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Author Topic: In astromomical terms , how big does a ball of gas have to be to become a star?  (Read 10147 times)

Offline John Chapman

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I vaguely understand the physics behind what makes a star 'burn'. It's huge gravity forces small atoms to combine, by a process of nuclear fusion, into larger atoms while releasing huge amounts of heat energy. I've heard that our sun combines about 300 million tons of hydrogen every second into about 297 million tons of helium (the 3 mill tons of 'lost' matter being converted directly into energy as described by Einstein's E=MC2). Later in it's life it will apparently combine helium atoms to produce larger atoms still and so on.

Orbiting around our Sun are several Gas Giants and much larger gas giants have been found orbiting other stars. According to Wiki, our own Jupiter and Saturn consist of mainly hydrogen and helium. So my question is this:

Is there any possibility that one of our gas giants could reach a critical size and spontaneously ignite? Has this happened elsewhere outside of our solar system? Can a small star orbit a larger star along with other planets? Do all stars start existence as gas planets and grow to become stars or are they spontaneously formed?


 

Offline DoctorBeaver

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Don't panic.

There is no chance that Jupiter or Saturn will become stars; their mass is way too low. In fact, I doubt whether any gas giant could grow enough for nuclear fusion to take place at its core. There simply would not be enough gas or space debris left over in the system for the planet to obtain sufficient mass. In the case of our own system, Jupiter has more mass than all the other planets in the system added together so even if they were all to accumulate into 1 super planet, its mass would not even double and would still be 30+ times too small to initiate nuclear fusion.

Stars form from clouds of gas that gradually condense under their own gravitaional force. Whether or not the cloud of gas condenses enough to form a star depends on its composition. If it has similar metallic ratio to our sun (our sun is a relatively new star with a high metallic content) then a mass of 75 times that of Jupiter will suffice. For low metal-ratio gas clouds the minimum mass is roughly 85 times that of Jupiter. The smallest star found so far is AB Doradus C which has a mass 93 times that of Jupiter. As that shows, a planet of 1 Jovian mass is only a midget by comparison.

There are many instances of low-mass stars orbiting larger-mass ones (Although that is not strictly true; they orbit around a common centre of gravity). In fact, it is thought that there are more binary stars than single stars. Don't ask me what would happen to any planets in such a system because the gravitational interactions are incredibly complicated.
« Last Edit: 22/02/2009 09:45:39 by DoctorBeaver »
 

Offline Vern

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This is a very interesting question. After a few minutes of Google exercises I came up with some info that may apply.

Quote from: mass requirement to become a star
The smallest theoretical mass for a star to support nuclear fusion is 0.07 or 0.08 solar masses, so smaller stars are out there.

Quote from: another source
A star's mass cannot be lower than 13 Jupiter masses, because below this critical point the core does not get hot enough by gravitational pressure to start the fusion of deuterium, which requires at least a temperature of more than 1500 to 4000 K (depending on metallicity), in combination with a mass of about 7% that of our Sun. A brown dwarf therefore is heavier than a gas-giant planet, but not quite massive enough to be a star.
 

Offline John Chapman

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Hi DoctorBeaver and Vern

As usual your answers are so complete and well structured that they've left me with nothing else to ask! However, I was considering posting a separate question involving the structure of planets which I may as well ask now!

I believe the solar system was formed originally from of a disc of dust and gas by the process of accretion. Bits stick together firstly by friction and then using gravity and those 'bits' turn into large lumps. Lumps attract more lumps and before you can say "naughty girls don't like scientists" you have a solar system. So why are some planets composed of gases while others are made from a variety of minerals? It's as if planets are selective in what the attract. Also why do we have an asteroid belt which, for some reason, has never coalesced? And what about the Kuiper Belt?
 

Offline LeeE

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I believe that Jupiter does actually generate some heat and radiates in the infra-red, just not very much though, and certainly not through sustainable fusion.

There are a couple of factors at play in the protoplanetary disk that lead to a kind of sorting of the material in the PP disk.  The solar wind from the star tends to push material in the disk out and away from the central star, whilst at the same time, gravity is pulling the same material in towards the central star.  However, the lighter elements are more easily pushed out by the solar wind than the heavier elements, due to their lower mass, which at the same time means they are less strongly pulled in by gravity, this force being dependent on the sum of the two masses being considered.

It's almost as though the star 'distills' the matter in the PP disk in to different fractions using the two mechanisms of solar wind and gravity instead of heat.
 

Offline John Chapman

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Fractional distillation? That's a nice, clear analogy. So is there a tendency for planets nearest their star to be made from lighter material while the ones furthest out are made of denser stuff?
 

Offline LeeE

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The tendency is the other way around; the heavier elements are found closer in and the lighter ones further out.

Mercury has a mean density of 5.427 g/cm³, Venus 5.204 g/cm³, Earth 5.5153 g/cm³ and Mars 3.934 g/cm³, whereas Jupiter is just 1.326 g/cm³, Saturn 0.687 g/cm³, Uranus 1.27 g/cm³ and Neptune 1.638 g/cm³, so you can see that the sorting is apparent but not perfect.

Edited to add the density of Mars.
« Last Edit: 22/02/2009 18:07:10 by LeeE »
 

Offline Vern

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The tendency is the other way around; the heavier elements are found closer in and the lighter ones further out.

Mercury has a mean density of 5.427 g/cm³, Venus 5.204 g/cm³, Earth 5.5153 g/cm³ and Mars 3.934 g/cm³, whereas Jupiter is just 1.326 g/cm³, Saturn 0.687 g/cm³, Uranus 1.27 g/cm³ and Neptune 1.638 g/cm³, so you can see that the sorting is apparent but not perfect.

