Orbiting a Black Hole

If a bunch of planets are orbiting a red giant and, all of a sudden, that red giant collapses into a black hole, would the planets continue to orbit the black hole? I'm kind of thinking that they would continue in their orbits because the mass of the red giant should be about the same as the black hole; it would just be that the black hole is much denser. Can anyone tell me whether this is right?

 

Log in or Sign up to hide all adverts.

Yes, you are right, as long as the mass remains the same the planets' orbits will not change.

 

Log in or Sign up to hide all adverts.

Thanks again, zanket!

Please Register or Log in to view the hidden image!

Now, practically speaking, I understand that when a star collapses, it spews out a lot of its mass and emits a lot of radiation, which is why supernovae are so bright. Would this loss of mass decrease the gravitational pull on the planets such that they would hurdle out of their orbits and into space?

 

Log in or Sign up to hide all adverts.

I was under the impression that in the proccess of becoming a black hole, the surrounding planets would be destroyed.



edit- forget it, thinking about it for a bit, I could see where a super giant with planets around it would not need to expand any further, but go right to a black hole. I wonder if the energy released as that occured would be enough to vaporise the planets? hmmmm

 

Chances are that the planets would be destroyed or blown out of orbit by the destruction of the parent star. If they weren't, howver, the remaining black hole or neutron star would indeed have less mass and would not hold onto its planets as well. The orbits would shift and either they would orbit closer to the star remnant or orbit more slowly where they are, or they could fall into the black hole/neutron star or be ejected from the system. Hard to say exactly... but the black hole has less mass than the star it formed from.

 

Yes, their orbits will change at a minimum, and if they have escape velocity they'll leave orbit. Let’s assume the planets remain intact. Initially they have the same velocity perpendicular to the orbital radius, but less attraction to the center of the system, so their perpendicular velocity dominates and carries them further away from the black hole. Unless they have escape velocity the continuous attraction from the black hole will eventually rein them in and they’ll stay in orbit. My guess is that if their orbits were circular before, they’ll be elliptical after, enclosing a greater area and touching the former orbit at two points (such that the former orbit just fits within the latter orbit). I’m no expert on this though.

 

Thanks for all your feedback. It has been most helpful.

Please Register or Log in to view the hidden image!

 

I believe that supernovas are a very different process from black holes. If a star collapses on itself to create a black hole, nothing escapes it. Super novas ocur for a different reason (I'm not entirely sure which).

 

that's not the way that i understand it. what collapses into a black hole is generally a supernova remnant. the core of the star. if it is more massive than, what?, 8-10 solar masses it will collapse into a black hole. if it's greater than 4 but less than 8 solar masses it will collapse into a neutron star.

that's my understanding, supernovae happen in the creation of neutron stars and black holes. otherwise you have a white dwarf.

i could be wrong... especially about the mass required to make a black hole.

 

I was merely stating my understanding of the subject. It is most likely that I was wrong, for I'm not a specialist in the subject (oh, I wish I was

Please Register or Log in to view the hidden image!

)

 

A supernova is the collapse of a star. As I understand it, a star that is less than 8 solar masses will leave behind a white dwarf. A star that is between 8 and 25 solar masses will leave behind a neutron star. A star that is more than 25 solar masses will leave behind a black hole.

The elements that have been blown apart from the star before its collapse then begin to form new stars and planets. It is believed that this cycle has happened numerous times since the beginning of the universe.

 

Around 1930, S. Chandrasekhar studied astrophysical models of white dwarf stars and came to the conclusion that no white dwarf can be more massive than about 1.2 solar masses. This became known as the Chandrasekhar limit. Chandrasekhar was awarded the Nobel Prize for these studies, and went on to study stellar structure, resulting in the concept of black holes.

Novae follow the transformation of matter from a main sequence star to a white dwarf companion in a binary system such that the mass of the white dwarf remains below the Chandrasekhar limit. They can change by 10-12 magnitudes in a few hours, and typically have absolute magnitude of -7.

