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walt fristoe
SFN Regular
USA
505 Posts |
 Posted - 12/11/2004 : 13:41:48
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I have a few questions that I'm hoping someone can help me with:
1. When the sun leaves the main sequence and becomes a red giant, and then subsequently a white dwarf, how will the orbits of the outer planets be affected, if at all, given that the sun's center of gravity will remain unchanged?
2. I've read that a star must be about 3.2 solar masses to become a black hole after it leaves the main sequence. Is this its mass before or after it sheds its outer layers during a supernova?
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Siberia
SFN Addict
Brazil
2322 Posts |
 Posted - 12/11/2004 : 16:32:10 [Permalink]
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quote:
Originally posted by walt fristoe
I have a few questions that I'm hoping someone can help me with:
1. When the sun leaves the main sequence and becomes a red giant, and then subsequently a white dwarf, how will the orbits of the outer planets be affected, if at all, given that the sun's center of gravity will remain unchanged?
K, I couln't find a coherent site that would tell me about orbit changes during the red giant phase. Some of them even differ on what would be the fate of Earth during that. Here's what I found:quote:
This is actually a complicated, poorly understood issue (i.e., a field of active research). The fate of earth's orbit (10 billion years down the road) depends on how rapidly the sun swells, and when and how it puffs its atmosphere off into space. If the mass loss is quick, the sun shrinks, as does its gravitational grip on earth. In this scenario, the earth's orbit will expand, getting it out of the way of the sun's expansion to the red giant phase. If there is no significant mass loss before the sun swells, earth could be engulfed. The drag forces on the earth would rob orbital energy, and the earth would likely spiral into the sun.
I'd say the outer planets would be pushed farther from the sun. On the first case, as Earth's orbit would expand, so would the other planets' orbits. On the second case, I don't know if the total mass of a post atmospheric shedding sun + Earth + Mercury + Venus would be the same as the sun's 'original' mass. Then again, I'm just guessing, I'm no astronomer, so don't take me too seriously
Source: Physics 110
[Edited to clean up quoting and link - Dave W.] |
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| Edited by - Siberia on 12/11/2004 16:37:12 |
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Ricky
SFN Die Hard
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4907 Posts |
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Siberia
SFN Addict
Brazil
2322 Posts |
Posted - 12/12/2004 : 06:31:20 [Permalink]
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Thanks Dave, I was struggling with that link, until I gave up  |
"Why are you afraid of something you're not even sure exists?"
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"People who don't like their beliefs being laughed at shouldn't have such funny beliefs."
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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
Posted - 12/12/2004 : 10:30:49 [Permalink]
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quote:
Originally posted by walt fristoe
2. I've read that a star must be about 3.2 solar masses to become a black hole after it leaves the main sequence. Is this its mass before or after it sheds its outer layers during a supernova?
I'll start with this one:
This is the weight of the body as it collapses. Usually this means that the main sequence star needs to be around 10 solar masses, as it looses much weight as it goes through the final stages of it's life. |
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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
Posted - 12/13/2004 : 09:07:10 [Permalink]
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quote:
Originally posted by walt fristoe
1. When the sun leaves the main sequence and becomes a red giant, and then subsequently a white dwarf, how will the orbits of the outer planets be affected, if at all, given that the sun's center of gravity will remain unchanged?
The Sun will loose some of it's mass through corona ejections and solar wind. Eventually, it is estimated to loose about 30% of it's mass.
(This number will vary somewhat depending on who is making the estimate, and when the book you're relying on was printing. In this case, I'm using "Our Cosmic Origin" written by Prof. Armand Delsemme, English translation published 1998)
When the Sun looses it's mass, the planet's total energy remains constant, but since the gravitational pull of the Sun decreases, the distance (height) will increase, as the orbital speed will be too high for that orbit. This means that the orbits will shift outward, as kinetic energy will be converted to potential energy until a new equilibrium is reached.
Perhaps you were thinking that the solar wind and mass-ejection will still have it's center of gravity at the center of the sun, and this is mostly correct, assuming a fairly homogeneous ejection.
However, since practically all the mass escaping from the sun will end up outside the planetary orbit, it will not contribute to the gross gravitational pull toward the Sun, since roughly half of that mass will be on the opposite side of the planet, as seen from the Sun.
