Not quite a star, but almost

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"?

A Planet becomes a true Star when it starts Fusion, then and only then does
it become a Star.


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wrote in message
oups.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"?

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
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

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

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
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

On Fri, 18 Aug 2006 16:06:59 -0700 (PDT), (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.

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
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

On Fri, 18 Aug 2006 15:23:47 -0700 (PDT), (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

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
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

On Fri, 18 Aug 2006 19:16:08 -0700 (PDT), (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.