PDA

View Full Version : Red Giant



AKONI
2004-May-05, 05:18 AM
Our Sun is supposed to be destined to turn into a Red Giant, engulfing the Earth in the process.

While I have read about this and seen it on numorus programs on the Science Channel, one thing is always left out... How long does the process take?

So here are my questions...


If the Sun began turning into a Red Giant today...

1. How long would it take before we first noticed this change?

2. How long would it take from the time we first felt the effects to the point where the earth was no longer habitable?

3. How long does the entire process take from beginning to completition (I'm assuming it's not done even when the Earth is turned into a burnt pot roast)?

Brady Yoon
2004-May-05, 05:43 AM
1. How long would it take before we first noticed this change?

The change into the red giant phase would take a long time. Not very long compared to its main sequence stage, but still very long in terms of a human lifespan. Noticeable changes would occur over several million years.


2. How long would it take from the time we first felt the effects to the point where the earth was no longer habitable?

I can't give you an accurate time, but again, probably several million years.


3. How long does the entire process take from beginning to completition (I'm assuming it's not done even when the Earth is turned into a burnt pot roast)?

The entire red giant phase, from when the star begins to run out of hydrogen to when the star throws off its outer layers as a planetary nebula, is about several hundred million years. This also includes the star when it is on the hydrogen shell burning stage, the horizontal branch, the helium flash, and the asymptotic giant stage.

Hope that helps. :)

Brady Yoon
2004-May-05, 05:48 AM
One more thing. According to current planetary science, the Earth will become inhabitable only one billion years from now. As the sun slowly increases in luminosity, the oceans will start evaporating, and the atmosphere will boil away, which will trigger a greenhouse effect. Earth will once again become sister planets with Venus.

Toutatis
2004-May-05, 06:42 AM
Inasmuch as 'evolution' to the 'red giant phase' must follow significant reduction in solar mass (as per m=E/c^2) and, hence, gravitation - It seems possible that Earth may (via assumption of a greater orbital radius) be spared absolute annihilation through 'immersion' in stellar plasma --- Still… I suspect the forecast will be right toasty!!! http://www.badastronomy.com/phpBB/images/smiles/icon_biggrin.gif

Just an idea???? --- Comments will be appreciated :-)

Best regards
Sarandon

Peter B
2004-May-05, 06:50 AM
According to current planetary science, the Earth will become inhabitable only one billion years from now.

Er, not to nit-pick, but to prevent any confusion, the Earth will become uninhabitable in about 1 billion years. :)

kenneth rodman
2004-May-05, 07:09 AM
From what I have read the sun wont swell into a red giant for another 5 Billion yes 5,000,000,000 years from now

Peter B
2004-May-05, 07:57 AM
From what I have read the sun wont swell into a red giant for another 5 Billion yes 5,000,000,000 years from now

That sounds about right, too.

The reason the Earth will become uninhabitable in about 1 billion years is not because the Sun will swell into a red giant. I forget the precise reason, but it's something more fundamental about the Earth itself.

eburacum45
2004-May-05, 08:01 AM
Yes, Kenneth, That is correct;
the Red Giant phase will not start until five billion years from now.

Nevertheless Peter B is correct- the Earth will be too hot in only one billion years , because our sun will increase slowly in brightness while remaining a yellow dwarf.

If humans or their descendants are still around, I expect they will be able to protect the Earth from a little bit of extra sunshine during this period- a sunshade in the L1 point has been suggested.

By the time the sun becomes a red giant even this sort of technology will be futile-
there is no other option but to leave the system altogether.

Eroica
2004-May-05, 11:27 AM
The Once and Future Sun (http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html)

Brady Yoon
2004-May-05, 05:26 PM
Er, not to nit-pick, but to prevent any confusion, the Earth will become uninhabitable in about 1 billion years.
Oops. #-o :D

Toutatis
2004-May-06, 03:36 AM
Er, not to nit-pick, but to prevent any confusion, the Earth will become uninhabitable in about 1 billion years.