So basically, the loss of weight will mean an increase in orbital radius.
I hope this answer your question. |
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Dude
SFN Die Hard
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6891 Posts |
Posted - 12/13/2004 : 11:19:38 [Permalink]
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quote:
2. I've read that a star must be about 3.2 solar masses to become a black hole after it leaves the main sequence. Is this its mass before or after it sheds its outer layers during a supernova?
Main sequence stars don't go supernova. They do the red-giant thing and end as a planetary nebula around a white dwarf. This is what generally is expected to happen to our sun in about 5 billion years.
Some newer thinking in the field suggests that our own sun is not quite massive enough to become a planetary nebula however...
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Dave W.
Info Junkie
USA
26022 Posts |
Posted - 12/16/2004 : 19:33:47 [Permalink]
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quote:
Originally posted by Siberia
Thanks Dave, I was struggling with that link, until I gave up
Just to let you know, Siberia, the problem was that you had http-slash-slash, and not http-colon-slash-slash, at the start of it. |
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Siberia
SFN Addict
Brazil
2322 Posts |
Posted - 12/17/2004 : 05:09:24 [Permalink]
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| Ooh. Thanks. I overlooked that. |
"Why are you afraid of something you're not even sure exists?"
- The Kovenant, Via Negativa
"People who don't like their beliefs being laughed at shouldn't have such funny beliefs."
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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
Posted - 12/17/2004 : 13:31:09 [Permalink]
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quote:
Originally posted by Dude
quote:
2. I've read that a star must be about 3.2 solar masses to become a black hole after it leaves the main sequence. Is this its mass before or after it sheds its outer layers during a supernova?
Main sequence stars don't go supernova. They do the red-giant thing and end as a planetary nebula around a white dwarf. This is what generally is expected to happen to our sun in about 5 billion years.
Actually, main sequence stars are stars located in the main sequence of the Herzsprung-Russel diagram, and does not indicate the star's mass. It only indicates that the star is currently in a stable state, burning hydrogen, a state it will have for the most part of it's life. Spica, Becrux, and Orionis C are examples of main sequence stars that will eventually go supernova (and end up as a black hole).
When a star have burnt most of it's hydrogen and starts to swell into a red giant, it will leave the main sequence. |
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Dave W.
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26022 Posts |
Posted - 12/17/2004 : 19:49:48 [Permalink]
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quote:
Originally posted by Siberia
Ooh. Thanks. I overlooked that.
"Overlooked" hell, it took me ten minutes of active searching to find it. |
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Siberia
SFN Addict
Brazil
2322 Posts |
Posted - 12/18/2004 : 05:58:24 [Permalink]
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quote:
Originally posted by Dave W.
<blockquote id="quote"><font size="1" face="Verdana,Arial,Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Siberia
Ooh. Thanks. I overlooked that.
"Overlooked" hell, it took me ten minutes of active searching to find it.
[/quote]
Aww. Poor Dave! |
"Why are you afraid of something you're not even sure exists?"
- The Kovenant, Via Negativa
"People who don't like their beliefs being laughed at shouldn't have such funny beliefs."
-- unknown
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walt fristoe
SFN Regular
USA
505 Posts |
Posted - 12/21/2004 : 11:30:20 [Permalink]
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Hey, thanks alot folks! Very interesting answers, just what I was hoping for.
Here's another question:
From what I understand, as viewed from a distance anything that falls into a black hole will appear to move slower in time than objects far from the event horizon, and that time would appear to completely stop for that object as compared to that of outside observers, such that outside observers would never be able to actually see anything cross the event horizon. Even the surface of the collapsing star can never be seen by outside observers to fall below the event horizon.
So, since the beginning of the universe, has anything ever actually fallen into (past the Schwartzchild radius) any black holes, anywhere?
And if nothing can have fallen into any black hole, would that mean that no singularities have been formed?
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Valiant Dancer
Forum Goalie
USA
4826 Posts |
Posted - 12/21/2004 : 12:23:24 [Permalink]
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quote:
Originally posted by walt fristoe
Hey, thanks alot folks! Very interesting answers, just what I was hoping for.
