Brown Dwarf: Star or Gas Giant?

[Plankowner]

Not a poll, but I thought I would throw this out the community for discussion.

Given that Traveller doesn't talk about Brown Dwarves very much (if at all) and that the current scientific communitee cannot make up it's collective mind so there there is no good definition, I want to ask the question. If you were going to revise your stellar/system generation method, would you include a Brown Dwarf as a Dim Star or as a Big Gas Giant? Which way should it be treated?

 

[Black Globe Generator]

Originally posted by Plankowner:
If you were going to revise your stellar/system generation method, would you include a Brown Dwarf as a Dim Star or as a Big Gas Giant? Which way should it be treated?

I don't know about "should," but I treat them as a big gas giants in accordance with Ken Pick's informative essay at the Freelance Traveller website.

 

[TheEngineer]

Hi !

AFAIK and according my favrite astro booklet and Wikipedia too the actual definition is, that if an object produces any fusion reaction but no hydrogen fusion its called a brown dwarf.
The lower mass limit is around 11 to 15 masses of Jupiter. If mass is lower they call it planet.

For generation purpose I would allow both, a brown dwarf as central "star", companion whatever, or as a super gas giant. The borderline is not fixed.

Anyway, Mals JTAS artical is a pretty good start, if You've got appropriate access.

regards,

TE

 

[mike wightman]

GURPS Space 4th ed. treats them as failed stars, occuring either as companions to an existing star or solo with the chance of its own satelite planet system.

 

[Border Reiver]

Failed stars. With a size of a large gas giant and a mass o the order of several Jupiters.

I'd play them as failed star with the possibility of fusion reactions still occuring and the associated dangers.

 

[princelian]

I would strongly recommend that for Traveller mapping purposes you categorize them as small stars, because the purpose in Traveller for putting a gas giant on the map is for fuel skimming.

You do not want to fuel-skim a brown dwarf. Full-fledged stars they may not be, but their black-body radiation maxes out in the IR band, and they are HOT HOT HOT, kiddies. The fuelling run would be death unless the dwarf was very very old. A young dwarf should have a surface temperature of over 1000K (~700 deg. C, ~1300 deg. F). While a starship in space can take those kinds of temperatures - direct sunlight is also hot - a lot more energy will be transmitted to the hull in a hot gaseous atmosphere than in space, where heat transfer is inefficient.

Furthermore, all brown dwarfs have roughly the same size as a largish gas giant, but have masses 10-90 times the mass of Jupiter. That means that the gravitational pull at the atmospheric interface would be 10-90 times that of a gas giant.

Finally, brown dwarves are fully convective, as are stars, as seen in this link. This means that the "wind currents" in the atmosphere would make Venus (500+ knot sustained winds) look tame.

So yes, you could refuel from a brown dwarf if you had to, but I don't wanna if I have a choice. Better to survey the dwarf's solar system and see if you can find an ice comet, carbonaceous asteroid with trapped water, or something like that.

However, a really massive gas giant (6-7 times Jupiter's mass) is kind of like a brown dwarf, as a small dwarf (9-12 times Jupiter's mass) is kind of like a big gas giant. So while it would be dangerous to try to skim one, it could theoretically be done.

But be prepared to flip to the Prior History section of your rulebook if you let the maniacs do it.

 

[Spinward Scout]

Would this be considered a proto-star? As in, if any more mass is added to the dwarf (like in 2010) that it may 'ignite' and became a star. If the dwarf is near a nebula, couldn't the free roaming matter gravitate to the dwarf?

 

[Icosahedron]

I would agree with The Engineer and Princelian on this one. If there is a fusion reaction in the core, technically it should be a star.

But for practical purposes, in Traveller, the question is 'can I skim it for fuel, or will it destroy my ship on approach?' That would be the real test.

 

[mike wightman]

Some links I've found interesting:

http://en.wikipedia.org/wiki/Brown_dwarf

http://astro.berkeley.edu/~stars/bdwarfs/

http://www.physics.upenn.edu/~inr/talks/bdpics/bd1.htm

 

[TheEngineer]

Hi !

I..ron, with a multiple of Jupiter masses and a usually higher surface temperature its perhaps not a good idea to try to skim fuel

Regards,

TE

 

[mike wightman]

Its gravity field would be much stronger as well, wouldn't it?

That could cause problems...

 

[Laryssa]

I don't know about a Brown Dwarf, or a Star, but Gas Giant you can smell by his flatulence from miles away.

 

[Plankowner]

Thanks for the comments!

Regarding the fusion thing...

Most Brown Dwarfs will be past their fusion stages and will just be really hot, if I read the references correctly. They will have fusion cores for several tens of millions of years (depending on initial mass), then the fusion process will stop and they will will slowly cool over the next few trillion years.

