Why are there black holes at the center of some galaxies?

[truetaurus]

How did they form? If a galaxy does not have a black hole, how does it keep its shape?

[JulianO]

How did they form?

They formed by lots of stuff settling in the centre, probably early on in their life. They also tend to funnel in gas and dust from other parts of the galaxy too so continue to grow.

If a galaxy does not have a black hole, how does it keep its shape?

Gravity!

A black hole is nothing special in terms of structure. Its just like a heavy sun. Things orbit around it. However things will happily orbit around any concentration of mass. If doesn't even need to have a dense centre, a set of particles will orbit around their common centre of mass.

People think of black holes as huge all consuming monsters. It's true they will consume matter but only if you get close enough to them. Just like if you get close enough to the sun it will consume you - as will Jupiter incidentally.

They are not some sort of cosmic vacuum cleaner sucking in matter from light years around - they only consume stuff that happens to fall into them. You could happily fly by a black hole and be fine as long as you weren't really close - just like you could with the sun. If you happened to stop by one, then it would drag you in through gravity, but then so would the sun, Jupiter and the Earth. On the last three you'd burn up or end up a pancake, on a black hole you'd disappear.

[Superewza]

They are not some sort of cosmic vacuum cleaner sucking in matter from light years around - they only consume stuff that happens to fall into them. You could happily fly by a black hole and be fine as long as you weren't really close - just like you could with the sun. If you happened to stop by one, then it would drag you in through gravity, but then so would the sun, Jupiter and the Earth. On the last three you'd burn up or end up a pancake, on a black hole you'd disappear.

It's worth noting that it's not just distance, it's speed. The faster you're going the closer to an object you can get without getting pulled in, as you have more energy with which to escape it's gravitational pull, or at least to put yourself in orbit.

That's also how they de-orbit spacecraft from Earth - they don't simply fire 'up' or point it nose down and fire the engines, they arrange it so that the engines are pointing in the direction of travel and then fire, slowing it down and putting it at a lower altitude (which would intersect the atmosphere, slowing it down ever more, and then the ground which... well, you can imagine ). Hence the term 'retro-burn'.

[lw24]

Black holes are essential to understanding how galaxies came to be, and how they formed.

Very early on in the Universes' life, there were few stars. Those that were, grew very big, and shone very bright. Imagine the first ever star to collapse in on itself, so dense that it creates a black hole. Now, nothing in the universe is denser than a black hole, and this is the first black hole ever to come into being in the Universe. Everything in it's close proximity starts to orbit it. And there you have it, an early galaxy. As more stars orbit the black hole, the more dense the galaxy is, and therefore the stronger the gravitational pull of the galaxy. More and more stars are 'pulled' into the galaxy until we have galaxies like today.

Clear Skies

Luke

[JulianO]

Yes - early stars lived fast and died young

One sort of misconception about black holes is that they are somehow more extreme than stars. As far as gravity goes, they are not - if you replaced our sun with a black hole of the same mass, we'd keep on orbiting and not notice... except it would be very cold and no day/night and other such incoveniences.

The early stars were probably much bigger than the biggest stars today, for a variety of reasons, but in general the black hole a star leaves behind at the end of its life is often much smaller in mass terms than the star that went pop.

For instance a star of 15 times the mass of the sun, might go supernova and leave behind a 3 solar mass black hole. So the resulting black hole is actually a fair bit less hazardous than the original sun - and so has less gravitational effect locally. Not that you'd want to go and try and stand on it though!

I'm not entirely sure how the earliest back holes formed, probably as described above, but then they often grow by consuming gas and other material that is slowed down by friction enough to deorbit into the black hole as Superewza described.

I don't know if you seen those tables they have in exploratory science museums, with a hole in the middle - oh

. Its a good way to describe it, but its not true to life. To get into the hole in the middle, unless you actually aim it straight at it, it needs to lose energy. It does that on the table by friction with the table and air resistance. In space there is no table, and little in the way of air resistance, so all things being equal stuff would just tend to orbit it.

[FraserClarke]

Very early on in the Universes' life, there were few stars. Those that were, grew very big, and shone very bright. Imagine the first ever star to collapse in on itself, so dense that it creates a black hole. Now, nothing in the universe is denser than a black hole, and this is the first black hole ever to come into being in the Universe.

