Can a collapsing star only form a black hole if it has a companion?

[TheBlackNinja]

I've ready about how they don't fit the expected and all that dark matter thing, so I'd like to know what kind of variability of those curves in function of some paramete3r has been found.

For example - Is it known if it varies with the galaxy's age?

 

[Drakkith]

I only know the basics, so I believe it only depends on the rotational speed compared to the amount of visible matter/mass we have determined the galaxy possesses. I don't think age has anything to do with it.

 

[ibysaiyan]

Welcome to PF TheBlackNinja !
One of the ways in which we assume the presence of dark matter is by the effect it has over the rotational velocity of spiral/elliptical galaxies (I think).When we plot a graph of rotational curve of a particular galaxy against the distance from the centre of the galactic bulge(I think you might have mis-interpreted galactic center to age,maybe).

What we observe is that near the proximity of visible galaxy the rotational velocity values for the i) Observed and ii) expected value appear to be very similar i.e negligible difference but as we go further away from the center to our surprise we note the expected value to be constant(why is it constant for the density is decreasing).This is where the mysterious,hypothetical matter comes into the picture.

At lower radius we get the relation of 'v' proportional to 'r' but on high radius we see 'v' proportional to 'r^-1/2' .
EDIT:As for the expected values,we get them by taking into consideration the whole baryonic matter ,the micro waves given off by the neutral hydrogen clouds,etc.
Regards,
ibysaiyan

 

[TheBlackNinja]

Welcome to PF TheBlackNinja !
One of the ways in which we assume the presence of dark matter is by the effect it has over the rotational velocity of spiral/elliptical galaxies (I think).When we plot a graph of rotational curve of a particular galaxy against the distance from the centre of the galactic bulge(I think you might have mis-interpreted galactic center to age,maybe).

What we observe is that near the proximity of visible galaxy the rotational velocity values for the i) Observed and ii) expected value appear to be very similar i.e negligible difference but as we go further away from the center to our surprise we note the expected value to be constant(why is it constant for the density is decreasing).This is where the mysterious,hypothetical matter comes into the picture.

At lower radius we get the relation of 'v' proportional to 'r' but on high radius we see 'v' proportional to 'r^-1/2' .
EDIT:As for the expected values,we get them by taking into consideration the whole baryonic matter ,the micro waves given off by the neutral hydrogen clouds,etc.
Regards,
ibysaiyan

Hi there! thanks for the reply and the welcomes

you seem to have swapped "observed" with "expected" where I highlighted.

I think I had understood that before, but I was just thinking that this "thing"(which they define as an non observable matter) may have have some typical dynamic.Like, its distribution inside the galaxy may change over the time.So I was thinking that observing galaxies at different ages could give some clue.
I had actually thought about lots of black holes before, because I had read the the youngest spiral galaxies had mostly O and B stars, and as they die fast they'd probably be black holes now.So in those young galaxies the rotation curve would be as expected, since they don't have those numerous black holes yet.
This idea may be naive but it should help to explain what I think.Maybe whatever is that dark matter was not dark at some instant, or it was not distributed that way that gives that characteristic rotation curve, or something like that

 

[Nabeshin]

I think I had understood that before, but I was just thinking that this "thing"(which they define as an non observable matter) may have have some typical dynamic.Like, its distribution inside the galaxy may change over the time.So I was thinking that observing galaxies at different ages could give some clue.

Dark matter is of course dynamic in these sense that it responds to gravitational potentials, but by the time a galaxy has formed the dark matter has already collapsed into a more or less stable configuration. So you don't really expect to see variation among galaxies -- before galaxies are formed there is a lot going on with the DM but that's another topic.

I had actually thought about lots of black holes before, because I had read the the youngest spiral galaxies had mostly O and B stars, and as they die fast they'd probably be black holes now.So in those young galaxies the rotation curve would be as expected, since they don't have those numerous black holes yet.