Where one billion = one thousand million (i.e. 10^9), that is... ;-) --- Don't laugh! I formerly resided in a land where convention had 10^9 = "one milliard" (not to be confused with mallard! ;-)) and one trillion (i.e. 10^12) = "one billion" http://www.rainbowpirates.com/forum/images/smiles/icon_confused.gif

Wadda a long strange trip it's been!!! http://www.badastronomy.com/phpBB/images/smiles/icon_eek.gif

TTFN
Sarandon

eburacum45
2004-May-06, 07:34 AM
I must remember to use gigayears in future- no chance of confusion there- except that people are not yet familiar with the term...

cyswxman
2004-May-06, 03:22 PM
The reason the Earth will become uninhabitable in about 1 billion years is not because the Sun will swell into a red giant. I forget the precise reason, but it's something more fundamental about the Earth itself.

I seem to recall that one thing is the continued slowing of the Earths' rotation making days (and nights) increasingly longer which will adversely affect our climate. I haven't done the math, but how long will a "day" be 1 billion years from now?

Russ
2004-May-06, 07:41 PM
Whatever time span is involved, I doubt that humanity will be around to concern themselves. IIRC no spiecies with more than one cell has lasted more than about 300 million years. Most much less than that. There may be genetic decendants of humanity out then, but they'll brobably resemble us as much as we resemble a vole. But, then again, who knows? :D :lol:

Dgennero
2004-May-06, 08:52 PM
This rule is IMO not applicable to humans - we are the first species on earth capable of actively changing our genetic material and conciously influence evolution or even create hybrid beings (biological/technological).
I think our species will not last as humans any more within no more than 10,000 years. There will be descendants, totally different beings. Our current ethical problems or revulsion felt by some will IMO then seen as outdated carbon-based chauvinism.

AKONI
2004-May-06, 09:25 PM
Thanks everybody.

I knew it didn't take a few seconds as shown in the graphics of the shows, but I didn't realize the final process took as long as it does...

But now I have a new qustion... In the final stage as shown in the link Eroica ... http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html provided, the white dwarf is centered in the solar system. It is much smaller - earth size, and yet the remaining planets continue their orbit.

How is this possible when it's mass is only 0.54 M sun?

Why do they not simply spiral out into space, or develope an orbit around one another or even Jupiter?

Yumblie
2004-May-06, 11:31 PM
.54 times the mass of the sun is still pretty hefty. It's still more massive than Jupiter, and it will still be closer as well. The orbit will just be farther out, since it is less massive. I'm not sure if because of the less massive sun that the other planets will perturb Earth's orbit a bit more, though it probably still won't have much of an effect.

AKONI
2004-May-07, 12:49 PM
Okay, thanks for the info.

tracer
2004-May-07, 02:35 PM
The change into the red giant phase would take a long time. Not very long compared to its main sequence stage, but still very long in terms of a human lifespan. Noticeable changes would occur over several million years.
I recall an article about a star that we were observing in the process of swelling from main-sequence to red giant. Based on the speed of the process as revealed by ground-based telescopes over a couple of decades, the evolution from luminosity class V (main sequence) to luminosity class III (giant) could take as little as 20,000 years.

Eroica
2004-May-07, 03:00 PM
But now I have a new qustion... In the final stage as shown in the link Eroica ... http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html provided, the white dwarf is centered in the solar system. It is much smaller - earth size, and yet the remaining planets continue their orbit.

How is this possible when it's mass is only 0.54 M sun?