Here's another question:
From what I understand, as viewed from a distance anything that falls into a black hole will appear to move slower in time than objects far from the event horizon, and that time would appear to completely stop for that object as compared to that of outside observers, such that outside observers would never be able to actually see anything cross the event horizon. Even the surface of the collapsing star can never be seen by outside observers to fall below the event horizon.
So, since the beginning of the universe, has anything ever actually fallen into (past the Schwartzchild radius) any black holes, anywhere?
And if nothing can have fallen into any black hole, would that mean that no singularities have been formed?
I really don't have the background to discuss advanced quantum mechanics concerning the surface of the collapsing star relative to the event horizon.
I believe the operative word here is "appears". The matter continues to be compressed by gravitation and does fall into a black hole. Evidence of this is apparent in the emitting of x-rays. So things do fall into the black hole.
Nothing has appeared to fall into a black hole since the formation of the universe except the merging of black holes. Since optics have not advanced to the point where the event horizon can be imaged, the hypothesis, based on advanced quantum mechanics and special relativity, cannot be tested.
It also only refers to the surface of the star, not the cores of stars. Singularities can definately exist within the confines of the inner guts of a star. Since the surfaces of these celestial bodies are usually preceeded by a swelling to the point of giant status, they should be visible. The supermassive black hole which lurks in the center of our galaxy near Sagitarius A cannot be resolved optically, yet we see a profound influence on surrounding stars and an additional black hole in close proximity. |
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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
Posted - 12/23/2004 : 01:06:25 [Permalink]
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quote:
Originally posted by walt fristoe
From what I understand, as viewed from a distance anything that falls into a black hole will appear to move slower in time than objects far from the event horizon, and that time would appear to completely stop for that object as compared to that of outside observers, such that outside observers would never be able to actually see anything cross the event horizon.
This is one of the quirks that relativistics shares with quantum mechanics: Common sense can not be applied to it reliably.
There are several factors that need to be considered when dealing with black holes.
quote:
Even the surface of the collapsing star can never be seen by outside observers to fall below the event horizon.
When a star collapses into a neutron star, the diameter differs by at least a magnitude from a white dwarf.
The same goes for a black hole. The diameter of neutron star is at least a magnitude greater than that of a black hole. When a neutron star accumulates enough mass to collapse into a black hole, the black hole forms in the centre of the neutron star, with an event-horizon with a ~3km diameter (or was that the radius, I can never recall which if it). At that point the inner pressure holding up the neutron star in a globe disappears and it's matter will, just like water in a sink when the plug is pulled, just get sucked into the hole.
quote:
Originally posted by Valiant Dancer
I believe the operative word here is "appears". The matter continues to be compressed by gravitation and does fall into a black hole. Evidence of this is apparent in the emitting of x-rays. So things do fall into the black hole.
I think that some clarification is needed at this point. The x-rays is emitted by the matter as it is approaching the black hole, way outside the event horizon, due to the extreme heat produced by friction in the compressed matter (mostly gaseous).
As light-emitting matter closes in on the event horizon another weird effect appears: gravitational red shift.
If you fire a gun straight up in the air, the bullet starts out with a lot of kinetic energy. As the bullet travels upward in the gravitational field it gains potential energy and looses kinetic energy. The same thing happens to photons (light). However, due to its nature, light does not loose speed, but it's kinetic energy is stored (in layman's terms) as its frequency. So, light that is emitted from matter close to the event horizon will loose frequency (increase in wave length) it will shift toward red spectrum as it leaves the vicinity of the black hole. The closer to the event horizon, the greater the effect. This is what defines the event horizon: it is the point of no return, even for light.
quote:
Singularities can definitely exist within the confines of the inner guts of a star.
I honestly can not see how that can be possible except at the instant of the "death" of the star.
A star, even a neutron star, is holding itself up thanks to the pressure inside of it. The inner heat makes the plasma try to expand and the gravitation holds the plasma back together. An equilibrium is keeping the star in it's current shape.
If a singularity is created, it will start sucking up atoms, or the elementary particles that is necessary for keeping the pressure up. It's like letting out the air out of a balloon. It will just collapse.
quote:
Since the surfaces of these celestial bodies are usually preceeded by a swelling to the point of giant status, they should be visible.
