# Blackholes and galaxies

I'm sure this is a stupid and/or already asked question. But I don't do astronomy.
If every galaxy has a black hole, and black holes suck everything in, does this mean that our galaxy is being slowly destroyed by black holes? (Everything just keeps getting sucked in) And if so, are we gonna die due to the massive gravitational pull, eventually? Or are the black holes, like, inactive or something? (can black holes BE inactive?)
And if this is all true, how come everything isn't already gone?

Last edited:

tiny-tim
Homework Helper
Welcome to PF!

Hi GreatEscapist! Welcome to PF!

No, black holes have no sucky-power.

Matter crashes into a black hole in exactly the same way that it crashes into a star of the same mass …

the only difference is that the event horizon of a black hole is slightly smaller than a star of the same mass, so there's more chance of the matter whizzing by and escaping!

Chalnoth

the only difference is that the event horizon of a black hole is slightly smaller than a star of the same mass, so there's more chance of the matter whizzing by and escaping!
Heh, slightly? Yeah, you're understating that just a little...the event horizon of a black hole the mass of our own Sun would be about 3km in radius. Our Sun's current radius is 700,000km. So yeah, most matter just misses black holes.

Sucky-power! LOL

phyzguy
If every galaxy has a black hole, and black holes suck everything in, does this mean that our galaxy is being slowly destroyed by black holes? :

The key word in this statement is "slowly". It's true that more and more of the mass of the universe is falling into black holes as time goes on. However, the time for this to happen is extremely long, far longer than the current lifetime of the universe. Long, long before this will happen, we have other things to worry about, such as the sun expanding into a red giant and consuming the earth.

Nabeshin
The key word in this statement is "slowly". It's true that more and more of the mass of the universe is falling into black holes as time goes on. However, the time for this to happen is extremely long, far longer than the current lifetime of the universe. Long, long before this will happen, we have other things to worry about, such as the sun expanding into a red giant and consuming the earth.

There are such things as gravitationally unbounded systems. So no, even given infinite time, a black hole will not accrete everything.

Chalnoth
There are such things as gravitationally unbounded systems. So no, even given infinite time, a black hole will not accrete everything.
Over very large time spans, nearly everything will eventually run into one or decay. We're talking around 10^40 years for this to happen, though, so yeah, a long way off:
http://en.wikipedia.org/wiki/Future_of_an_expanding_universe

Nabeshin
Over very large time spans, nearly everything will eventually run into one or decay. We're talking around 10^40 years for this to happen, though, so yeah, a long way off:
http://en.wikipedia.org/wiki/Future_of_an_expanding_universe

Wikipedia said:
The end result is that most objects are ejected from the galaxy, leaving a small fraction (perhaps 1% to 10%) which fall into the central supermassive black hole.

So much more matter will simply go off and decay in the middle of nowhere than will be consumed by the black holes.

Once central galactic black holes suck in all immediate matter in the area, everything will for the most part, become stable. There are I think 10 or so stars observed orbiting our central black hole, and are all in a relatively stable orbit. I will see if I can find the data.

tiny-tim
Homework Helper
Heh, slightly? Yeah, you're understating that just a little...the event horizon of a black hole the mass of our own Sun would be about 3km in radius. Our Sun's current radius is 700,000km. So yeah, most matter just misses black holes.

Hi Chalnoth!

I was thinking more of comparison with a neutron star

from http://en.wikipedia.org/wiki/Neutron_star" [Broken] …
A typical neutron star has a mass between 1.35 and about 2.1 solar masses, with a corresponding radius of about 12 km if the Akmal-Pandharipande-Ravenhall (APR) Equation of state (EOS) is used.

… so a neutron star of the same mass would have about 4 times the radius of a black hole, which is nearer "slightly" than about 200,000.
Sucky-power! LOL

Hi graybass!

I always use that word, because I honestly can't think of anything more sensible.

"Attraction" won't do, since gravity attracts anyway … I need a word that suggests more that the average atttractiveness … a sort of go-out-and-getishness rather than a just-lying-in-wait-ishness.

Hence "sucky-power" !!

Last edited by a moderator:
Chalnoth
Fair enough :)

I would add, that from the point of view of a poor sucker in the FAAAAR future... into black holes, or ejected to decay, is 6 of one, a half dozen of the other. Given time, BHs will start to slooowly radiate, shrink, and radiate faster. The end result (should) be the same... diffuse radiation.

Kinda kills the notion of immortality as "desirable" doesn't it?

I feel so mortal now.
Black holes makes me feel so tiny...and scared.

I feel so mortal now.
Black holes makes me feel so tiny...and scared.

Cosmology has a way of doing that.

EDIT: All kidding aside, if you're being at all serious, there is no reason to be afraid. As others have said, they don't present a particular threat, and is thinking about the end of the cosmos as we know REALLY any more terrifying than contemplating your own death? I find that imponderables tend to evoke similar reactions of dread and fear. Recognize it, and be free to pursue a good and ethical life as best you can. After all, nothing matters except that it effects the living, and absolutes only exist if you believe in them (or by some chance, there is a deity(s)) and want to follow them. For the rest, concern yourself with the living, the present, and the future you CAN effect... the rest is for later, or not (and we'll find out... or not).

Last edited:
Yeah, I guess that's all I can do.
Black/white holes are so friggin interesting.