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Date: 18 Aug 2006 15:11:29
From:
Subject: Not quite a star, but almost
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All this talk of "what is a planet?" got me to thinking about large
planets that are not quite massive enough to be stars.
What would be the nature of such an object? Would it be hot? Do large
planets form a continuum with small stars, or is there a point at which
"ignition" occurs? If so, what would be the nature of this "ignition"?
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Date: 18 Aug 2006 15:15:43
From: Starlord
Subject: Re: Not quite a star, but almost
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A Planet becomes a true Star when it starts Fusion, then and only then does
it become a Star.
--
The Lone Sidewalk Astronomer of Rosamond
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<allisonki@IGNmail.com > wrote in message
news:1155939089.604983.231030@m79g2000cwm.googlegroups.com...
> All this talk of "what is a planet?" got me to thinking about large
> planets that are not quite massive enough to be stars.
>
> What would be the nature of such an object? Would it be hot? Do large
> planets form a continuum with small stars, or is there a point at which
> "ignition" occurs? If so, what would be the nature of this "ignition"?
>
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Date: 18 Aug 2006 15:23:47
From: Brian Tung
Subject: Re: Not quite a star, but almost
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Allison Kirkpatrick wrote:
> All this talk of "what is a planet?" got me to thinking about large
> planets that are not quite massive enough to be stars.
>
> What would be the nature of such an object? Would it be hot? Do large
> planets form a continuum with small stars, or is there a point at which
> "ignition" occurs? If so, what would be the nature of this "ignition"?
The bottom is generally regarded as deuterium fusion. Deuterium nuclei
contain a neutron in addition to the proton. This has two effects with
respect to nuclear fusion. First, the nucleus is more massive, so that
electromagnetic repulsion does not affect it as readily as it does the
hydrogen nucleus. Secondly, the neutrons provide another source of
nuclear binding.
The upshot is that deuterium fusion can take place at lower pressures
and temperatures than ordinary hydrogen fusion. While about 80 Jovian
masses are needed for a gas giant to produce the pressures and
temperatures required to fuse hydrogen, only about 13 Jovian masses are
needed to fuse deuterium.
Such a deuterium-fusing brown dwarf would indeed glow in the visible.
It would be hot. It would, however, not be long-lived, because there
is much less deuterium than hydrogen in the universe, and the deuterium
would not be enough, generally speaking, to fuse the jump-start the
fusion of ordinary hydrogen. After a relatively short time, the
deuterium would run out (having fused into ordinary helium), and the
dwarf would cool down.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 19 Aug 2006 02:47:23
From: Chris L Peterson
Subject: Re: Not quite a star, but almost
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On Fri, 18 Aug 2006 15:23:47 -0700 (PDT), brian@isi.edu (Brian Tung)
wrote:
>The bottom is generally regarded as deuterium fusion...
>
>Such a deuterium-fusing brown dwarf would indeed glow in the visible.
>It would be hot. It would, however, not be long-lived, because there
>is much less deuterium than hydrogen in the universe, and the deuterium
>would not be enough, generally speaking, to fuse the jump-start the
>fusion of ordinary hydrogen. After a relatively short time, the
>deuterium would run out (having fused into ordinary helium), and the
>dwarf would cool down.
Okay, let me connect this with the planet definition thread (and take
this a little farther yet, knowing we're just having fun)...
If this object is in a system with a normal hydrogen fusor, does the
system go from a double star system to a single star system, and the
object from a star to a planet? What about any large moons that orbit
it? Are they planets because the object was a fusor "at one point in its
lifetime"? Maybe they were planets and then became moons once the
deuterium fusion stopped?
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 18 Aug 2006 20:02:41
From: Brian Tung
Subject: Re: Not quite a star, but almost
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Chris L Peterson wrote:
> If this object is in a system with a normal hydrogen fusor, does the
> system go from a double star system to a single star system, and the
> object from a star to a planet? What about any large moons that orbit
> it? Are they planets because the object was a fusor "at one point in its
> lifetime"? Maybe they were planets and then became moons once the
> deuterium fusion stopped?
