to consider- If you're in stable orbit you don't actually fall inwards.
I think you've misunderstood my posts. I agree that a pointlike object in a circular orbit will remain in a circular orbit, absent any external factors. However, a non-pointlike object is actually in a range of orbits - different orbits for different parts of the object - and will drift apart unless held together. (Note that the separate parts, after this, will still be in orbit.) This is the same effect [wikipedia.org] that produces the tides on Earth; when an orbiting body is close enough to be torn apart by this effect, it's the Roche limit [wikipedia.org]. (You'll see a derivation equivalent to mine in both of those articles.)
Another way of thinking of it: as you say, it's not a case of the black hole pulling more than the star at a given distance. Instead, it's a matter of the difference between the pull of the black hole on different parts of the star being greater than the gravity of the star holding itself together.
I'm not sure where you got the idea that I was talking about the star being pulled into the black hole. That happens, certainly, but through a range of other effects: primarily, I would guess, through friction between the star and other material in orbit around the black hole. (You ascribed it to charged particles from closer to the event horizon, but these are emitted in jets perpendicular to the accretion disk, rather than omnidirectionally.)
Finally, I apologise for making an argument from authority, but I am an astronomer, though this isn't my exact field of research. I don't expect you to take my word for it, but I hope this will persuade you to read my posts in enough detail to understand the point you've missed.
Re:How can you see something going into a black ho (Score:5, Interesting)
I apologize if it's a dumb question, but isn't the whole point of a black hole that not even light escapes?
The gravity tore apart the star before it entered the black hole. Watching all the videos about black holes and space might lead one to think that orbits are easy to achieve, but after I ran some particle simulations [google.com] using simple Newtonian physics in my game engine, I noticed that most particles will slingshot around a source of heavier gravity when they approach, and be flung too far away for gravity to recapture it. In a stellar nursery this sling shot effect places a limit on the star's size, the other main contributing factor being initial density of the nebula. This is true for black holes as well as planets or asteroids approaching a star. So, although some of the star will fall into the black hole, a lot more of it gets flung away from the black hole -- It's a classic case of Conservation of angular momentum...
They're seeing what happens when something gets close to a black hole, not goes into it. You can see things "going into a black hole" before they've reached the event horizon. Also: In my sim, elliptical orbits that didn't result in the object being flung away became tighter and rounder orbits over time.
That schools don't have kids play with simple sims like these in class is Ridiculous! My high-school age little brother hasn't played a traditional game in three weeks. Since I gave him the gravity sim (particle engine stress test) to play with -- all he does is simulate solar systems and formation of stars, or big stars eating little stars, etc. It's the first time I've ever seen him interested in space beyond the Halo Universe! He asked me about Quantum Physics yesterday!
Re:Will black hole devour dark matter, anti-matter (Score:5, Interesting)
Well, we've created antimatter in the lab and it seems to behave very much like normal matter, it just has the opposite charge (for protons/electrons) and Baryon number (a QM property). So I suspect it would behave very much like normal matter, in fact I doubt we can actually tell whether a celestial object/event involves matter or antimatter, though it seems fairly likely that all the "native" matter in a particular galaxy will be the same type, otherwise it would have mutually annihilated whenever a gas cloud of one kind interacted with it's opposite, though a matter galaxy could conceivably capture a rogue star from a passing antimatter galaxy - as long as the rogue star never exploded or hit something directly it would likely be indistinguishable except for a *very* faint and diffuse halo where its antimatter-based solar wind contacted and annihilated the interstellar medium.
Dark matter though... that's an interesting question. As far as we can tell it only interacts gravitationally so it will never glow or collide with anything, since both are EM interactions. The Bullet Cluster would seem to indicate that it even passes right through other dark matter. Which raises an interesting question, while it could presumably be sucked into a black hole's event horizon it might continue to behave just as bizarrely, possibly even being able to escape again somehow. We just have no idea what the stuff is, it's even possible that it's not matter at all, but rather a phenomena symptomatic of a fundamental misunderstanding of the nature of reality, much as black-box radiation in the 1800s led to the development of quantum mechanics and radically altered our understanding of the universe. It was widely believed at the time that we basically understood everything about physics, with just a few loose ends still to tie up (BB radiation, the cause of spectral lines, and a couple others). Instead those loose ends led to the unraveling of virtually everything we thought we knew and opened the door to something far stranger.
There's also the possibility that black holes don't exist at all and the question is nonsensical. We have evidence of ultra-massive non-luminous objects, but little if any for the existence of the defining characteristic of black holes, an event horizon. We assume they are black holes because our theories say that anything that massive would collapse into a singularity, but think about it - we're postulating that a body can become so dense that it creates a region of space where the laws of physics themselves to break down! There are several competing theories that make such a situation impossible, one that I like is based on the fact that Einstein treated gravity as a special case - all other energy fields generate a gravitational field based on their energy density. Einstein felt that it would be "double dipping" to have gravitational fields do so and discarded the idea. However, if we rework the equations assuming that they do in fact do so then we find that as the gravitational field strength becomes extreme the "secondary" gravity generated by the extreme energy density of the "primary" field pulls back against the primary source, causing the field strength to plateau at a level less than that required to create an event horizon, regardless of the density of the central object. If that, or some other mechanism, puts an upper limit on gravitational field strength it seems likely that the ultramassive objects are simply some sort of exotic quark-degenerate matter that happens to be non-luminous. As far as I can remember photons are radiated when (1) charges accelerate through space (as with radio transmissions), (2) electrons descend to a lower orital, and (3) nuclear processes result in lower binding energies. I don't know much QM, but it seems likely that (4) quark bindings and transmutaions that result in "left-over" energy would be a final source, and the only one that might apply to a neutron star, which are apparently directly observable (I couldn't find much in the way of de