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I have a question, imagine a Galaxy with a supermassive black hole smack in the middle, with a mass of a billion suns, devowering stars at an incredible rate and relatively growing as it consumes stars.

A black hole that large in the center of our galaxy, “Sagittarius A” it’s location is believed to be, in the densest part of our galaxy (I mean concentration and proximity of stars) would be swallowing up stars at an enormous rate would it not?.

Astronomers say it is possible that all, most, many? Galaxies have a supermassive black hole at their cores, let’s assume this is fact, eventually an entire galaxy can be consumed by a black hole that size. The more it consumes, the greater it’s gravitational pull, it eventually grows exponentially until the whole galaxy is snuffed out?.

Having said that, there surely must be examples somewhere of galaxies in the process of being swallowed up, in their final death throws, the remnants of spiral arms and the few millions of stars left on their final plunge into blackness and infinite, invisible density. 

Or,  supermassive black holes may well be a relatively new phase in a galaxies life cycle? are we living at the point where these gargantuan black holes are coming into existence, and maybe it will be billions of years more before galaxies begin to fall prey to their own black holes?. Are there any examples of galaxies much older than ours where evidence shows they are being consumed slowly?

Something as massive, and destructive, as a black hole with a billion star mass, smack in the middle of a galaxies densest region must eventually become a runaway, growing exponentially and consume an entire galaxy, would it not?. Is it possible that our present stelliferous age will meet its end by way of ever expanding black holes?. 

I would love to hear your thoughts!

 

 

Edited by Sunshine
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Interesting thoughts.

Now, you can put me somewhere below ‘complete numpty’ level in terms of my knowledge here, but I seem to remember that because gravitational strength falls of by the inverse square law, the influence of the black holes gravity actually falls off relatively fast. I think that is why galaxies are not just collapsing into their central black holes; the centripetal forces (right term?) tending to fling the stars away from the centre must be balancing the gravitational force of the central black hole and surrounding core stars.

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1 hour ago, Sunshine said:

Having said that, there surely must be examples somewhere of galaxies in the process of being swallowed up, in their final death throws, the remnants of spiral arms and the few millions of stars left on their final plunge into blackness and infinite, invisible density.

If there were such galaxies in their "final death throws", I am not sure we would be aware of them. Yes, there might be quite a lot of stars in a particular pattern, but take a galaxy in the Virgo cluster (which cosmically speaking is pretty close). How many individual stars can we see in one of those galaxies (excluding things like supernovae)? Or, at the other end of the scale, consider how faint local 'dwarf' galaxies are when you are looking at individual stars rather than the combined glow of trillions.

Now, consider trying to identify structure in something as faint as one of our dwarf galaxies at the distance of the Virgo cluster ....

The absence of evidence is not necessarily evidence of absence ...

Edited by Demonperformer
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22 minutes ago, Stu said:

Now, you can put me somewhere below ‘complete numpty’ level in terms of my knowledge here, but I seem to remember that because gravitational strength falls of by the inverse square law, the influence of the black holes gravity actually falls off relatively fast. I think that is why galaxies are not just collapsing into their central black holes; the centripetal forces (right term?) tending to fling the stars away from the centre must be balancing the gravitational force of the central black hole and surrounding core stars.

This could be true, but there is evidence I gather that these supermassive black hole may just play a bigger part in the evolution of galaxies than previously thought. Apparently, there is evidence that the rotational speed of galaxies is directly related to the size of its black hole as measured by the speed of its outermost stars. If this is true then the effect that the black hole has on its parent galaxy is far more than we may think.

If this is true then a galaxy with a supermassive BH at its core is essentially like a toilet bowl on the flush, just a matter of time before it’s gone.

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The runaway effect you describe need not take place at all, if there is no drag due to e.g. gas and dust. Suppose I have a black hole of ten million solar masses, with a roughly spherical distribution of stars orbiting it. Now take a single star in an orbit at a safe distance around it. Its orbit is determined by that of the black hole at the centre and the sum of masses of the stars within its orbit. If the black hole swallows any number of stars, the total mass within the orbit of our "safe" star doesn't change, so its orbit doesn't change.

There are indications that the capture rate of stars is higher during early stages of galactic evolution, which is why quasars are generally far away, and nearby Seyfert galaxies are relatively uncommon, and that once all stars in unstable orbits near the hole have been swallowed, the black hole can become relatively quiet, swallowing only the odd star from time to time. Burst of activity might occur when orbits are perturbed, e.g. during a merger. 