Whilst it is true that the first stars were probably very massive (none have been detected yet, so we're not quite sure!), they are very unlikely to be the source of the massive black holes at the centre of galaxies. The stellar black holes are 10's to maybe 100's of solar masses. The black holes in the centre of galaxies are 100000 solar masses or more. We know these formed early on, because we can see quasars right out to redshift 6 or more, and you need a massive black hole to make a quasar. There probably isn't enough time then to make these massive black holes from lots of smaller black holes because, as Julian says, it actually takes a long time to fall into a black hole unless you have some source of fricition.

More likely the black hole is somehow linked to the process of galaxy formation, and starts life much earlier on. The galaxies come together from mass fluctuations in the early universe, which causes matter (normal and dark) to start gathering together (a positive feedback progess; the more matter you have, the heavier you are, so the more matter you can attract, so the heavier you get, etc...). Once you've got a high enough gas density (and the Universe has got cold enough), you can start forming stars. It is still an open question though how you can form such massive black holes so early in the Universe.

[DarkStar7]

Black holes are always in the centre of the galaxies because they are the oldest parts of it and formed first. The further stars are away from the nucleus the younger they are. My Alternative to the Big Bang hypothesis thread predicts this.Further the proto galaxies the Globular clusters have a intermediate sized BH at their centres and and surrounded by the first generation of metal poor stars.

[JulianO]

Black holes are always in the centre of the galaxies because they are the oldest parts.

Just because something is old, it doesn't mean it will form a black hole. You need the required density, and so you have to explain how all the mass got together into a small space. There are some very old globular clusters for instance with no indication of black holes.

The further stars are away from the nucleus the younger they are.

This simply isn't true, or is a gross generalisation. Stars form throughout galaxies. It's true there is often a lot of star formation towards the edge of galaxies, but there are definitely stars recently formed very much towards the centre of the galaxies, and in contrast very old stars towards the edge.

Stars form for lots of reasons, shocks in dust, mergers, feedback loops - all sorts of reasons, so I don't think you can generalise. Stars can also migrate outwards and inwards within the galactic disk. Galaxies are not static things, and new stars form wherever the conditions are favourable.

[NeutrinoWave]

Black holes are always in the centre of the galaxies because they are the oldest parts of it and formed first. The further stars are away from the nucleus the younger they are. My Alternative to the Big Bang hypothesis thread predicts this.Further the proto galaxies the Globular clusters have a intermediate sized BH at their centres and and surrounded by the first generation of metal poor stars.

Not sure here, as Global Clusters contain some of the oldest stars, and GCs are found in the spherical halos of Galaxies, ie very far from the nucleus. Now these very old stars are very near their local black hole (local nucleus)within the GC itself, and these BHs are termed IMBHs, Intermediate Sized Black Holes.

[DarkStar7]

Yes Julinano, what I stated was an over generalisation, but on average the older stars, and higher masses objects will be towards the centre. Rem my hypothesis all objects will get more massive, due to either accumulation of newly produced hydrogen from vacuum of space or through being consumed from nearby BHs There is still new hydrogen being produced near the centre of galaxies but because the objects around that area are much older there are many more high mass objects there. These objects are surrounded by lower mass objects which themselves have lower mass objects orbiting them.

Think about a lonely White dwarf star. It will never get any less massive. One day it will accumulate enough hydrogen to go nova, and may perhaps collapse into a neutron star. Given long enough it too will be a BH. I'm not stating all stars produce BH this slowly, but that all objects can only increase in Mass over time, from protons in vacuum of space to Supermassive BHs whilst the rest of universe expands in all directions.

[DarkStar7]

Neutrinowave, I hypothesise that Globular clusters are the predecessors to the Supermassive BHs. The Globular clusters will grow and after several encounters with other Globular clusters will eventually form into a new spiral galaxy. That is if it not consumed by a local monster galaxy like our Milky Way Galaxy first.

[ScubaMike]

There is still new hydrogen being produced near the centre of galaxies

I would love to see a reference for this statement.

Think about a lonely White dwarf star. It will never get any less massive. One day it will accumulate enough hydrogen to go nova, and may perhaps collapse into a neutron star. Given long enough it too will be a BH. I'm not stating all stars produce BH this slowly, but that all objects can only increase in Mass over time, from protons in vacuum of space to Supermassive BHs whilst the rest of universe expands in all directions.

Untrue, all stars *lose* mass over time, either by the process of converting mass to energy,(at a very slow rate relative to the mass of the star), or via mass ejections.

[DarkStar7]

Before Juliano corrects me...Protons accumulate Mass by Nuclear Synthesis or Molecular bonding.

[DarkStar7]

Untrue, all stars *lose* mass over time, either by the process of converting mass to energy,(at a very slow rate relative to the mass of the star), or via mass ejections.

