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What do galaxies orbit?

 
 
aperson
 
Reply Fri 8 Sep, 2006 11:06 pm
I'm not a scientist, but I was just wondering, if a moon orbits a planet, a planet orbits a star, and a star orbits a black hole, what do the black holes in the centre of galaxies orbit in the clusters of galaxies, and what do the objects in the middle of clusters of galaxies orbit. I know that they orbit the centre of the universe, but what exactly causes them to do this?
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Type: Discussion • Score: 11 • Views: 28,936 • Replies: 151
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stuh505
 
  1  
Reply Fri 8 Sep, 2006 11:15 pm
...and I'm not an astrophysicist, but here goes. You can't say that "stars orbit around black holes" to start with. Maybe some do but some stars orbit stars too, or nothing. A black hole is just a remnant of a star after going supernova. A black could could be less massive than a star. A star has enough gravity to have an event horizon, too...it's just not very dense, so the event horizon is less than the visible radius. After exploding in a supernova most of the mass is lost, but what's left is more dense, and if the resulting radius is less than the event horizon (which is now smaller than it would have been before) then voila it's a black hole now. So black holes aren't just at the center of galaxies. Now you ask, what do galaxies orbit? I suppose they would orbit each other given enough time, but I don't think the universe has been around long enough for much of that to happen.
aperson
 
  1  
Reply Fri 8 Sep, 2006 11:30 pm
Stars can't just orbit nothing, unless they're floating on their own out there in the middle of intergalactic space, which I don't think is possible. And even one star orbits another, they both orbit the centre of their galaxy i.e. a black hole. Or even if they both orbit a reletively small black hole, that black hole is orbiting the centre of the galaxy. And galaxies exist in clusters, with, I assume of a great number of other galaxies. They can't all be orbiting eachother, unless the do so in some extremely intricate pattern, which I don't think they do. And clusters orbit the centre of the universe - you haven't answered or even refered to my question about what causes them to do so.
stuh505
 
  1  
Reply Fri 8 Sep, 2006 11:45 pm
Quote:
Stars can't just orbit nothing, unless they're floating on their own out there in the middle of intergalactic space, which I don't think is possible.


The state of the universe is defined by energy. The universe is just a bunch of energy crammed into 1 roughly spherical area that is expanding. This is stationary. That's all there really is. We can look inside the universe and say, "hey look at this, there are lots of bits of energy close together here...let's give this blob a name, like "star" or "black hole" or whatever...but that doesn't make it a discrete thing.

When you look at clusters of energy in the universe, you will find that they are moving around...but that's just when you look at little pieces. As a whole it's stationary but expanding.

The only thing you need to explain why things orbit is to understand this fact: all energy is attracted to all other energy, with a force proportional to the distance between any 2 units of energy.

That is what gravity is. And that simple law, combined with conservation of momentum -- which says that things in motion tend to stay in motion, and things at rest tend to stay at rest, CAUSES orbits to happen.

If you take two objects that are attracted to each other and start them moving at the proper direction, gravity will accelerate the objects towards each other and if done in the correct way you get an orbit. Just think about it it's pretty simple. If you still don't get it why don't you download the gravity simulator I posted a while ago and observe how it works for yourself.
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timberlandko
 
  1  
Reply Sat 9 Sep, 2006 12:03 am
Anything which orbits orbits a center of mass. An atom's electrons orbit the atom's center of mass, its nucleus. A satellite, natural or artificial, orbits a center of mass which is the planet around which the satellite orbits. The center of mass for a planet's orbit is the star at the center of the planet's orbit. A normal black hole in a galaxy orbits just the same as do that galaxy's stars; around the galaxy's center of mass. It also may orbit another black hole, or may have stars orbiting it.

The stars within a galaxy orbit the super massive black hole which is the galaxy's center of mass. Relative to the galaxy of which it is the center, a super massive black hole orbits nothing; the galaxy orbits it. However, a galaxy itself may orbit some other center of mass, such as a cluster of galaxies.

Gravity is the mechanism - though at the atomic level, all 4 fundamental forces (Strong, Weak, Electromagnetic, and Gravity) each play a part. Though the weakest of the 4 fundamental forces, gravity's range of influence is unlimited, and as gravity is a function of mass, the more mass you assemble in one place, the greater will be the gravitational attraction of that mass. The Earth's moon is an appreciable mass, with significant gravitational attraction of its own; that's what causes tides in Earth's oceans, and the moon's gravity also is slowing the Earth's rotation.