Double star system, by the rules we've set up so far.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 19 Aug 2006 14:19:57
From: Shawn Curry
Subject: Re: Not quite a star, but almost
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Chris L Peterson wrote:
> On Fri, 18 Aug 2006 15:23:47 -0700 (PDT), brian@isi.edu (Brian Tung)
> wrote:
>
>
>>The bottom is generally regarded as deuterium fusion...
>>
>>Such a deuterium-fusing brown dwarf would indeed glow in the visible.
>>It would be hot. It would, however, not be long-lived, because there
>>is much less deuterium than hydrogen in the universe, and the deuterium
>>would not be enough, generally speaking, to fuse the jump-start the
>>fusion of ordinary hydrogen. After a relatively short time, the
>>deuterium would run out (having fused into ordinary helium), and the
>>dwarf would cool down.
>
>
> Okay, let me connect this with the planet definition thread (and take
> this a little farther yet, knowing we're just having fun)...
>
> If this object is in a system with a normal hydrogen fusor, does the
> system go from a double star system to a single star system, and the
> object from a star to a planet? What about any large moons that orbit
> it? Are they planets because the object was a fusor "at one point in its
> lifetime"? Maybe they were planets and then became moons once the
> deuterium fusion stopped?
I'd use the "Naval Aviator" definition here. Someone who has safely
flown planes onto a carrier will always have a special position of honor
in aviation. He or she is a Naval Aviator, and that never changes
regardless of length of service or age.
Once a star, always a star. Once a planet, always a planet, except for
Pluto ;-)
Shawn
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Date: 18 Aug 2006 23:04:02
From: SkySea
Subject: Re: Not quite a star, but almost
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The nature of the object would be warm to very hot. Even Jupiter
releases more energy than it receives from the Sun. With more massive
objects, various forces and factors (including friction from original
condensation) work to heat up the material.
Depending on the composition of the material, you eventually end up
with enough heat from those factors to start a fusion process,
commonly when hydrogen atoms fuse to release energy and form helium
atoms. I seem to have a feeble recall that the ignition process is
rather chaotic, rather than a complete conversion of the whole body.
The large, warm objects are often called "brown dwarfs". This does
raise the issue of the diffference between a planet and a brown dwarf.
I expect that a brown dwarf would be classified as a type of planet.
>Allison Kirkpatrick wrote:
> All this talk of "what is a planet?" got me to thinking about large
> planets that are not quite massive enough to be stars.
>
> What would be the nature of such an object? Would it be hot? Do large
> planets form a continuum with small stars, or is there a point at which
> "ignition" occurs? If so, what would be the nature of this "ignition"?
=============
- Dale Gombert (SkySea at aol.com)
122.38W, 47.58N, W. Seattle, WA
http://flavorj.com/~skysea
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Date: 18 Aug 2006 16:06:59
From: Brian Tung
Subject: Re: Not quite a star, but almost
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SkySea wrote:
> The nature of the object would be warm to very hot. Even Jupiter
> releases more energy than it receives from the Sun. With more massive
> objects, various forces and factors (including friction from original
> condensation) work to heat up the material.
I wouldn't call it friction so much as compression. When you compress a
gas, it heats up.
Jupiter releases more heat than it gets from the Sun in large part
because of its distance from the Sun. If it were only 1 AU from the
Sun, it would receive far more light/heat than it emits. In contrast,
if it were massive enough to fuse deuterium, it would (for the short
time that it fused deuterium) be far more energetic than it would be
just below that cutoff--say, at 10 Jovian masses.
> Depending on the composition of the material, you eventually end up
> with enough heat from those factors to start a fusion process,
> commonly when hydrogen atoms fuse to release energy and form helium
> atoms. I seem to have a feeble recall that the ignition process is
> rather chaotic, rather than a complete conversion of the whole body.
