Center of Our Galaxy - Nebula Orbits & Questions

[doc.madani]

I was wondering what is at the center of our galaxy? could it possibly be large amounts of nebula's in which we are orbiting around?

 

[Glennage]

I was wondering what is at the center of our galaxy? could it possibly be large amounts of nebula's in which we are orbiting around?

It's a supermassive black hole.

 

[doc.madani]

It's a supermassive black hole.

can black holes absorb other black holes?

 

[Glennage]

can black holes absorb other black holes?

Yes, they gain more mass, therefore it's event horizon(area of influence) would increase.

 

[doc.madani]

does time exist in black holes?

 

[Glennage]

Yes I think it does, but say you have a "clock" outside a black hole, it would be ticking normally, at normal speed, but as you fell into a black hole, time would appear to go slower. But I believe time would "stop" at the event horizon..Not 100% sure but that's what I remember anyways. Trying to keep it simple for you.

 

[willem2]

does time exist in black holes?

If you jump in, time exists at least as long as you do.

for someone outside, events that might happen inside a black hole do not happen at any time outside. I think that means that time doesn't exist in a black hole for someone outside.

 

[sylas]

It's a supermassive black hole.

And here are some pictures. We can't see the center of the Milky Way in visible light, but it can be observed in other frequencies. Here's a famous 2003 X-ray image from the Chandra space telescope. (reference link). The black hole itself is "Sagittarius A*" or "Sgr A*".

See also this Chandra image from 2007, showing light echos from when the black hole was thought to have consumed a mass about the size of the planet Mercury.

A nice summary of evidence that the whole galaxy is pretty much orbiting this thing can be found in Is there a Supermassive Black Hole at the Center of the Milky Way?, by Mark Reid, in Int. J. Mod. Phys. D18:889-910, 2009, doi:10.1142/S0218271809014820, arXiv:0808.2624v1 [astro-ph]

Felicitations -- sylas

 

[Dmitry67]

I assume that the Black Hole in our galaxy is 'dead', I mean, it had already consumed all the nearby stars. So it does not work as quasar and does not generate jets.

But what happens when it 'swallows' a star? From time to time there must be stars flying too close? And how often does that happen?

 

[doc.madani]

i was reading some articles and found that the centre of our galaxy is made up of dark matter and dark energy,, and these dark matter and dark energy is what keeps our galaxy together,, through its enormous gravitation.. what are these dark matter and energy?

another thing,, does time require light?? in which light is the only constant in our universe therefore time can only be calculated through measuring and recording light??
so.. light does not exist in black holes therefore time wouldn't exist too?

 

[sylas]

Two responses in one...

I assume that the Black Hole in our galaxy is 'dead', I mean, it had already consumed all the nearby stars. So it does not work as quasar and does not generate jets.

But what happens when it 'swallows' a star? From time to time there must be stars flying too close? And how often does that happen?

Our black hole is not dead; just comparatively quiet when compared with what is seen in some galaxies. It has flared up a number of times; though what we see is radiation from material falling towards the horizon, not material from the hole itself. It is a very strong source of X-rays. I gave a link previously (here it is again) for light echos which are from major outbursts quite recently; and here is a report of a major outbust 300 years ago.

When we say a certain number of years ago, this actually means how long ago the outburst would have been seen at Earth, and then it would be another 26,000 years earlier still to the outburst itself. What we see now are echos of older outbusts.

We've also seen some flares directly quite recently. See Milky Way's Black Hole Sending Out Flares at UniverseToday (18 Nov 2008). A star falling into the hole would give a very large flare. According to this article at space today, such an event should occur about once every million years or so, although for a neutron star or a smaller black hole. That is quite often, on the time scales of a galaxy; not often enough for astronomers to expect to see it.

However, we can see such events in other galaxies. A possible such flare up is reported in New Scientist: Black hole seen devouring star in best detail yet (New Scientist 7 December 2006). This article suggests that a quiet black hole might swallow a star every 10,000 years or so.

i was reading some articles and found that the centre of our galaxy is made up of dark matter and dark energy,, and these dark matter and dark energy is what keeps our galaxy together,, through its enormous gravitation.. what are these dark matter and energy?