Why do they not simply spiral out into space, or develope an orbit around one another or even Jupiter?
Good question! If the Earth "settles" into its final orbit at a distance of 1.85 AUs from the centre of the Sun, it must be orbiting more slowly than today. So what slows it down? :-k

Dgennero
2004-May-07, 06:38 PM
Kepler says so :D
My explanation: The earth (a little bit like an electron rising to a higher orbital) gets somewhat more "out" of the sun's "gravitational pot" and thus to a higher energetic state; this gained energy has to be lost somewhere else - in that case, the earth's kinetic energy/momentum/speed.
But better get a second opinion, for I'm not absolutely sure.

lek
2004-May-07, 07:02 PM
Interesting point... that website just states something like earths orbit gets bigger to match the smaller sun. Which doesnt make any sense imo. For stable orbit earths velocity would have to drop to roughly half for that given mass of sun and distance, right?

Sun losing weight explains earth moving away from the sun but what reduces earths velocity to stabilize it?

I got the impression that those predictions were based on some simulation so i would imagine its not just a little flaw in it, more like crap/missing explanation to question: why?

Avatar28
2004-May-07, 07:23 PM
I'm wondering that myself. SOMETHING has got to slow down the earth. Maybe it's drag from the planetary nebula or something. Who knows.

That's another question. Would not the earth and the other planets accrete a LOT more mass if almost half the sun's mass has been blown off around it? Heck, I could almost see Jupiter sucking up enough to possibly become at least a brown dwarf.

lek
2004-May-07, 07:42 PM
edited out the rubbish... not much left :(

JohnOwens
2004-May-07, 08:00 PM
Anybody thought of applying conservation of angular momentum to the Earth's larger orbit? :D

Eroica
2004-May-07, 08:55 PM
Ah, I see. Are you saying that what the Earth gains in angular momentum by revolving about the Sun in a bigger orbit it must lose by revolving more slowly?

Eroica
2004-May-08, 08:36 AM
But why wouldn't it lose the extra momentum by rotating on its axis more slowly? :-k

lek
2004-May-08, 09:01 AM
...
crap/missing explanation to question: why?

* Shakes fist at professor who doesnt explain everything to his students and instead assumes they know some basic physics... :oops: :lol:

I Think i forgot this thing called "gravity", which seems to be guilty of stealing earths kinetic energy... #-o

Eroica
2004-May-08, 09:15 AM
I Think i forgot this thing called "gravity", which seems to be guilty of stealing earths kinetic energy... #-o
Explain. The gravitational force is at right-angles to the orbital velocity, so it can possibly increase or decrease it (unless it has something to do with the Earth's tidal bulge?)!

lek
2004-May-08, 09:56 AM
Sun loses mass --> centripetal force exceeds now weaker force of gravity--> earth moves away from sun.
Angle between velocity and force of gravity isnt perpendicular any more.
Centripetal force comes from earths kinetic energy, part of which is now doing work against gravity as earth moves to higher orbit.

Even if quite a big lump of sun magically disappeared in an instant, it wouldnt send earth spiralling away, it would just settle into new orbit...


edit: cant help but wonder why nobody responded yet :D.
Dodgy use of terms like force and work, i think i (once again) mixed centripetal and centrifugal.. i suppose "as earth moves away from sun, gravity reduces its velocity=>converts earths kinetic energy to potential energy" would have been too simple... One day i hope to be able to write without shutting down the brain ;)

Eroica
2004-May-08, 11:32 AM
Thanks, lek, I get it now.

And another thing just occurs to me. The Earth's orbit is elliptical. So the distance between the Earth and Sun is continually changing, and as it does so, the Earth's orbital speed changes. #-o

Grand_Lunar
2004-May-09, 01:06 AM
The Once and Future Sun (http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html)

A terrific site on the sun's life cycle. But I have a question about it.
The white dwarf phase is said to last "forever'. I thoguht a white dwarf will cool and become darker and darker, eventually becoming a black dwarf; the burnt out core with no light of its own. Sure, this will take a while, but forever? :-k I don't think so.

Yumblie
2004-May-09, 01:20 AM
The Once and Future Sun (http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html)

A terrific site on the sun's life cycle. But I have a question about it.
The white dwarf phase is said to last "forever'. I thoguht a white dwarf will cool and become darker and darker, eventually becoming a black dwarf; the burnt out core with no light of its own. Sure, this will take a while, but forever? :-k I don't think so.

