Can anyone enlighten me?
What is it that distinguishes a brown dwarf from a large gas-giant planet?
Scientists perusing data collected by NASA's Wide-field Infrared Survey Explorer (WISE) have spotted some really cool stars – brown dwarfs with an atmospheric temperature as low as an agreeable 25°C. Dubbed "Y dwarfs", these objects have hitherto eluded astronomers hunting them at visible wavelengths, although WISE has finally …
I think there's some debate over whether or not there is a continuum with stars at one end and gas giants at the other. The key definition of a star, though, is presumably that it has sustained fusion in its core at some stage. Deuterium fusion (D +p -> 3He + gamma) requires objects of around 15 Jovian masses, IIRC.
Size, and whether or not hydrogen fusion occurs within it.
Taking some figures from our favourite on-line encyclopaedia, a stellar body needs to have a mass of approximately 8% of that of our Sun for the core pressure and temperature to get high enough for gravity-induced hydrogen fusion to occur; this also tends to "burn" lithium as well.
However, the mass of a sub-stellar body may yet be high enough for other forms of fusion to occur, using "heavy" isotopes of hydrogen - primarily deuterium (hydrogen-2) - and also possibly "burning" lithium, usually when the it is younger and hotter (from gravitation condensation/contraction).
If anyone can correct any mistakes I've made, please do so! :)
Thinking the exact same thing when I read this. So I did some research (not a lot I admit)
http://atramateria.com/the-coolest-neither-planet-nor-star-brown-dwarf/
and
http://en.wikipedia.org/wiki/Brown_dwarf#Distinguishing_low-mass_brown_dwarfs_from_high-mass_planets
Should help explain a bit.
Explosion: because brown dwarfs would like to start a reaction.
The classification originates from those smarty pants at Harvard:
http://en.wikipedia.org/wiki/Stellar_classification
It's been extended over the years and, like many classification schemes in astronomy (in my experience), it's a living thing as more objects are discovered that don't quite fit.
http://en.wikipedia.org/wiki/Brown_dwarf
Covers your very question - it suggests (although no reference given) that an object must have sustained fusion at some point to be counted as a star - seems reasonable.
"NASA explains the purple hue shown in its artist's impression of a Y dwarf was chosen mainly for artistic reasons".
In other words, here is a picture illustrating something that noone knows what it looks like.
Oh, hum, wait, what?
If people are rewarded according to their imagination, the guy in charge of the budget is on a 7 figures salary.
Sorry to hear that you need the help of a NASA artist to grab the concept of a star being a sphere. If it helps, you can create your own explanatory star picture by choosing the "circle" tool from Paintbrush.
In addition to shape, the point of dinosaurs illustration is to convey a sense of scale, which this pictures also fails to convey.
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Finding a brown dwarf closer than our closest star would be quite a coup considering the closest known star is only 1 AU away from us. One would think that an even closer brown dwarf would have already been detected as a consequence of its gravitational influence. (Such as hurling the Earth out if its nice stable orbit into the deep cold of interstellar space.)
Interesting to note that such a "failed star" might become the last refuge of life in a dying universe, tens or hundreds of billions of years from now. Like the sun, they stay warm by nuclear fusion, but at such a low rate that they'll probably be the last places left where liquid water (and therefore life as we know it) can exist.
I find it incredibly hard to believe that life will be anything like it is now by the point we're approaching the heat death of the universe. In just 3.5 billion years we've gone from a single anaerobic bacteria to the duck-billed platypus*, discarding countless other forms, and possibly many different basic biochemistries, along the way, and we're not even halfway through the life of this single planet.
* platypus = obviously the most complex form of life on the planet. Unnecessarily complex.
We don't know much about what was on the surface of the earth before large parts of it got covered with liquid water, but there's no evidence that life can exist without water.