I think you have mixed up your article. Dark matter is believed to be an important part of our galaxy, but not in the center. It is thought to be associated as a large "halo" around the entire galaxy. Dark energy is not a part of the galaxy, but something hypothesized to be spread throughout all the space of the universe, and tends to accelerate cosmological expansion.

another thing,, does time require light?? in which light is the only constant in our universe therefore time can only be calculated through measuring and recording light??
so.. light does not exist in black holes therefore time wouldn't exist too?

No. Light is not the only constant (though what you might mean by that is very unclear). Light does exist within a black hole, in the sense that photons have no trouble passing across the boundary of the hole after which they proceed rapidly to the central singularity. Light can't get out of the hole, but that is a different question.

 

[doc.madani]

why does our galaxy become more denser and greater in diameter towards the centre?

can it be due to inactive black holes? or are there clumps of massive neutron stars

 

[Dmitry67]

sylas, thank you for such detailed answer! I am checking the links.

 

[doc.madani]

what is our universe expanding into? if we are expanding into nothing,,, what is the definition of nothing?

 

[sylas]

what is our universe expanding into? if we are expanding into nothing,,, what is the definition of nothing?

It is not expanding into anything. It is not expanding into nothing. There is no "into". It is just expanding. It means over time, there's more space between things.

 

[doc.madani]

hmmm yes,, but i still don't understand how the universe is expanding,, hard to explain,, if we were to travel to the edge of the universe what would we see on the other side?

 

[sylas]

hmmm yes,, but i still don't understand how the universe is expanding,, hard to explain,, if we were to travel to the edge of the universe what would we see on the other side?

There is no edge to the universe, as far as we know. There's no reason to think there is an edge.

 

[Dmitry67]

Here is a good link:
https://www.physicsforums.com/showthread.php?t=261161

 

[doc.madani]

oh, does expanding of the universe simply mean the increase in space between other matter? if so, if we continue to expand at this rate, can the universe collapse on its self?

 

[russ_watters]

How does expanding imply collapse?

 

[matt.o]

How does expanding imply collapse?

You misunderstand the question.

oh, does expanding of the universe simply mean the increase in space between other matter? if so, if we continue to expand at this rate, can the universe collapse on its self?

Yes, the expansion means that distances between objects increase with time. Whether the expansion will cease and reverse depends on the matter density in the Universe. You might be interested in the Swinburne Astronomy Online Encyclopedia entry on the http://astronomy.swin.edu.au/cosmos/D/Density+Parameter". Presently, we think there is far too little matter in the Universe to reverse the expansion and the Universe will continue to expand. In fact, as has been previously pointed out, the rate of expansion is currently accelerating.

 

[doc.madani]

^^ thanks for the links checking them out now

 

[matt.o]

No worries. The Swinburne Astronomy Online Cosmos Encyclopedia is written by professional astronomers and there are many more useful articles than the ones I linked to!

 

[doc.madani]

A little to special relativity, if one spaceship traveling at 0.9c north passes another spaceship traveling at 0.9c south,, wouldn't the ship traveling north see the spaceship traveling south to be traveling at 1.8c,, but that's impossible.. how did Einstein explain this theory?

 

[Fuz]

A little to special relativity, if one spaceship traveling at 0.9c north passes another spaceship traveling at 0.9c south,, wouldn't the ship traveling north see the spaceship traveling south to be traveling at 1.8c,, but that's impossible.. how did Einstein explain this theory?

No. Nothing travels, or appears to travel, faster than the speed of light in any reference frame. When something is traveling that fast (or any velocity) the amount of time that passes, changes. Its called time dilation. No matter what your velocity, you will always see light travel the same speed, c.

 

[Fredrik]

A little to special relativity, if one spaceship traveling at 0.9c north passes another spaceship traveling at 0.9c south,, wouldn't the ship traveling north see the spaceship traveling south to be traveling at 1.8c,, but that's impossible.. how did Einstein explain this theory?

It would see the other ship moving at approximately 0.994475c. In units such that c=1, the general formula is

\vec u\oplus\vec v=\frac{1}{1+\vec u\cdot\vec v}\bigg(\vec u+\vec v+\frac{\gamma_{\vec u}}{1+\gamma_{\vec u}}\vec u\times(\vec u\times\vec v)\bigg)

where \gamma_{\vec u}=\frac{1}{\sqrt{1-v^2}}

When the velocities are parallel, this reduces to

u\oplus v=\frac{u+v}{1+uv}

The \oplus notation isn't standard. It's just what I like to use.