I think they just mean compositionally. I believe once the sun becomes a white dwarf, it will not undergo any change except in temperature. A black dwarf is just a white dwarf that has completely cooled down. Astronomy semantics strikes again.

Kaptain K
2004-May-09, 03:00 AM
As was mentioned, it is a matter of semantics. The lifetime of the whte dwarf stage of a star like the Sun is several thousand times the current age of the universe. It may not (technically) be for ever, but it might as well be!

Grand_Lunar
2004-May-10, 01:56 PM
As was mentioned, it is a matter of semantics. The lifetime of the whte dwarf stage of a star like the Sun is several thousand times the current age of the universe. It may not (technically) be for ever, but it might as well be!

That's probably why no one has detected a black dwarf yet.
I'm curious why it takes so long though.
Am I correct in assuming that a white dwarf starts out at the temperature of the core of the dead star? Millons of degrees celsius? Near the temp that the fusion occurred at? That'd make sense. And perhaps the material doesn't radiate heat too well. Would that be right?
And I thought neutron stars, black holes, and supernova were complicated! Now these!

Kaptain K
2004-May-10, 02:47 PM
It is simply a matter of black body radiation. You have an object with a mass on the order of that of the Sun, but a surface area on the order of that of the Earth. It takes a long time to radiate that much energy through that little area.

Grand_Lunar
2004-May-11, 02:05 AM
It is simply a matter of black body radiation. You have an object with a mass on the order of that of the Sun, but a surface area on the order of that of the Earth. It takes a long time to radiate that much energy through that little area.

I thought as much that white dwarf material (what would that be called? super dense matter? I know matter from neutron stars is sometimes called neutronium) would be a poor radiator of heat (or would conductor be more correct? I say radiator, since in space, there's nothing much to conduct too). Am I also right in assuming the initial temp is in the millions? Hot stuff!
All this shows why I'm the Grand Lunar and not the Grand Solar or Grand Stellar! 8-[

Mars
2004-May-11, 02:22 AM
The day increases because of the moon.

Conservation of angular momentum would cause the Earth's year to increase as it drifted to a further orbit. I believe with the White Dwarf's mass being lower, the tug of gravity would be less and Earth's momentum would push it into a larger orbit.

Mars
2004-May-11, 02:31 AM
The explanation about the moon slowing the Earth's rotation is in BA's book. Pg. 74

Kaptain K
2004-May-11, 04:01 AM
It is simply a matter of black body radiation. You have an object with a mass on the order of that of the Sun, but a surface area on the order of that of the Earth. It takes a long time to radiate that much energy through that little area.

I thought as much that white dwarf material (what would that be called? super dense matter? I know matter from neutron stars is sometimes called neutronium) would be a poor radiator of heat (or would conductor be more correct? I say radiator, since in space, there's nothing much to conduct too). Am I also right in assuming the initial temp is in the millions? Hot stuff!
All this shows why I'm the Grand Lunar and not the Grand Solar or Grand Stellar! 8-[
The matter in white dwarfs is called "degenerate matter" or more specifically, "electron degenerate matter". Electrons and the nuclei are in a plasma state. The repuslsion between electrons keeps the star from collapsing any farther. This state holds up to about 1.4 solar masses. Above this threshold, gravity overcomes the electrostatic repulsion and the electrons combine with the protons to form neutrons and the star collapses to a neutron star. The strong nuclear force holds the neutrons apart until the mass reaches about 3.2 solar masses and the neutron star collapses to a black hole.

White dwarfs and neutron stars lose heat by radiation. Black body radiation is determined soley by temperature and surface area. The higher the temp, the faster they lose energy. The lower the surface area, the slower heat escapes.