I like the article on Freelance Traveller and I had seen it before, but I don't like having to roll on 6 different tables to determine the size of my gas giant.

I was really on the fence when I asked the question, but I am now leaning towards having BD's be a class of star that can be rolled, replacing some of the M5v stars.

That might also help with the ongoing problem that Traveller has with stellar density, if a lot of the non-habitable systems were orbiting Brown Dwarfs, then it might make the "stars" a bit farther apart, but that means that most "stars" have habitable planets, which is a whole nother issue...

If you classify BD's as stars, then only a soon-to-be dead man (or woman or sophont) would try to skim fuel from it.

 

[Icosahedron]

Originally posted by Plankowner:
I was really on the fence when I asked the question, but I am now leaning towards having BD's be a class of star that can be rolled, replacing some of the M5v stars.

That might also help with the ongoing problem that Traveller has with stellar density, if a lot of the non-habitable systems were orbiting Brown Dwarfs, then it might make the "stars" a bit farther apart, but that means that most "stars" have habitable planets, which is a whole nother issue...

If you classify BD's as stars, then only a soon-to-be dead man (or woman or sophont) would try to skim fuel from it.

IMTU I solve all of these problems with a single handwave. Blank hexes IMTU may or may not contain very cool red or brown dwarfs. These do not form a normal accretion disc and hence have no planets or gas giants. As a result, they are as useless as empty space and are simply not recorded as starsystems. They do, however, solve the number density problem and the number of stars with habitable planets problem.

 

[Laryssa]

What is the minimum jump distance of a black hole?
A neutron star?
A White Dwarf?
A Brown Dwarf?
A Main sequence star?
A Red Giant?

Seems to me that if you precipitate out of jump space at the 100 diamter limit of a black hole, your ship will be torn apart by the black hole's tidal forces and quaicky accelerated to near the speed of light and sucked into the envent horizon, all faster than you can blink an eye.

 

[Anthony]

Hm. At 100D from a 5 stellar mass black hole (about 15,000 km, have fun even plotting that course) orbital velocity is about .07c, local gravity is about 500,000 Gs. If you actually came in at zero relative velocity, it would take you less than 2.5 seconds to fall into the star. If you come in at orbital velocity, you wouldn't fall into the star, and tidal force would be about 0.07G/m, which would be a problem for a large ship, but not a big problem for a small ship.

 

[princelian]

This is actually a problem, because the gravitational force felt by the sun at Earth's orbital distance (1 AU) is far greater than the force of the Earth at 100 D, so gravity cannot be what you're getting away from with the requirement to get to 100D.

Let's take Earth, 1 g at surface, which is ~6,300 km from center-of-mass. At 100D distance from surface, you're at ~1,250,000 km, and the gravity you are feeling is
</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;">1 gee / ((1.25E6/6.3E5)^2) = 2.54E-5 gee</pre>[/QUOTE]This is one 39 thousandth of a gee, or 0.00025 meters per second squared.

For the sun, with a gravitational force of 274 m/s^2 at the surface (7.0E8 meters radius), to reach the same 0.00025 m/s^2 force requires you reach a distance of 733 million kilometers (which also happens to be ~100D, btw). That distance also happens to be about 5 AU, which tells you that if you're 100D from Earth you're still far, far inside that radius for Sol.

Hmmm....so if it's not gravity per se that's the deciding factor, why do you have to reach 100D?

Or should you have to reach 100D of the primary (an Honorverse-esque "hyper limit of the star")? I like that idea...but what are the implications for the typical Traveller space travel procedure?

To answer your question, Laryssa, 100D probably works for stars, too, but that distance for most of them is probably far beyond the habitable zone.

And black holes and neutron stars curve space so much that, well, I just wouldn't go that close to them anyhow, N-space or J-space-wise.

 

[Plankowner]

Try Gravity Gradient (the CHANGE of gravity per unit distance) it goes with the CUBE-root of the distance, not the square-root so it drops off much faster. If you use that, I believe that the sun's gradient where it matches the Earth at 100D is inside 1AU. Haven't done the math though...

 

[Laryssa]

What if I did this? Calculate the gravity at 100 diameters of Earth and then calculate the gravity at 100 diameters of Earth + 1,000 km, and subtract the gravity at the further distance from that at the closer distance? Now what if I tried to look for this same gravity differential with respect to the Sun? with a difference of 1,000 km. When I have time maybe I'll check this out. Diameter is meaningless when dealing with a black hole.

What about a Red Supergiant? Would it be possible to emerge from jump space inside its atmosphere? Take a red supergiant like Betelgeuse for example?

 

[Anthony]

Originally posted by Laryssa:
What if I did this?<snip>

That's the gravity gradient mentioned above.