There are three types of supernovae:

1. Speculated to be caused by the transfer of matter from a main sequence star to its white dwarf companion in a binary system (such that the white dwarf gains a mass greater than the Chandrasekhar limit ).
2. Caused by the formation of an iron core; leaves behind a neutron star or black hole.
3. Caused by carbon detonation; leaves no stellar remnant.
Neutron Stars are supported by Inverse Beta Decay when the mass of the star is too great for Electron Degenerancy to have an effect. This is typical of stars from 1.4 - 3 solor masses

Over ~3 solar masses, and the gravitational force of the mass of the star overpowers the outward force of common pressure, electron degenerancy, or Inverse Beta Decay. Thus, above ~3 solar masses, is where we begain to see black holes forming where nothing can escape, not even light.

Oringinally, it was theorized that there could exist black stars. Simply, the mass was so great, that the escape velocity excedded the speed of light. This was shown to be impossible when the mass of the star could not exist and still be stable.

Mass, Charge, and Angular Momentum are the only things that a black hole can posses, thus, everything that can ever be discovered or calculated about a black hole has come from assumptions on each of the three points and together as well.

Robert Penrose concluded in 1964 that in every imploding star there must be a singularity and thus in every black hole there must be one.

Also, there is a Cosmic Censorship Hypothesis. This states that no "naked" singlarity exists in nature due to the event horizon which does not let light escape. Thus, a singularity will never be directly observed, only theorized to exist and mathematically calculated to exist.


Hope this helps. I suggest "Black Holes And Time Warps" By Kip Thorne if you are the least bit interested in the history and fact of black holes.

Later,
T

Please Register or Log in to view the hidden image!

 

For clarification, I should point out that the Chandrasekhar limit (which is about 1.4 solar masses) and the other figures Tristan was using refer to the material that is remaining after the supernova. The numbers that I referred to in my previous post referred to the total mass of the star before the supernova.

 

If both gravitational forces are the same then why is light able to escape from a Red Giant and not from a Black Hole?
And if light really is the fastest speed in the universe then why can’t it escape from a black hole?
They’re the two questions I want to know.

 

Let the mass of a red giant and black hole be the same. Their gravitational force is the same at some radius x, where x is at or above the surface of the red giant. At that radius for both the red giant and the black hole light can escape. But at some lesser x—below the surface of the red giant—you’ll reach a radius where light cannot escape from the black hole. The imaginary sphere at that radius is called an event horizon.

Gravity has an effect up to an infinite speed. Light moves at the fastest speed directly measurable; it isn’t the fastest possible speed. Check out this thread for more info.

 

light orbiting a black hole

a footnote to what zanket said:

the radius of the event horizon is 2GM/c^2
(for an ordinary uncharged unrotating black hole)

and also there is a distance from the center
at which light can actually orbit the hole----go
in a circle around it.

This is outside the event horizon and the distance
is 3GM/c^2
the sphere with that radius is sometimes called the
photon sphere
I find it amazing that a ray of light can be bent into
a circle so that photons are actually orbiting something

the original poster asked about stuff orbiting a black hole,
so this footnote is meant to extend that to include light as well

 

I've never thought about light orbiting a black hole before. Thanks for the insight, Mark.

 

My pleasure, Jade Squirrel.

The sphere around a black hole which is the right distance for
light to circle the hole is called the "photon sphere".
If you are in that sphere you can see the back of your own head.


I just googled with the keywords "photon sphere"
and found quite a lot of stuff including
some movies showing views from outside and inside the photon sphere of a black hole.
But something's wrong with my computer and it will not
play movies---I have to stick with still photos and diagrams.
I do not know if these movies are any good. they are probably just short animated diagrams. Here is the link---

http://antwrp.gsfc.nasa.gov/htmltest/gifcity/gotops.html

This movie is part of something called "virtual trips to black holes"

http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html

Again, I do not know if these are any good because I cannot watch them until I get something fixed in software.

The "photon sphere" is the set of points a distance 3GM/c^2
away from hole center. Light orbits at that distance.