Grand_Lunar
2004-May-11, 08:36 PM
I've heard of degenerate matter, but didn't know it was the term for a white dwarf's composition.
I wondered if there was a limit to a neutron star's matter. Had to be, or else where hence does a black hole form?
So something the size of earth, but the mass of the sun, and starting at millions of degrees takes a long time to cool down then. Cool stuff!
I didn't know a neutron star would loose heat. Thought it produced it, since they emit strong radio waves. Or is that just from its magnetic field? Or something else?
Now a question about red giants, which might have been answered before. On that website that shows the sun's cycle, it seems that the sun first becomes a red giant, but then shrinks when it stabilizes after burning heluim. I thought the additional heat would keep the sun at a swollen size. What happens?

Brady Yoon
2004-May-11, 10:50 PM
I wondered if there was a limit to a neutron star's matter. Had to be, or else where hence does a black hole form?
As Kaptain K said, the mass limit of a neutron star is around 3.2 solar masses. However, if there is sufficient rotation or magnetism that would counteract gravity, the limit would be higher.


I didn't know a neutron star would loose heat. Thought it produced it, since they emit strong radio waves. Or is that just from its magnetic field? Or something else?
The neutron star would still radiate normally. Light emitted from a neutron star can escape easily because its escape speed is lower than light speed. There would be a small but measurable gravitational red shift, though.


Now a question about red giants, which might have been answered before. On that website that shows the sun's cycle, it seems that the sun first becomes a red giant, but then shrinks when it stabilizes after burning heluim. I thought the additional heat would keep the sun at a swollen size. What happens?
Good question. I'll do some research. :D

Grand_Lunar
2004-May-24, 09:27 PM
In a move to keep this thread alive......

What other kinds of giant stars are there? I've heard of the red supergiants of course (like Betalgeuse), but are there other colors of giants? Yellow giants? Blue giants? Orange? Green?
And since I mentioned it, are there any stars larger than Betalgeuse?

Hamlet
2004-May-25, 02:29 PM
In a move to keep this thread alive......

What other kinds of giant stars are there? I've heard of the red supergiants of course (like Betalgeuse), but are there other colors of giants? Yellow giants? Blue giants? Orange? Green?
And since I mentioned it, are there any stars larger than Betalgeuse?

Eta Carinae (http://www.seds.org/messier/xtra/ngc/etacar.html) is one of most massive stars that we know. However, in cosmic term, it won't hold this title for very long. It's expected lifetime is only about a million years and will most likely end up as a supernova.

eburacum45
2004-May-25, 02:54 PM
Yeah; Eta Carinae is a Luminous Blue Variable;
http://homepage.fcgnetworks.net/rduch/LBVS.htm

which means a blue giant.

There are smaller blue giants, like Deneb, and Naos around;
quite a few red giants, the biggest example I can think of being Betelgeuse;
the few yellow and orange giants like Enif and Sadalmelik are pretty puny by comparison.

eburacum45
2004-May-25, 03:07 PM
but no green giants, jolly or otherwise.

earthman2110
2004-May-25, 03:12 PM
so if the white dwarf stage of a star lasts "forever", wouldnt it be nice to build a civilization around one of these little guys? would that present any major problems?

Kullat Nunu
2004-May-25, 03:22 PM
so if the white dwarf stage of a star lasts "forever", wouldnt it be nice to build a civilization around one of these little guys? would that present any major problems?

No and yes. White dwarf stage does not last forever. Eventually they will become black dwarfs (none yet exist) which emit no light or heat. Lack of radiation may be problem with younger white dwarfs also. A habitable planet orbiting white dwarf must be so close that it would become tidally locked.

Kullat Nunu
2004-May-25, 03:54 PM
What other kinds of giant stars are there? I've heard of the red supergiants of course (like Betalgeuse), but are there other colors of giants? Yellow giants? Blue giants? Orange? Green?
And since I mentioned it, are there any stars larger than Betalgeuse?