So what happens to the White Dwarfs and Neutron stars etc.. They float around forever then? Yes Stars try to lose mass... but they fail. Gravity wins.

[JohnnyD333]

I would like to think that these black holes are related to keeping these galaxies together. Isnt the reason we speak of theories such as dark matter and dark energy is because gravity cannot hold galaxies together on its own, they simply fall apart when we create them on computers. Maybe the black holes in the center are somehow related to keeping galaxies from flying apart. Just a theory:)

[DarkStar7]

[quote=ScubaMike "I would love to see a reference for this statement."

Didn't Richard Feynman Quote, something along the lines of... "There is enough vacuum energy in a cup to boil all the oceans of the world"

Interstellar space is a bit bigger than a cup,lol

There is plenty of energy out there to produce the occasional hydrogen atom. I would of thought most of the hydrogen out there using the Standard model would have been used up by now. It ain't going to be a nice future if standard model is right and it does run out Thankfully the big bang model is on shaky ground. Perhaps it's the fact that children like big bangs, because they are fun,that blinds people to truth that is a an erroneous theory. It is not all wrong however, just about 50% wrong. Let's list its failings:

Inflation does not produce the smooth universe required

There is no before. Inflation has to produce something from nothing to the whole universe in a split second...lol, galaxies and clusters at the supposed early universe epoch are too mature...and more massive than near by galaxies. Gamma ray bursts come from huge massive objects, dark energy, dark matter, dark flow... Shall I go on...

[JulianO]

Thankfully the big bang model is on shaky ground. Perhaps it's the fact that children like big bangs, because they are fun,that blinds people to truth that is a an erroneous theory. It is not all wrong however, just about 50% wrong. Let's list its failings:

There are issues with the big bang, and it is likely to be refined, but I don't think your list covers it's major flaws.

Inflation does not produce the smooth universe required

Inflation does produce a smooth universe - it was one of the attractions of the theory - that it explained a number of problems, such as the horizon problem and the smoothness.

There is no before. Inflation has to produce something from nothing to the whole universe in a split second...lol,

Actually there is a before, inflation kicks in about 10^-38 to 10^-32 seconds into the BB, which is after the BB happens. Not very long, I'll grant you, but the earliest epoch in the BB is 10^-43s or so. A few other events happen in the intervening time.

What caused the BB is another question - something the brane worlds have suggestions about.

galaxies and clusters at the supposed early universe epoch are too mature...and more massive than near by galaxies.

I'd like to see evidence for that. Distant galaxies are so hard to get any information about that its difficult to say anything definitive about them. Anything beyond redshift of 3 or so is incredibly fuzzy even in the biggest of telescopes, so you can't say much about their structure. The earliest galaxies are just a single dot.

Gamma ray bursts come from huge massive objects,

Colliding neutron stars I think you mean, so whats the issue with those?

dark energy, dark matter, dark flow... Shall I go on...

Dark matter is on fairly firm ground. We don't know exactly what it's made of, but we can model it and observe its effects. A bit like understanding how to make electric motors and magnets before we knew the electron existed.

Dark energy is less well known about, as is dark flow. Excellent opportunities for research though!

[ScubaMike]

"I would love to see a reference for this statement."

Didn't Richard Feynman Quote, something along the lines of... "There is enough vacuum energy in a cup to boil all the oceans of the world"

Interstellar space is a bit bigger than a cup,lol

There is plenty of energy out there to produce the occasional hydrogen atom.

So, the "occasional hydrogen atom" will offset the loss of mass due to fusion?

By extrapolation, your theory indicates any astronomical body will gather enough mass to ignite into a star unless it is absorbed by or combines with another...

[DarkStar7]

So, the "occasional hydrogen atom" will offset the loss of mass due to fusion?

By extrapolation, your theory indicates any astronomical body will gather enough mass to ignite into a star unless it is absorbed by or combines with another...

Perhaps more than an occasional hydrogen atom. Interstellar is not swept clear by the Suns solar wind. As we see there is plenty of gas produced in the galactic disk to produce nebulae and stars out there.

Yes, if the Earth was moved well away from the influence of Stella winds it would eventually gather enough gas to become a brown dwarf, then a low mass Star, it would be a very slow process, but given enough time if it wasn't consumed by a greater mass companion it would eventually become a BH. Remember however there is nothing in my hypothesis preventing higher mass Stars forming directly from Nebulae with a much faster life cycle, in the typical way we are used to. Let me turn the question on it's head... What eventually happens to all the dead Stella remnants? They can't get any less massive. All things in the universe have a life cycle. Nothing lasts forever, everything including protons, planets Stars and and the very space they occupy have to be recycled. I think it is a very simple and elegant hypothesis that everything returns back to the quantum foam via Black holes, to produce a new generation of expanding space.