Perhaps, but as I mentioned in my post, it is not ordinary hydrogen
that gets fused, but deuterium. It isn't hot enough to fuse more than
a tiny fraction of the ordinary hydrogen.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 18 Aug 2006 21:12:54
From: William Hamblen
Subject: Re: Not quite a star, but almost
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On Fri, 18 Aug 2006 16:06:59 -0700 (PDT), brian@isi.edu (Brian Tung)
wrote:
>Jupiter releases more heat than it gets from the Sun in large part
>because of its distance from the Sun. If it were only 1 AU from the
>Sun, it would receive far more light/heat than it emits. In contrast,
>if it were massive enough to fuse deuterium, it would (for the short
>time that it fused deuterium) be far more energetic than it would be
>just below that cutoff--say, at 10 Jovian masses.
Any object that is cooling transfers more heat out than transfers in.
Bud
--
The night is just the shadow of the Earth.
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Date: 18 Aug 2006 19:16:08
From: Brian Tung
Subject: Re: Not quite a star, but almost
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I (Brian Tung) wrote:
> Jupiter releases more heat than it gets from the Sun in large part
> because of its distance from the Sun. If it were only 1 AU from the
> Sun, it would receive far more light/heat than it emits. In contrast,
> if it were massive enough to fuse deuterium, it would (for the short
> time that it fused deuterium) be far more energetic than it would be
> just below that cutoff--say, at 10 Jovian masses.
William Hamblen wrote:
> Any object that is cooling transfers more heat out than transfers in.
Err, OK--what's the relevance to what I said, though? I'm missing
something.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 19 Aug 2006 00:54:17
From: William Hamblen
Subject: Re: Not quite a star, but almost
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On Fri, 18 Aug 2006 19:16:08 -0700 (PDT), brian@isi.edu (Brian Tung)
wrote:
>I (Brian Tung) wrote:
>> Jupiter releases more heat than it gets from the Sun in large part
>> because of its distance from the Sun. If it were only 1 AU from the
>> Sun, it would receive far more light/heat than it emits. In contrast,
>> if it were massive enough to fuse deuterium, it would (for the short
>> time that it fused deuterium) be far more energetic than it would be
>> just below that cutoff--say, at 10 Jovian masses.
>
>William Hamblen wrote:
>> Any object that is cooling transfers more heat out than transfers in.
>
>Err, OK--what's the relevance to what I said, though? I'm missing
>something.
It's just that an object that is radiating more energy than it
receives is doing nothing special.
--
The night is just the shadow of the Earth.
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Date: 19 Aug 2006 00:19:59
From: Brian Tung
Subject: Re: Not quite a star, but almost
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Bud Hamblen wrote:
> It's just that an object that is radiating more energy than it
> receives is doing nothing special.
Ahh, OK. I suppose that what is often meant by that statement with
respect to planets like Jupiter is that they generate more heat in their
interior (by continuing compression?) than they receive from the Sun.
Actually, I really don't know what they mean, precisely.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 19 Aug 2006 08:27:31
From: William Hamblen
Subject: Re: Not quite a star, but almost
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On Sat, 19 Aug 2006 00:19:59 -0700 (PDT), brian@isi.edu (Brian Tung)
wrote:
>Bud Hamblen wrote:
>> It's just that an object that is radiating more energy than it
>> receives is doing nothing special.
>
>Ahh, OK. I suppose that what is often meant by that statement with
>respect to planets like Jupiter is that they generate more heat in their
>interior (by continuing compression?) than they receive from the Sun.
>Actually, I really don't know what they mean, precisely.
Jupiter is still cooling from it's formation almost 5 billion years
ago. It radiates about 60% more heat than it receives from the Sun.
Saturn also radiates about 60% more heat than it receives from the
Sun. Uranus and Neptune must still be cooling off, but I don't know
their heat budgets.
Bud
--
The night is just the shadow of the Earth.
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