Giants are stars that have ended hydrogen fusion in their cores.
So for example a really bright hydrogen-burning main-sequence (aka dwarf) O star is much brighter than the Sun in the red giant phase.

There are "different colored" stars depending on their surface temperature:
- O stars are white-blue and much hotter than the Sun
- B stars are white-blue too, but less hot
- A stars are white like Sirius
- F stars are white-yellow, a bit hotter than the Sun
- G stars are yellow like the Sun
- K stars are orange
- M stars are red and cool (actually still quite yellow-orange)

K and M class giant stars are much more common than other giants of other classes.

Giants have been classified depending on their luminosity:
- Subgiants (luminosity class IV): stars that have just ended hydrogen fusion in their cores and are becoming real giants. Examples: Procyon (F5 IV), Merope (B7 IV).
- Giant (luminosity class III) is the endpoint of a low mass star like the Sun. Examples: Arcturus (K1 III), Aldebaran (K5 III).
- Bright giants (class II) are brighter and more massive than Sun. Examples: Canopus (F0 II), Albireo (K3 II).
- Supergiants (class Ib & Ia): endpoint of massive stars. Examples for less bright supergiants Ib: Enif (K2 Ib) and Sadalmelik (G2 Ib) eburacum45 already mentioned.
- Bright supergiants (class Ia) are extremely bright stars, well visible across thousands of light years. Examples: Rigel (B8 Ia), Deneb (A1 Ia).
- There is also more or less unofficial class, hypergiants (class 0) which are even (tens of times) brighter than bright supergiants. Examples: Eta Carinae (B0 0), Pistol Star.

Grand_Lunar
2004-May-25, 04:12 PM
Yeah; Eta Carinae is a Luminous Blue Variable;
http://homepage.fcgnetworks.net/rduch/LBVS.htm

which means a blue giant.

There are smaller blue giants, like Deneb, and Naos around;
quite a few red giants, the biggest example I can think of being Betelgeuse;
the few yellow and orange giants like Enif and Sadalmelik are pretty puny by comparison.

I looked at an image of Eta Carinae. It doesn't look blue at all; more reddish. Or is that just because the image shows the star surrounded by its own guts?
I've thought that stars of 20 to 30 solar masses were big. Isn't that about what Betalgeuse is? Seeing that Eta Carinae is at least 100 solar masses is mind boggling! What diameter would such a star be? How far would it streatch if placed where the sun is? Earth, I know, would be a goner if Betalgeuse was there. Imagine this one!
When this star goes "KA-BOOM!!!" (or actually, " !!!") wouldn't it form a black hole? I'm sure it would.

Kullat Nunu
2004-May-25, 04:14 PM
And to make things more unclear, there is a peculiar variable star V838 Monocerotis which suddenly expanded, brightened to supergiant status and cooled down to about 1000 K, much cooler than any other supergiant. It was then very deep red class L supergiant! Usually spectral classes L and T belong to very dim brown dwarfs.

earthman2110
2004-May-25, 04:46 PM
No and yes. White dwarf stage does not last forever. Eventually they will become black dwarfs (none yet exist) which emit no light or heat. Lack of radiation may be problem with younger white dwarfs also. A habitable planet orbiting white dwarf must be so close that it would become tidally locked.

do we know how much heat or light is radiated by a white dwarf?

eburacum45
2004-May-25, 04:52 PM
so if the white dwarf stage of a star lasts "forever", wouldnt it be nice to build a civilization around one of these little guys? would that present any major problems?

No and yes. White dwarf stage does not last forever. Eventually they will become black dwarfs (none yet exist) which emit no light or heat. Lack of radiation may be problem with younger white dwarfs also. A habitable planet orbiting white dwarf must be so close that it would become tidally locked.

