Astronomers may have witnessed a star torn apart by a black hole | Hacker News Reader

Astronomers may have witnessed a star torn apart by a black hole

blogs.discovermagazine.com

94 pointswglb27 comments

Here is followup: http://blogs.discovermagazine.com/badastronomy/2011/04/07/fo...

This occured just on two weeks ago, for definitions of two weeks that encompass plus or minus 4 billion years.

Is this not yet a Well-Known Fact that astronomers say "we observed it two weeks ago" and We The Laymen know that Great Distances are involved and the event itself happened "long, long ago, in a galaxy far, far away?"

The concept is mind boggling and very humbling due to the time scales involved. We can talk about an event that won't happen (we won't observe) till a thousand years from now although it already happened 6000 years ago by inferring from physical laws. Most of the universe around us is in snapshots from long ago.

It was announced in 2007 that the Pillars of Creation were destroyed by a supernova's shockwave about 6,000 years ago.[8] Because of the limited speed of light, observers on Earth currently see the shockwave approaching the pillars, and will not see the destruction occur for about another 1,000 years

http://en.wikipedia.org/wiki/Pillars_of_Creation

Something I have wondered before, if we lived in a universe in which an observation from essentially any distance was observable near instantaneously would this help or hinder astronomy?

I would say hinder. If we assume that interesting events are distributed equally throughout space (i.e. the universe is uniform) but differ over time (the early universe was obviously different to the current universe) then astronomers currently get to observe a wide range of events throughout both space and time. If events were observed immediately from all distances, then astronomers would only be able to observe current events.

More of a problem: the edge of the observable universe is simply the edge beyond which light hasn't yet reached us. If the universe is flat and infinite (as it currently appears to be) then in a universe with an infinite speed of light there would be no edge to the observable universe. We would see everything, and the whole night sky would shine like the sun. This would seem to be a hindrance.

edit: On the other hand if the universe just wraps around, then... actually I'm not sure what happens, but I guess every ray of light emitted from a star gets absorbed by something, which would still make the Earth (if planets could even exist) far too hot.

Yes. It would be immensely more difficult if we could not look back in time. There would be no microwave background, for example, so no knowledge of the fluctuations in density soon after the Big Bang.

As a comparison, do you think it would be easier for an alien to figure out the life cycle of humans if they could see a few humans at every period of life (like in astronomy) or if they only saw middle-aged humans, but many more of them?

On the other hand, it would be possible to effectively communicate with distant species, who in turn might have existed for a long time and have records of past events.

We would also be able to receive messages from distant species before they sent them.

Well the concept of observation and causality would be broken/meaningless because this would imply an infinite speed of light. Hence there would be no limit to the speed with which information could be transmitted. We would have much more interesting things to worry about, like if and how we could exist.

If this were the case, every single aspect of our world would be radically different. It is an impossible question to answer satisfactorily. It is debatable if life as we know it would evolve in such a universe. Among other things, there might be no relationship between cause and effect. It rapidly becomes impossible to even talk about the consequences of such a setup, we do not have the required vocabulary, nor the cognitive structured needed to even contemplate it.

I don't have a great understanding of relativity, but doesn't general relativity state that things happen relative to the observer's frame of reference. So, in fact, this event did occur either 2 weeks or 4 billion years ago, or anywhere in between, depending on where you're standing?

Can anybody clarify or correct me?

The only answer you've gotten so far is garbled nonsense ... and I don't want to make similar mistakes. So I'll stay very simple:

1. You don't need general relativity to talk about this. Special relativity (that's special as in "special case") suffices.

2. It's true that in general, you can't talk sense about the idea that two events happened at the same time; in this case, say, that you looking at your watch two weeks ago and the explosion "really" happening were simultaneous.

I defer to a more effortful write-up for more: http://en.wikipedia.org/wiki/Relativity_of_simultaneity

Relativity is about the perceived passage of time as observed in reference frames traveling at different velocities relative to one another. So it would have happened two weeks ago to someone traveling very (very) fast, but that's got nothing to do with how far they are from the event itself (i.e. where they're standing).

The frame of reference here is not one of location but of relative velocity. You are allowed to account for the distance the light had to travel before it reached you (and you observed the event) in Relativity, just as in anything else as far as I'm aware.

It should also be pointed out that two observers moving at different speeds would not agree on how far away the black hole is.

everything in the universe, including the things like now and two weeks ago, travels not faster than the speed of light

so it happened two weeks ago

I was confused about which were actual images taken of the star and black hole. The first one looks like an "artist's impression" - is it just the grainy picture at the end which actually captures it?

IANAA (I am not an astrophysicist) but isn't the accompanying image very different from what actually happens when a star crosses the event horizon of a black hole?

I thought that when an object crosses the event horizon of a black hole, to an observer outside the event horizon, it will look like the object just slowed down and sort of "suspended" there. I didn't think it would emit a burst of radiation.

(This is just from reading Brian Greene and Michio Kaku, so my understanding could be way off the mark.)

I think you may be confusing the event horizon with the Roche limit. The event horizon is the distance where not even light can escape. The Roche limit is the closest a body can come to another (more massive) body such that it can stay intact with itself, as opposed to being torn apart by tidal forces in the massive object's gravitational field.

I suspect, but do not know for sure, that the Roche radius for a black hole and regular star is much larger than the event horizon for the same black hole.

They are actually somewhat independent. The Roche limit depends on both body's mass and volume, where the event horizon relates to a single body mass and volume.

EX: Cold Brown dwarfs can orbit closer together than a brown dwarf and a earth like planet.

PS: Some real satellites, both natural and artificial, can orbit within their Roche limits because they are held together by forces other than gravitation. Jupiter's moon Metis and Saturn's moon Pan are examples of such satellites, which hold together because of their tensile strength.

Interestingly the equation for the Roche limit has a term for the radius of the primary, but not the satellite. Only the density of the latter matters, not its size.

Basically correct (except the concept of volume doesn't hold up in a black hole). It is theoretically possible to have a black hole big enough that its Roche limit is inside its event horizon; but a black hole that big is probably at least vanishingly rare in the observable universe.

A black hole has a volume less than it's event horizon. So, it's not really undefined as much as ill defined. Also, a black hole's event horizon is not necessarily a perfect sphere ex: two black hole's orbiting each other.

Aha, that makes sense.

And yes, some quick Googling turns up several papers that talk about the Roche limit for a black hole and a star being well outside the black hole's event horizon.

http://www.google.com/search?sourceid=chrome&ie=UTF-8...

Objects would almost never fall straight in to a black hole. In the case where there is relative angular velocity, they spiral in, and then the physics of accretion disks takes over. This involves the release of massive amounts of energy, particularly along radial jets, as the stuff in the disk spirals inwards. The energy release happens well before the infalling matter gets close to the event horizon.

There's nothing overly special about an object disappearing across an event horizon, observationally. However, blackholes happen to have extremely severe tidal forces that border the event horizon, and objects passing through this region (even stars) will be torn apart and smashed back together and will be raised to incredible temperatures and release massive amounts of energy. It is this process which is being observed, and it is actually one of the most efficient processes for converting mass to energy in the Universe.

See: http://en.wikipedia.org/wiki/Black_hole#Accretion_of_matter

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