The Earth, along with all the other planets, exerts gravitational attraction on the Sun, too - though Earth's mass is such that the gas giants, notably Saturn and Jupiter, have a stronger effect on the Sun than does the Earth. In fact, the first hard evidence we had of planets orbiting other stars was the observation of a star's "wobble", induced by the gravitational effect of very massive bodies - planets more massive than either Saturn or Jupiter - orbiting and "tugging on" their central star.

As for "the center of the universe", well, that's problematic; we have no real idea where or what or even if that is. To practical purpose, the Earth would be the center of its observable universe, but that is simply a function of the limits of observation. We can work out to within very strong probability that the observable universe expanded from a single dimensionless point some 14 Billion or so years ago, but just where in relationship to the current position of the Earth within the unverse that initial dimmensionless point might have been located is yet beyond our capability to pinpoint.
Brandon9000
 
  1  
Reply Sat 9 Sep, 2006 12:05 am
aperson wrote:
Stars can't just orbit nothing, unless they're floating on their own out there in the middle of intergalactic space, which I don't think is possible....

Bingo! The stars are just floating out there in space. Although a few stars might be orbiting something, in general, they're just out there floating.
aperson
 
  1  
Reply Sat 9 Sep, 2006 12:15 am
Alrighty.

I thought that galaxies or clusters or whatever orbited the centre of the mass of the universe?
0 Replies
 
aperson
 
  1  
Reply Sat 9 Sep, 2006 12:17 am
Brandon9000 wrote:
aperson wrote:
Stars can't just orbit nothing, unless they're floating on their own out there in the middle of intergalactic space, which I don't think is possible....

Bingo! The stars are just floating out there in space. Although a few stars might be orbiting something, in general, they're just out there floating.


You're saying that the stars in galaxies which orbit the centre of their galaxy are just a portion of the total number of stars, and that theres a whole lot more out there.
0 Replies
 
Brandon9000
 
  1  
Reply Sat 9 Sep, 2006 12:37 am
aperson wrote:
Brandon9000 wrote:
aperson wrote:
Stars can't just orbit nothing, unless they're floating on their own out there in the middle of intergalactic space, which I don't think is possible....

Bingo! The stars are just floating out there in space. Although a few stars might be orbiting something, in general, they're just out there floating.


You're saying that the stars in galaxies which orbit the centre of their galaxy are just a portion of the total number of stars, and that theres a whole lot more out there.

I'm saying that apart from planets and moons, in general, celestial objects don't orbit anything.
0 Replies
 
aperson
 
  1  
Reply Sat 9 Sep, 2006 12:41 am
Which is still saying that there are a lot of stars that are not in galaxies.
0 Replies
 
Brandon9000
 
  1  
Reply Sat 9 Sep, 2006 12:52 am
aperson wrote:
Which is still saying that there are a lot of stars that are not in galaxies.

There's no relation between the two things. In general, celestial objects are not in orbit around other masses, although they're influenced by them.
0 Replies
 
g day
 
  1  
Reply Sat 9 Sep, 2006 03:08 am
Did I ever mention my hobbies are astronomy and physics?

First you need to know not all things in space are in gravitational bound structures - don't assume gravity has both radiated everywhere or where it has reached it has enough strengh to be observable by any means we have today.

Second lets talk about structures in space in increasing size:

Planet -> Solar System -> Galactic spiral -> Galaxy -> Local Group -> Cluster -> Super Clusters

So Galay is in a group of 5 called the local group, which is part of the Virgo cluster which is part of the Great Attactor.

So as you zoom out your scale you get areas that are definitely gravitational bound, and areas that discernibly aren't (i.e. great voids of space that are truly empty of energy or matter).

Third recognise that spacetime itself isn't constrained by Relativity to expand at less than lightspeed, because the dimension itself is streching rather than travelling outwards. Space that is not subject to gravity (i.e. exceptionally large empty voids between clusters can't be considered gravitational bound, so isn't much subject to the laws of relativity.

Now onto your question.

We are currently onto 3rd generation stars. The first proto-stars where at least hundreds of times larger than typical suns today, and they only lasted one tenth of the lifetime of stars we have today.

