Brown Dwarf Stars - Interesting and quite unknown stellar object

[Smaug]

What is it?

In order to understand what is a brown dwarf, we need to understand the difference between a star and a planet. It is not easy to tell a star from a planet when you look up at the night sky with your eyes. However, the two kinds of objects look very different to an astronomer using a telescope or spectroscope. Planets shine by reflected light; stars shine by producing their own light. So what makes some objects shine by themselves and other objects only reflect the light of some other body? That is the important difference to understand -- and it will allow us to understand brown dwarfs as well.

As a star forms from a cloud of contracting gas, the temperature in its center becomes so large that hydrogen begins to fuse into helium -- releasing an enormous amount of energy which causes the star to begin shining under its own power. A planet forms from small particles of dust left over from the formation of a star. These particles collide and stick together. There is never enough temperature to cause particles to fuse and release energy. In other words, a planet is not hot enough or heavy enough to produce its own light.

Brown dwarfs are objects which have a size between that of a giant planet like Jupiter and that of a small star. In fact, most astronomers would classify any object with between 15 times the mass of Jupiter and 75 times the mass of Jupiter to be a brown dwarf. Given that range of masses, the object would not have been able to sustain the fusion of hydrogen like a regular star; thus, many scientists have dubbed brown dwarfs as "failed stars".

So why would we care about brown dwarfs? It is possible that a great deal of the mass in the universe is in the form of brown dwarfs, and since they do not give off much light, they could constitute part of the "missing mass" problem faced by cosmology.

Curitosity:

The coolest Brown Dwarf ever discovered has a temperature of only about 300 K (27ºC), its name is WISE 1828+2650.

Comparison between Stars, Planets and Brown Dwarfs:

http://4.bp.blogspot.com/-NG9Fn7OBddQ/TZswu9J7FkI/AAAAAAAAAAM/YWt6EPSwsIo/s1600/Super%2BCool%2BBrown%2BDwarfs0001.jpg
http://www.daviddarling.info/images/brown_dwarf_size.jpg

Ilustrative image of a low-energy emissivity Brown Dwarf:

http://astro.berkeley.edu/~basri/bdwarfs/sciamh.JPG
http://upload.wikimedia.org/wikipedia/commons/thumb/1/11/Y_Dwarf_Chillin_in_Space.jpg/800px-Y_Dwarf_Chillin_in_Space.jpg

[Countess Grishnack]

Very interesting threat, we only should worry if sun becomes a brown dwarf then the earth would freeze and in this case we would be in a serious danger but this will take a lot of time.

[Virtuous]

I suppose at that size even if considered as a planet there can't be rocky/liquid Brown Dwarves, only gas, right? (Noob physics question, I know).

[Pretan]

Very interesting threat, we only should worry if sun becomes a brown dwarf then the earth would freeze and in this case we would be in a serious danger but this will take a lot of time.

Our Sun will become a white dwarf not a brown dwarf.

The Earth will be burnt to a crisp long before that happens however.

[Virtuous]

Very interesting threat, we only should worry if sun becomes a brown dwarf then the earth would freeze and in this case we would be in a serious danger but this will take a lot of time.

Brown Dwarves are only "Stars" that didn't have enough mass/energy to form. I don't think a star can ever go into that state. Either red giant or supernova...or the blue thingies.

[Pretan]

I suppose at that size even if considered as a planet there can't be rocky/liquid Brown Dwarves, only gas, right? (Noob physics question, I know).

They're basically an intermediate between Gas Giants (Jupiter) and Small Stars.

If Jupiter was much larger it would have a much higher pressure in the centre of it (due to increased gravity), this high pressure would allow Nuclear fusion on a low scale thus making it a brown dwarf star. Normal Stars, like the Sun, are basically enormous gas giants that have been turned into enormous Nuclear Power Stations. Neither would produce Nuclear energy if they were rocky or liquid, therefore they wouldn't be Stars/Dwarf Stars.

I'm currently writing up about this sort of stuff at Uni. But I am terrible at explaining my thoughts.

[Loki]

I suppose at that size even if considered as a planet there can't be rocky/liquid Brown Dwarves, only gas, right? (Noob physics question, I know).

No, a planet is an object circling a star - regardless of size. A star can be of any size.

[Virtuous]

No, a planet is an object circling a star

Not necessarily, there are binary stars :D


regardless of size. A star can be of any size.

Not really, as far as I know there can't be stars smaller than a brown dwarf or as large as a planet.

(alright, brown dwarfs are failed stars but you get what I mean)

[Mago]

Interesting. I was watching something about that on the Science Channel. Love that kind of stuff. If anyone wants to learn about the universe and us spend some time watching Carl Sagan's Cosmos, it will blow your mind away.

[Virtuous]

Yeah, Carl Sagan is like the Cosmo-Physics-Poet.

[Smaug]

No, a planet is an object circling a star - regardless of size. A star can be of any size.

Circling a star is one of the itens in the checklist to an object be considered a star. There are other criteria that must be taken into consideration, like mass, size, etc...

The moon for example has enough mass to be considered a planet, but it isn't because it circles a planet, not a star. Pluto is circling a star, but doesn't have enough mass, and also, it is part of the Kuiper Belt. An object to be considered a planet must also have a "clean" neighbourhood, free of asteroids and other space trash.