The "event horizon", which you must not touch, is closer in at radius 2GM/c^2. Nothing can escape that has come in that close.

But 50 percent further out is still safe, that is the photon sphere distance where you can see the back of your head.

If you find any good visuals please share the links
thx,
Mark

 

getting back on topic---supernovae

Oops, I just happened into this thread and added a footnote to what someone had said black holes, but the real topic is supernovas.

Jade S---you said chandrasekhar limit is 1.4 solar masses and that is what I remember too.

You also gave mass ranges for stars that determine how they end. IIRC these numbers are right. If a star is under 8 solar while it is still on the main sequence (normal life of star) then when it goes red giant and blows off its outer layers what is left will
be LESS than 1.4 and so will be a cooling white dwarf.

It might, for example, be mostly carbon (fusion having gone from H to He to C and no further). Since it is less than 1.4 it can't go supernova. Nothing can happen to it (unless it has a binary companion to dump mass on it and build up to 1.4!)

As you suggested, if the star begins as 8-25 then it CAN go supernova eventually and leave a neutron star remnant. Like, it fuses H to He to C....all the way to Iron, in the core. And it blows off most of its outer material. And what is left is this Iron core which is 1.4 or more.

Fusion up to iron yields energy but beyond that it is endothermic (absorbs energy) so nothing is gained by fusing above iron.
So this big ball of iron is helpless and as it cools it has nothing to prevent it from crushing itself by its own weight.

I think you know all this Jade Squirrel but I am telling the story just to get it laid out in this thread. The interesting thing is this:
when a ball of cooling iron begins to crush the electrons and iron nuclei down into pure solid neutron matter there is a sudden reduction in volume by something like a thousand-fold. Roughly.
A sudden huge decrease in volume.

A huge mass is suddenly in free fall towards its own center.

What causes the explosion????

This is interesting to me. Do you have an answer?

A lot of gravitational potential energy is released for sure.
And a lot of neutrinos---an iron nucleus has like 26 protons
and each of those gets squashed together with an electron and turns into a neutron, to join the neutron star forming at the very
center, but also a neutrino is released. So 26 neutrinos fly out for each iron nucleus that gets squashed. I think maybe THEY could blow a star apart, there being so many of them---a storm of neutrinos.
But also there is the socalled BOUNCE effect. I have seen people explain the explosion in various ways. how do you think it works?

And then, as you say, if in normal life the star was over 25 solar masses then the radius of the destined neutron star remnant (which is smaller with larger mass) is less than 2GM/c^2 and GLUPPP!
a black hole is formed. The neutron star that would have formed was so massive that it was too dense to exist.
And this event of collapsing into a black hole seems to be
the biggest fireworks of all since charged material accelerates as it falls into the hole and releases a burst of gamma rays. I am told that up to 10 percent of the mass of the collapsing object can be converted into gammaray. The biggest gammaray burst to date was observed March 29 of this year, I think. More violent than a supernova. I forget the estimated release of energy.

Your numbers 8 and 25 sound right, reminiscent of a general astro course. But I'm not altogether sure. Do you happen to have
a web reference? Essentially right in any case.

BTW I can show you a way to calculate the Chandra limit. Would you like to know it? It is easy. So you don't have to rely on remembering that it is 1.4 solar masses.

Supernovas are interesting. And then there is that thing that all the stuff in our bodies comes from supernovas. Because they blow the carbon and nitrogen and oxygen that has been cooked inside stars out into space where it can condense again into stars and planets. You mentioned this. Without supernovas there would be no life as we know it. You mentioned this "generation" stuff. yes.

 

Mark: I'm afraid we're getting a little out of my league here. I don't really have much of a background in science, although I find cosmology fascinating to research on my own time. But the knowledge I have in this field is very limited compared to others who regularly post in this forum. Perhaps zanket may be able to answer some of your questions. He's not a physicist, but he is a lot more knowledgeable than I am in this subject, and he seems to know what he's talking about.