Edited to add the density of Mars.
Great post LeeE; you must have an encyclopaedia in your back pocket.:)   
 

Offline DoctorBeaver

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Vern - I think your reference to brown dwarves may be a bit misleading. An accretion of gases of that mass will not initiate fusion. As far as I am aware, brown dwarves are the result of more massive stars having lost mass while the fusioning of material continues, albeit at a slower rate.
 

Offline Vern

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Vern - I think your reference to brown dwarves may be a bit misleading. An accretion of gases of that mass will not initiate fusion. As far as I am aware, brown dwarves are the result of more massive stars having lost mass while the fusioning of material continues, albeit at a slower rate.
I'll research it some more; but I got the impression from the Wikki article that Brown Dwarf also applied to sub-stars not quite massive enough to sustain fusion. The old burned out Brown Dwarf stars are now known as Black Dwarf stars -- again from that article. I'll go back and look at it again; thanks for pointing out a possible error :)
 

Offline Vern

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I'm not sure this makes it any more clear, but this is the source of my understanding of a Brown Dwarf.

Wikkipedia treatment of Brown Dwarf Star

Quote from: Wikkipedia
Brown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest mass stars; this upper limit is between 75[1] and 80 Jupiter masses (MJ). Currently there is some debate as to what criterion to use to define the separation between a brown dwarf from a giant planet at very low brown dwarf masses (~13 MJ ), and whether brown dwarfs are required to have experienced fusion at some point in their history. In any event, brown dwarfs heavier than 13 MJ do fuse deuterium and those above ~65 MJ also fuse lithium. The only planets known to orbit brown dwarfs are 2M1207b and MOA-2007-BLG-192Lb.
 

Offline DoctorBeaver

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Vern - you're right, it isn't overly clear from that extract.
 

Offline John Chapman

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Thanks LeeE, DrB and Vern

This has been a very informative thread. And I'm eagerly lapping all this up.
 

Offline John Chapman

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Oh, but there is one thing you haven't answered:

Why hasn't the asteroid belt coalesced into a planet? And what about the Kuiper Belt? In fact what (and why) exactly is the Kuiper Belt?
 

Offline DoctorBeaver

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Tidal forces between the sun & Jupiter prevented the material in the asteroid belt coalescing. I'm not sure about the origin of the Kuiper belt or the Oort cloud.
 

Offline John Chapman

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Tidal forces?
 

Offline DoctorBeaver

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Gravitational forces are sometimes referred to as tidal forces. My apologies, I should have made that clear.
 

Offline Vern

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Tidal forces between the sun & Jupiter prevented the material in the asteroid belt coalescing. I'm not sure about the origin of the Kuiper belt or the Oort cloud.
From my research today using Google, I glean from the various papers that the Kuiper belt and the Oort cloud are remnants of the original accretion disk. They are the remains of the original accretion of matter from which the solar system formed.
 

Offline DoctorBeaver

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Vern - Do you know why they formed so far out when most of the heavy stuff stayed in the inner confines of the solar system?
 

Offline Vern

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Vern - Do you know why they formed so far out when most of the heavy stuff stayed in the inner confines of the solar system?
I get the feel from the research that the Kuiper belt and Oort cloud do contain mostly "dirty snowball" kinds of light material. The occasional heavy stuff probably comes from past collisions between stuff closer in IMHO. Water vapour may have been pushed around by the solar wind where heaver rocks and metals were not.
« Last Edit: 23/02/2009 15:10:15 by Vern »
 

Offline DoctorBeaver

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Vern - Do you know why they formed so far out when most of the heavy stuff stayed in the inner confines of the solar system?
I get the feel from the research that the Kuiper belt and Oort cloud do contain mostly "dirty snowball" kinds of material. The occasional heavy stuff probably comes from past collisions between stuff closer in IMHO.

That would make sense. Certainly most of the material that ended up at the far reaches of the solar system was gaseous - hydrogen, helium, methane, etc..
 

lyner

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Gravitational forces are sometimes referred to as tidal forces. My apologies, I should have made that clear.
To avoid confusion, I feel that the term 'tidal' should be used to describe differential forces on a single body. It would be much more consistent to describe effects of two masses on a diffuse array of objects as just 'gravitational'.
After all, the tides on Earth (ocean or subterranean) are caused by rotation as much as by gravity.
 

Offline DoctorBeaver

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SC - Thank you for pointing that out. On reflection I do believe you are correct.
 

Offline LeeE

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The tendency is the other way around; the heavier elements are found closer in and the lighter ones further out.

Mercury has a mean density of 5.427 g/cm³, Venus 5.204 g/cm³, Earth 5.5153 g/cm³ and Mars 3.934 g/cm³, whereas Jupiter is just 1.326 g/cm³, Saturn 0.687 g/cm³, Uranus 1.27 g/cm³ and Neptune 1.638 g/cm³, so you can see that the sorting is apparent but not perfect.

Edited to add the density of Mars.
Great post LeeE; you must have an encyclopaedia in your back pocket.:)  

Not at all.  I have a very poor memory for things that don't seem vitally important and rely upon remembering just the important bits, from which I work-out/deduce the other details.  It can mean I end up with different answers at different times though - Oops  ;)

All I did there was remember that wikipedia is good on details like that, so I don't have to remember them, just where to find them.  I had to remember the principles involved, but then I think they're important enough to be worth remembering :)
 

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