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View Full Version : A star can be ripped apart in a matter of days?

beajerry
02-19-2004, 10:01 AM
http://www.cnn.com/2004/TECH/space/02/18/shc.blackhole.star/index.html

about a star the size of our sun "ripped to shreds over hours or days" by a black hole. Did the writer get this right?

That's a lot of matter to be 'shredded'! How can it be done over such a small amount of time? One would think it would take years or even centuries for such an event to happen.

jjimm
02-19-2004, 10:08 AM
Depends where you're looking at it from. IIRC, if you're on the star/planet it will take centuries. If you're observing the event from billions of miles away, it could take just days, I presume. It's all relative.

ftg
02-19-2004, 10:39 AM
Well, how long the event took in our time frame is all that matters. Secondly, from the time frame of the ripped star, the slowing due to being in the high gravity field of the black hole wouldn't be large enough to change such a broad range as hours or days. From the perspective of the black hole, well, there isn't one.

The event took place during one swingby of the star past the blackhole. The star appears to have come in on a hyperbolic trajectory, swung past the black hole and zipped out along the exit line of the curve, losing a lot of mass to the black hole in the process.

To figure out how long this event took, you need to know the original path of the star, its speed, the masses of the two objects. Note that there is a lot of interaction on the data. The bigger the black hole, the closer the star gets and the faster it passes by. Based on the info give, clearly the astronomers had approximate figures for all these and thus could deduce a time range in which it occurred.

Comets are routinely broken up by close approaches to the Sun. The time period for such breakups can easily be hours. But, our Sun is not a black hole. A black hole can rip far larger objects apart, like stars, if they get close enough. (Even two planets will tear each other apart if they get close enough. Google "Roche Limit.")

If a star was moving reasonably fast, and just skimmed by the event horizon of a black hole, it could indeed be ripped apart in an hour.

It would only take centuries if that was how long it took for the star to pass by. Given that that implies distances on the scale of solar system diameters, it would be unusual for a star to be broken apart at such distances.

In short, if the star is close enough and the black hole big enough to rip it apart, it isn't going to take long.

Chronos
02-19-2004, 01:01 PM
Days actually seem like an unreasonably long time to me. The black hole in question was 100 million solar masses, and the minimum timescale for something like this to happen would be comparable to the black hole's mass (in the appropriate units). Converting 100 million solar masses to time units gives us just over 8 minutes.

jjimm, you have the time dilation backwards. From the perspective of someone staying safely outside the hole, an object never quite finishes falling through the event horizon, but for an observer falling in, it takes a finite (and very short) amount of time to cross the horizon and reach the singularity (at which point it's definitely curtains). Regardless, time dilation can't have been too significant in this case, or we wouldn't have seen the light from it.

MC Master of Ceremonies
02-19-2004, 01:19 PM
jjimm, you have the time dilation backwards. From the perspective of someone staying safely outside the hole, an object never quite finishes falling through the event horizon, but for an observer falling in, it takes a finite (and very short) amount of time to cross the horizon and reach the singularity (at which point it's definitely curtains). Regardless, time dilation can't have been too significant in this case, or we wouldn't have seen the light from it.

Correct me if I'm wrong Chronos, but surely that's only ture for a remote observer rather than a realistic observer.

rsa
02-19-2004, 01:27 PM
Days actually seem like an unreasonably long time to me. The black hole in question was 100 million solar masses, and the minimum timescale for something like this to happen would be comparable to the black hole's mass (in the appropriate units). Converting 100 million solar masses to time units gives us just over 8 minutes.

But isn't that making the assumption that this was more-or-less a "head-on" collision?

In this case, the star had a very large tangential velocity and only about 1% of the stars mass was actually consumed by the black hole. The remainder of the star's mass was flung off into space by momentum and the radiation pressure from the mass which did fall into the black hole.

02-19-2004, 01:43 PM
Relativity aside, doesn't a supernova only take hours to days? Granted, that leaves a small stellar remnant behind, but it blows up most of a star.