If we ever arrive at a white dwarf with planets perhaps we could use the mass of those worlds to build orbital habitats near the star;
here is one I made earlier...
http://www.orionsarm.com/worlds/Ouaddai.html

Hamlet
2004-May-25, 06:47 PM
No and yes. White dwarf stage does not last forever. Eventually they will become black dwarfs (none yet exist) which emit no light or heat. Lack of radiation may be problem with younger white dwarfs also. A habitable planet orbiting white dwarf must be so close that it would become tidally locked.

do we know how much heat or light is radiated by a white dwarf?

They start off with a surface temperature of about 100,000K. After about 4 - 5 billion years the white dwarf has cooled to about the surface temperature of our Sun at 5,800K.

Brady Yoon
2004-May-26, 04:34 AM
They start off with a surface temperature of about 100,000K. After about 4 - 5 billion years the white dwarf has cooled to about the surface temperature of our Sun at 5,800K.

Wouldn't they start off at temperatures close to the core temperature of a red giant star, maybe 100 million K? :-k

earthman2110
2004-May-26, 04:59 PM
so how much heat is given off by a "brown dwarf"

what about grumpy, sneezy, and bashful? :o

Hamlet
2004-May-26, 05:47 PM
They start off with a surface temperature of about 100,000K. After about 4 - 5 billion years the white dwarf has cooled to about the surface temperature of our Sun at 5,800K.

Wouldn't they start off at temperatures close to the core temperature of a red giant star, maybe 100 million K? :-k

I don't think so. The sources I have read show a cap at about 100,000K for white dwarf surface temperatures. Once nuclear reactions stop, the core will cool and collapse until it reaches electron degeneracy and becomes a white dwarf. Once in a degenerate state the white dwarf will continue to radiate its remnant heat and eventually become a black dwarf.

Brady Yoon
2004-May-26, 11:46 PM
so how much heat is given off by a "brown dwarf"

Not much. A brown dwarf with the temperature of 1000 K and a 1/5 of the radius would have a luminosity of only 3*10^-5 solar luminosities. In other words, the sun is over 32,000 times brighter.

eburacum45
2004-May-27, 05:41 AM
One thing I didn't realise about brown dwarfs till recently is that they only fuse deuterium (heavy hydrogen) for a few million years, then they go out; they will then be heated only by gravitational collapse, which is what heats Jupiter.

So for most of their lifetime a brown dwarf will hardly glow at all.
The gravity of a brown dwarf is phenomenal, as well; I have seen figures of about six solar gravities, making the surface gravity about 140 gee.

ach! a very inhospitable type of world; they will be difficult objects to find a use for in the long term.

Hamlet
2004-May-27, 02:55 PM
Brady Yoon's question about white dwarf temperatures made me think and so I did a little research last night. It seems that a white dwarf is almost isothermal. The degenerate electrons are good thermal conductors and so the temperature is nearly uniform throughout the entire sphere. However, there is a thin layer, about 50km thick, at the surface that isn't degenerate and acts like a thermal insulation layer. This barrier impedes the diffusion of radiation from below and effectively slows down the cooling rate.

So, for example, a white dwarf may have a sub-barrier temperature of around 10 million K, but the surface temperature may only be 10,000K. This effect coupled with the small surface area helps explain why it takes so long for white dwarfs to cool off.

Another tidbit. It seems that early on in its formation, the white dwarf generates a large number of neutrinos. Since neutrinos are weakly interacting with matter, most are lost to space, taking away a lot of energy and rapidly cooling the white dwarf. Eventually the white dwarf loses too much energy and can no longer generate neutrinos leaving radiative cooling to finish the job.

I know this is getting a bit OT, but I wanted to follow-up.

Grand_Lunar
2004-Jun-05, 08:31 PM
Hey Hamlet, I think I know a buddy of yours; his name is Riblet. Har har! 8-[
Seeing how white dwarves are the end result of a red giant, you're not too off topic.
Here's something I think I asked before: a star become giant (or supergiant) size because of the greater amount of heat made in the core, right? And that the heat makes the over envelope expand, resulting in a bloate star?

EDIT: edited for spelling and clarity.