[DarkStar7]

[quote=JulianO

Inflation does produce a smooth universe - it was one of the attractions of the theory - that it explained a number of problems, such as the horizon problem and the smoothness."

I'll have to correct you inflation does not produce the smooth CMB radiation we see. This is why Professor Alan Guff had to delay the trigger of inflation to fudge the result. It still doesn't work, how can opposite sides of the Universe be so similar in temperature. The CMB is obviously, at least to me all around us now! I bet the CMB can be measured between the Earth and the Moon. The Moon should completely block the CMB right? I challenge anyone in this forum to prove to me that this is the case.

[NeutrinoWave]

[quote name=JulianO

Inflation does produce a smooth universe - it was one of the attractions of the theory - that it explained a number of problems, such as the horizon problem and the smoothness."

I'll have to correct you inflation does not produce the smooth CMB radiation we see. This is why Professor Alan Guff had to delay the trigger of inflation to fudge the result. It still doesn't work, how can opposite sides of the Universe be so similar in temperature. The CMB is obviously, at least to me all around us now! I bet the CMB can be measured between the Earth and the Moon. The Moon should completely block the CMB right? I challenge anyone in this forum to prove to me that this is the case.

CMB is everywhere and you will notice this from anywhere ie isotropic. However someone in the far distance and far future relative to us may see the radiation red shifted further, as we see the matter decoupling(380,000 years after BB) from thermal radiation redshifted up till now in the microwave, hence CMB.

Whether the universe is homogeneous on large scales, that's another question, as there are recent discoveries of large voids.

SuperMassive Black Holes, IMBHs, and Stellar Black Holes are localised small scale structure, and a completely different kettle of fish.

I am wondering now how this Black Holes thread went off on a tangent into Cosmology, but hey ho, its part of the fun on SGL

[JulianO]

I'll have to correct you inflation does not produce the smooth CMB radiation we see. This is why Professor Alan Guff had to delay the trigger of inflation to fudge the result.

We'll have to agree to disagree then.

"Cosmological inflation has the important effect of smoothing out inhomogeneities, anisotropies and the curvature of space. "

"This process would smooth out spacetime to make the Universe flat, and would also resolve the horizon problem by taking regions of space that were once close enough to have got to know each other well and spreading them far apart, on opposite sides of the visible Universe today."

There are other theories besides inflation that explain the smoothness too - but inflation is currently the dominant one.

It still doesn't work, how can opposite sides of the Universe be so similar in temperature.

Because they were once very very close - before inflation. Its like a infinitely stretchable bit of rubber. Two dots on it just the width of an atom apart, then suddenly pulled out to the width of the solar system. They were very close, now they are very far apart. See above.

The CMB is obviously, at least to me all around us now! I bet the CMB can be measured between the Earth and the Moon. The Moon should completely block the CMB right? I challenge anyone in this forum to prove to me that this is the case.

I don't really see what you mean. If you point a microwave detector at the moon it won't detect CMB? I've no idea - it may well scatter some, in any case I'm not really sure what it proves.

[DarkStar7]

Neutrinowave, sorry mate, yes, Why the Black hole at the centre of Galaxies. Think I heard there is some correlation between Supermassive blackhole size and galactic disc size.

[FraserClarke]

The CMB is obviously, at least to me all around us now! I bet the CMB can be measured between the Earth and the Moon. The Moon should completely block the CMB right? I challenge anyone in this forum to prove to me that this is the case.

The moon is pretty hot compared to the 2.7K CMB, so all you'd detect is thermal radiation from the moon. It's a bit like asking whether the Sun blocks out the light from stars behind it...

In any case, look up "Sunyaev-Zel'dovich effect". This is the scattering of CMB photons as they pass through hot gas in galaxy clusters. Basically, relativisitic electrons 'bump into' CMB photons and give them a bit of boost of energy, making them slightly higher frequency. The effect gives a very characteristic shape change (relating to the electron density and temperature in the cluster) to the CMB spectrum along that line of sight. The SZ effect has been detected for many many galaxy clusters now; proving that the CMB photons originate from further away than the galaxy clusters (z>5).

[drdavekaplan]

Thanks for the info about retroburn

DrDave

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