Black holes, or any compact object, do not at all alter the rotation curves of galaxies. For one, the percentage of the mass contained in these objects is quite small. But the bigger issue is that, in a global gravitational sense, a black hole behaves no differently from a star, a clump of gas, or anything else. The mass is mass as far as the galactic rotation curve is concerned -- the only differences arise when you're very very close to the hole, and that's obviously not the case when we're speaking of things on galactic length scales.

 

[ibysaiyan]

Hi there! thanks for the reply and the welcomes

you seem to have swapped "observed" with "expected" where I highlighted.

I think I had understood that before, but I was just thinking that this "thing"(which they define as an non observable matter) may have have some typical dynamic.Like, its distribution inside the galaxy may change over the time.So I was thinking that observing galaxies at different ages could give some clue.
I had actually thought about lots of black holes before, because I had read the the youngest spiral galaxies had mostly O and B stars, and as they die fast they'd probably be black holes now.So in those young galaxies the rotation curve would be as expected, since they don't have those numerous black holes yet.
This idea may be naive but it should help to explain what I think.Maybe whatever is that dark matter was not dark at some instant, or it was not distributed that way that gives that characteristic rotation curve, or something like that

Hi,
Oh silly me! Thanks for the correction ,I hope that made sense because I had it written down at about 3.00am.Just as Nabeshin mentioned earlier black holes don't appear any exotic it's only when objects go pass the event horizon which is what separates black hole to the rest.

Of the WIMPS candidates we have axions,neutralinos(cold) and neutrinos.I was under the impression that not all stars transit into black holes(For the super giants once it has gone through type II supernovae the leftover remnant /core becomes a neutron star,which further collapses only if the Tolman Oppenheimer-volkoff limit is surpassed also not all black holes survive,they evaporate too due to entropy(I might be wrong). This is as much I can recall.
Regards,
ibysaiyan

 

[Nabeshin]

I was under the impression that not all stars transit into black holes(For the super giants once it has gone through type II supernovae the leftover remnant /core becomes a neutron star,which further collapses only if the Tolman Oppenheimer-volkoff limit is surpassed

That's right. In fact, very very few starts collapse directly into a black hole. The most promising mechanism is you have a medium-mass star (~15-20 solar masses) which collapses into a neutron star with a companion nearby. Then you slowly accrete gas until you pass the TOV limit and collapse to a BH. It's really difficult to do without a companion, since to directly collapse into a BH you need something ~100 solar masses.

also not all black holes survive,they evaporate too due to entropy(I might be wrong).
ibysaiyan

While it is true that black holes evaporate, any that have formed as a result of stellar collapse must have roughly stellar masses (take 1 solar mass). The lifetime for a black hole this size is many orders of magnitude larger than the current age of the universe (Not to mention the fact that the CMB alone provides more than enough of an energy influx to counteract the radiation). So evaporation is not an issue for stellar mass holes -- only very small black holes have this problem.

 

[ibysaiyan]

That's right. In fact, very very few starts collapse directly into a black hole. The most promising mechanism is you have a medium-mass star (~15-20 solar masses) which collapses into a neutron star with a companion nearby. Then you slowly accrete gas until you pass the TOV limit and collapse to a BH. It's really difficult to do without a companion, since to directly collapse into a BH you need something ~100 solar masses. While it is true that black holes evaporate, any that have formed as a result of stellar collapse must have roughly stellar masses (take 1 solar mass). The lifetime for a black hole this size is many orders of magnitude large than the current age of the universe (Not to mention the fact that the CMB alone provides more than enough of an energy influx to counteract the radiation). So evaporation is not an issue for stellar mass holes -- only very small black holes have this problem.

Thanks for your informative post.I see, so in a sense this mechanism which you mentioned above is very similar to type Ia supernovae,wait is this the accretion induced collapse? I read about it a while back where a carbon-oxyrgen W.D becomes a BH only when found in a binary system.

- ibysaiyan