Iteki
02-19-2004, 02:16 PM
Can I mini-hijack just to ask *when* this happened?
They were talking about it on the news this morning, and saying it happened 700 million light years away (or whatever). At first I thought they had viewed it in regular terms (which, with my very limited understanding, would mean it happened about 700 million years ago? ), but they "viewed" it by the xrays generated, how fast do they travel? When was the star actually destroyed?

Grey
02-19-2004, 02:45 PM
X-rays are simply a form of light with a wavelength smaller than visible light. They travel at c.

DarrenS
02-20-2004, 01:11 AM
Apparently a collision could easily cause this in a few hours. From here (http://www.sciamdigital.com/browse.cfm?sequencenameCHAR=item2&methodnameCHAR=resource_getitembrowse&interfacenameCHAR=browse.cfm&ISSUEID_CHAR=855D8E17-6E81-4F00-93FF-B9570962BBE&ARTICLEID_CHAR=A19DC0AC-D8D8-48A1-B474-815C9C3A58C) (full article costs money)It would take only an hour for the white dwarf to smash through, but the damage would be irreversible. The superheated sun would release as much fusion energy in that hour as it normally does in 100 million years. The buildup of pressure would force gas outward at speeds far above escape velocity. Within a few hours the sun would have blown itself apart. Meanwhile the agent of this catastrophe, the white dwarf, would continue blithely on its way-not that we would be around to care about the injustice of it all.

Napier
02-20-2004, 03:14 AM
Actually, according to recent work on gamma ray bursters as described in the latest issue of Sky and Telescope, substantial parts of a star can be consumed by a black hole forming at its center in as little as part of a second. The mechanisms aren't all understood, but there have been GRBs that short.

Some of these energetic stellar events can be pretty impressive. I read about these pillars that form on the surfaces of neutron stars with stellar neighbors getting consumed and raining down on the neutron star. The stuff rains down so hard that it radiates thermal xrays with such intense radiation pressure that they drive the stuff back up, maybe a couple hundred feet. Then it falls in again and repeats the cycle. The pillars are hundreds of feet across, and this cycle happens a thousand times a second. The average energy released at the surface is equivalent to a billion hydrogen bombs per second per square meter, IIRC.

jjimm
02-20-2004, 09:31 AM
I got it the wrong way round, didn't I. Sorry.

beajerry
02-20-2004, 06:42 PM
Ok, but let's say you are in a cozy boxseat only a few million miles away with your beer and peanuts and indestructable radiation-shield sunglasses (don't forget the SPF#2500 sunscreen) watching the star get ripped apart.
Nevermind watching the starstuff swirl down the black hole for eternity, would you actually see the star be ripped apart in a matter of hours? Right there in front of your eyes?

I mean we're talking a lot of matter here! Would the process start out painfully slow and speed up to a 'flash' of an event?

Cartooniverse
02-20-2004, 08:07 PM
Depends where you're looking at it from. IIRC, if you're on the star/planet it will take centuries. If you're observing the event from billions of miles away, it could take just days, I presume. It's all relative.

Your answer makes me think that time is compressed depending on space and distance. I didn't believe this to be so. In other words, an event that takes a week, but happened 120 million years ago, will find us in due time and appear to our deep-space telescopes- but wouldn't it occur in "real time" and take no less or more time than it took in it's own space and moment, when the incident first took place, 120 million years ago? It travels to our solar system at 186,000 mps. but I don't understand how it can be compressed as far as our view of an event.

Cartooniverse

jjimm
02-21-2004, 09:14 AM
Your answer makes me think that time is compressed depending on space and distance. I didn't believe this to be so. No, I meant time would be compressed as the star accelerated => c as it approaches the event horizon.

eburacum45
02-21-2004, 06:20 PM
The interesting thing to me is that most of the mass did not get sucked in;

black holes are excellent destructive tools, and you can use them to convert any solid matter into energy; only a tiny amount of the matter need be lost into the hole, if you aim it right .

There may be a few black holes in the local area of space-
several candidates are known within 1kpc;
instead of avoiding them in fear we should make use of their potential as generators.

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