Yes, black holes are identified by matter spiralling down toward the event horizon. The event horizon itself can not be detected.
quote:
The super-massive black hole which lurks in the centre of our galaxy near Sagitarius A cannot be resolved optically, yet we see a profound influence on surrounding stars and an additional black hole in close proximity.
The event-horizon of a star is calculated to be about 1 km per solar mass. Since the smallest mass of black hole is 3-4 solar masses, the diameter would be 3-4 km. The diameter of the sun is ~700'000km. A super-massive black hole, of 700'000 solar masses would have an event horizon the same diameter as our sun. Milky-way's super-massive black hole estimated to have 1-3 million solar masses would have an event horizon up to five times as large. Not more than that. Hoping to get a 'visual' of any event horizon is just wishful thinking.
While stars are appearing is disks on star charts and photos, it's just an effect created by the saturation in the photographic media. It does not represent the actual diameter of the star viewed, only how many photons from the star hit the film.
For having such a huge impact of it's surroundings, a black hole is very small. And the singularity, well, it's designation speaks for itself.  |
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| Edited by - Dr. Mabuse on 12/23/2004 01:10:05 |
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Valiant Dancer
Forum Goalie
USA
4826 Posts |
Posted - 12/27/2004 : 12:04:33 [Permalink]
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<blockquote id="quote"><font size="1" face="Verdana,Arial,Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dr. Mabuse
<blockquote id="quote"><font size="1" face="Verdana,Arial,Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Singularities can definitely exist within the confines of the inner guts of a star. [/quote]I honestly can not see how that can be possible except at the instant of the "death" of the star. [/quote]
Thanks for the clarifiaction. I thought I might be doing a disservice to the work by Hawking. Yes, I was referring to the instance of complete collapse into a black hole from a huge star and the coining of a new singularity. I do, however, believe that special relativity breaks down to a point near the event horizon evidenced by work by Hawking and others constituting the bulk of black hole research. There's also some work by Hawking detailing how a form of radiation (Hawking radiation to be specific) can and does escape black holes.
<blockquote id="quote"><font size="1" face="Verdana,Arial,Helvetica" id="quote">quote:<hr height="1" noshade id="quote">
A star, even a neutron star, is holding itself up thanks to the pressure inside of it. The inner heat makes the plasma try to expand and the gravitation holds the plasma back together. An equilibrium is keeping the star in it's current shape.
If a singularity is created, it will start sucking up atoms, or the elementary particles that is necessary for keeping the pressure up. It's like letting out the air out of a balloon. It will just collapse.
<blockquote id="quote"><font size="1" face="Verdana,Arial,Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Since the surfaces of these celestial bodies are usually preceeded by a swelling to the point of giant status, they should be visible. [/quote]Yes, black holes are identified by matter spiralling down toward the event horizon. The event horizon itself can not be detected.
<blockquote id="quote"><font size="1" face="Verdana,Arial,Helvetica" id="quote">quote:<hr height="1" noshade id="quote">The super-massive black hole which lurks in the centre of our galaxy near Sagitarius A cannot be resolved optically, yet we see a profound influence on surrounding stars and an additional black hole in close proximity.[/quote]
The event-horizon of a star is calculated to be about 1 km per solar mass. Since the smallest mass of black hole is 3-4 solar masses, the diameter would be 3-4 km. The diameter of the sun is ~700'000km. A super-massive black hole, of 700'000 solar masses would have an event horizon the same diameter as our sun. Milky-way's super-massive black hole estimated to have 1-3 million solar masses would have an event horizon up to five times as large. Not more than that. Hoping to get a 'visual' of any event horizon is just wishful thinking.
While stars are appearing is disks on star charts and photos, it's just an effect created by the saturation in the photographic media. It does not represent the actual diameter of the star viewed, only how many photons from the star hit the film.
For having such a huge impact of it's surroundings, a black hole is very small. And the singularity, well, it's designation speaks for itself.
[/quote]
The black hole is very small but the effect of it's gravitation is profound. I should have talked about black holes not being able to be optically resolved due to the warping of light around it which would mask any event horison. |
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| Edited by - Valiant Dancer on 12/27/2004 12:21:41 |
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