Is really a night side for brown dwarfs?

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danielj
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Is really a night side for brown dwarfs?

Post #1by danielj » 18.05.2004, 23:22

I was astonished that brown dwars has a night side.I though that they woul glow feebly.So they are iluminated by the primary star and glow at the night side?

eburacum45
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Post #2by eburacum45 » 19.05.2004, 10:07

Brown dwarfs apparently don't fuse deuterium for very long; a few tens of million years perhaps, longer the bigger they are. While they are fusing, they will glow red; when the deuterium runs out they will still glow dully from gravitational collapse like Jupiter does, although I don't really know how that works.

(edit)- whoa! I've just been reading up on gravity in brown dwarfs; seems that they have very high gravity, which changes over time as they shrink;

six times the surface gravity of the Sun is one figure I have found, expressed as log g = 5.2 whatever that means (that is about 160 gee!)

pretty weird places, eh?

ajtribick
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Re: Is really a night side for brown dwarfs?

Post #3by ajtribick » 19.05.2004, 16:01

danielj wrote:I was astonished that brown dwars has a night side.I though that they woul glow feebly.So they are iluminated by the primary star and glow at the night side?


Think of it this way: the brown dwarf is at such a low temperature that it will be radiating mainly in the red part of the spectrum, so any illumination from the brown dwarf will be red. If the brown dwarf is in orbit around, say, a G-type star, the star is radiating right across the visible spectrum. So the "day" side of a brown dwarf may appear different to the night side as the light from the star will illuminate colours apart from red on the brown dwarf's "surface".

julesstoop
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Post #4by julesstoop » 20.05.2004, 11:43

If the 'surface' is a sufficiently thick ionized layer, it won't reflect much light at all.
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Evil Dr Ganymede
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Post #5by Evil Dr Ganymede » 20.05.2004, 19:36

eburacum45 wrote:Brown dwarfs apparently don't fuse deuterium for very long; a few tens of million years perhaps, longer the bigger they are. While they are fusing, they will glow red; when the deuterium runs out they will still glow dully from gravitational collapse like Jupiter does, although I don't really know how that works.

(edit)- whoa! I've just been reading up on gravity in brown dwarfs; seems that they have very high gravity, which changes over time as they shrink;

six times the surface gravity of the Sun is one figure I have found, expressed as log g = 5.2 whatever that means (that is about 160 gee!)

pretty weird places, eh?


Wait til you see how dense they are (and how dense late M V dwarf stars are....). They're several tens of thousands of kg/m3!

Starman
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Post #6by Starman » 20.05.2004, 22:28

Microblackhole material?

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Evil Dr Ganymede
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Post #7by Evil Dr Ganymede » 20.05.2004, 23:52

Not even close. But significantly denser than rock on average. ISTR average densities for brown dwarfs of around 50,000 kg/m3...


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