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5 hours ago, michael.h.f.wilkinson said:

The runaway effect you describe need not take place at all

I guess my err was overestimating the proximity of stars in a galactic core, and sheer distances regardless of how they look in pictures, where stars seem to be on top of one another. 

you are correct, in the beginning a black hole would have much more to feed off of but eventually an equilibrium would be reached, whereby stars within the safe zone would fall into orbits as usual with the exception of one here and there.

It still is stimulating to say the least to ponder, considering there is still so much left to discover about them.  Every time I think about what could possibly be occurring in the depths of a black hole “singularity” as it is called, my brain ties itself in a knot and begs for mercy lol.

Edited by Sunshine
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The Sun's orbit is determined by the total mass inside its orbit. How much off that mass is concentrated in Sagittarius A* is not relevant to the Sun's orbit. 

It's the amount of mass inside an orbit that matters, not how concentrated it is toward the centre.

Only if the total mass inside the Sun's (or any other stars') orbit increases will the gravity on the Sun increase (or on that other star).

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There are stars more than 13 billion years old in the Milky way so we know  that galaxies  are not consumed by their central black holes. This implies the rate at which stars fall into black holes must be extremely slow. (There is no surprise here, Mercury has been orbiting close to the Sun for billions of years without being consumed) When stars occasionally do fall into black holes, they release large amounts of energy, producing a so called tidal disruption event (TDE) which releases a lot of energy, similar to a supernova explosion. These are thought to be very rare per typical galaxy, perhaps 1 per hundred thousand years. See here for example

https://www.gaia.ac.uk/alerts/what-and-why/why-are-we-looking/tde-agn

An event like this in our galaxy would  be obvious.

You can see the stars near to Sag A orbiting our black hole here for example

 

Robin

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Material (gas and dust) orbiting close to a galaxy central black hole can produce large amounts of energy through friction (heat). We see these as active galaxies and quasars, which are some of the most powerful objects in the universe. Even here though, the amount of material actually being consumed by the black hole is still very small compared with the mass of the galaxy and the radiation from these objects varies significantly when there is more or less material close enough. (There is not enough material close enough to our blackhole currently to produce the amount of radiation seen from a Quasar, though Sgr A is a strong source of radio emission)

Robin

Edited by robin_astro
Sgr A not Cas A
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  • 2 months later...

Supermassive B?lack holes massive as billóns of suns and the the gravitational pull is very strong.

Galaxies consumed by its Supermassive blackholes in  its core  its possible if the galaxy is too small and its density is too low but we need a proof  and cases of it

This idea still a science fiction for now.?

 

 

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  • 1 month later...

I'm still not convinced the universe wasn't brought into existence from the collapse of a black hole they can only get so big before something freaky happens 2+ trillion galaxies worth of mass in one place that's a lot of gravity eventually something has to give.

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1 hour ago, Bovski said:

I'm still not convinced the universe wasn't brought into existence from the collapse of a black hole they can only get so big before something freaky happens 2+ trillion galaxies worth of mass in one place that's a lot of gravity eventually something has to give.

Where was that place if the universe did not exist yet?

Regards Andrew

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19 minutes ago, andrew s said:

Where was that place if the universe did not exist yet?

Regards Andrew

Space :)

I'm not letting out any secrets you cant catch me out that easily. LOL

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On 22/03/2018 at 09:43, michael.h.f.wilkinson said:

The runaway effect you describe need not take place at all, if there is no drag due to e.g. gas and dust. Suppose I have a black hole of ten million solar masses, with a roughly spherical distribution of stars orbiting it. Now take a single star in an orbit at a safe distance around it. Its orbit is determined by that of the black hole at the centre and the sum of masses of the stars within its orbit. If the black hole swallows any number of stars, the total mass within the orbit of our "safe" star doesn't change, so its orbit doesn't change.

There are indications that the capture rate of stars is higher during early stages of galactic evolution, which is why quasars are generally far away, and nearby Seyfert galaxies are relatively uncommon, and that once all stars in unstable orbits near the hole have been swallowed, the black hole can become relatively quiet, swallowing only the odd star from time to time. Burst of activity might occur when orbits are perturbed, e.g. during a merger. 

Excellent!

Well, that's all, really....

?lly

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This is an interesting topic, I'm not qualified to answer the question but a good book on black holes is "Gravity's Fatal Attraction" by Begelman and Rees.

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