Proto-stars died in massive super novae (creating much of the heavy metals (denser than iron) we experience today. Gold or Uranium for instance only comes from Super Novae. Now the cores of these massive stars may have formed giantic blackholes.

Such a body could form a central hub for a galaxy. We believe at the centre of our galaxy is a mass smaller than our Solar System but wieght 30,000 times that of our Sun.

Now galactic clusters are gravitationally bound, around the centres of mass (point of suspension).

But gravity takes time to radiate out. After the Big Bang gravity wasn't everywhere, the universe is expanding and most of the universe can't feel the gravity of most of its clusters. The outward rush of energy from the big bang during inflation was likely 50,000 faster than lightspeed, so the expansion of spacetime outraced creation's gravity wave (else creation would have halted instantly - the event horizon of the Universe - were all mass in one small spot - would be about 95% of its diammeter today!).

So your question is actually very simple but the answer is anything but!
aperson
 
  1  
Reply Sat 9 Sep, 2006 03:26 am
THANK YOU G__DAY!!! Very Happy

Finally someone who can answer my questions without going off on great tangents!
0 Replies
 
aperson
 
  1  
Reply Sat 9 Sep, 2006 03:38 am
I just happened to have read an article about the dark era of the universe and the super stars that existed within it in a Time magazine.
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g day
 
  1  
Reply Sat 9 Sep, 2006 05:27 am
The early universe probably wasn't transparent until 300,000 years after the big bang. The average energy levels were too high until then for a stable medium to exist to allow light to travel unimpeded.

Anyway for a Sun to form you need a cloud of atomic hydrogen to collapse under its own weight - say a few million years.

If only atomic hydrogen is available Suns get very, very big - and last 100 million to 1 billion years (vs 5 - 12 billion for 3rd generation Suns).

There are likely gravitational models for this - but you need a supercomputer to model it. Otherwise try maybe freeware like gravit to see a simpler model!
0 Replies
 
rosborne979
 
  1  
Reply Sat 9 Sep, 2006 07:11 am
g__day wrote:
Now galactic clusters are gravitationally bound, around the centres of mass (point of suspension).


I believe that the collective center of a gravitationally bound group is called a barycenter.

Galaxy clusters which are bound would have a barycenter.

Our solar system has a barycenter which is not always at the center of the sun (in fact, it almost never is). Our sun happens to be very close to the barycenter because it contains a majority of the mass, but if we had two suns of similar size (and no planets), then the two suns would orbit a point (barycenter) which was between them.

The fact that stars and planets orbit a barycenter (and not the exact center of the central star) causes stars to appear to "wobble" when observed from outside the system. This is one mechanism that we use to detect planets orbiting distant stars.
0 Replies
 
littlek
 
  1  
Reply Sat 9 Sep, 2006 07:47 am
So, would the star nurseries of nebulae demonstrate to aperson that stars sometimes just hang out? How about constellations?

This is a great thread!
0 Replies
 
rosborne979
 
  1  
Reply Sat 9 Sep, 2006 08:10 am
littlek wrote:
So, would the star nurseries of nebulae demonstrate to aperson that stars sometimes just hang out? How about constellations?


Constellations are often composed of stars which are nowhere near each other. The relative magnitude of the stars in a constellation makes them appear to be in the same plane (distance from us), but in many cases they are separated by gigantic distances and are not gravitationally associated at all.

For instance, Betelgeuse and Bellatrix are both in the constallation Orion, but one is nearly twice as distant as the other.

A distinction is often made between things which are gravitationally bound and things which are not. For example, "Open Clusters" are not gravitationally bound, whereas "Globular Clusters" are.

Stars within a globular cluster are gravitationally bound to each other, but the cluster itself is also bound to its galaxy. And an open cluster is also bound to its galaxy, even though its component stars are not bound to each other.
0 Replies
 
littlek
 
  1  
Reply Sat 9 Sep, 2006 08:16 am
Rosborne - so, the stars in the constellations - are they all orbiting in gallaxies?

So, the idea of open clusters would help aperson see that not all stars are in orbiting gallaxies?
0 Replies
 
littlek
 
  1  
Reply Sat 9 Sep, 2006 08:20 am
Hmmm... from wiki: An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity. Open clusters are found only in spiral and irregular galaxies, in which active star formation is occurring.

Sounds like the open clusters are part of galaxies.
 

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