All terrestrial life still extant shares a common basic biochemistry, with features such as RNA coding for proteins built from a common set of amino-acids, ATP energy-transport, lipid membranes, and an aqueous support medium. There must have been simpler life-systems leading up to this system (think scaffolding), but we have no evidence of what it might have been. I think it extremely unlikely that whatever it was, it did not require liquid water to function. Water is a lowest-common-requirement for all the more complex subsystems.
I'm guessing that complex organisms find it hard or impossible to evolve in the atmospheres of gas giants or cool brown dwarfs. So they might remain at the single-celled stage "forever" until the brown dwarf no longer provides liquid water. Or until some exceptionally unlikely event happens, and multicellular or even intelligent life arises in the dark cold tail-end of a dying universe.
BTW if you envisage galaxies as having been "mined-out" by interstellar-scale intelligences, then think of a brown dwarf ejected from its galaxy and drifting forever alone and undetectable through one of the voids in intergalactic space. That would, in fact, be a more stable environment than one stil in a chaotic orbit around the centre of a galaxy.
The most complex form of life on the planet is surely some sort of insect. Butterfly: Egg, caterpillar, chrysalis ... complete dissolution of the caterpillar to a sort of living soup, and re-birth as a butterfly. Or spider-hunting solitary wasp. Somewhere in the egg is a program which allows it to hunt and paralyze spiders without becoming prey, dig a burrow, install the spider, lay an egg. I wish someone could tell me where and how.
Thats just mean - the categories used to be Wow Oh Be A Fine Girl Kiss Me Right Now Sweetie
(hmm, maybe the best way to find these things is to "look out the corner of your eye").
brown dwarf star => nuclear reactions started upon gravitational collapse but then ran out of oomph
gas giant planet => not big enough for collapse to initiate nuclear reactions
e.g. jupiter ~ 1 order of magnitude too small to be any kind of star
Jupiter emits more radiation than it recieves from the sun. Fusion at a very low rate is the probable source of the excess heat. Jupiter's core is believed to be mostly hydrogen in its theoretically predicted high-pressure metallic form.
The Earth also emits more heat than it receives. In this case we have good reason to believe that the sources are radioactivity and tidal friction, and possibly also ongoing crystallisation of the Earth's solid inner core from its liquid outer core.
For the Earth, the excees heat may have been the difference between a living planet and a snowball, in the early days when the sun was somewhat cooler and the moon was a lot closer.
I studied astrophysics in grad school back in the early 90s and one day asked this. I said that, given that most of the light is in O and B stars, but the M stars, though individually small, are so much more numerous that they contain most of the mass, couldn't small M dwarfs, neutron stars, cooled white dwarfs, and brown dwarfs account for some significant portion of the "missing mass"? I was scoffed for thinking like a stellar astronomer.
Could much mass be in lumps of cool matter (dark rocky basketballs)? Nice question - was asked just this on an astrophysics test years ago.
One kind of test made since: looking for "MACHO"s, massive compact halo objects. If much of our galaxy's missing mass were in brown-dwarf or other planet-to-star-sized lumps, they'd be detectable by gravitational lensing when one passed between us and a distant star. MACHO searches stared at huge fields of stars for the right kind of variability - brightening then fading over a day or so, with characteristic wavelength-independence that grav. lensing would do. Some have turned up, but not enough to be a large fraction of our Milky Way's gravitating matter. (But, lensing searches wouldn't see basketballs.)
The more fundamental limit on the amount of ordinary matter (protons &c.) comes from "big-bang nucleosynthesis". As I (vaguely) understand, the density of ordinary (subject-to-nuclear-reactions, "baryonic") matter in the very early hot universe determines the ratios of helium/deuterium/hydrogen/lithium/photons that were left over once things cooled off. Those ratios - 'primordial abundances' - are more or less measurable and set limits on those early-time densities.
Result: even if lots of today's ordinary matter were in the form of cold dark invisible basketballs, that matter would have had to be present during Big Bang times too. Observed primordial abundances and known nuclear physics say there was 'way too little of that, no matter what form it takes today, to explain the amounts of 'dark matter' which is detected by its gravity in holding together galaxies and galaxy clusters today.
(Presumably the mysterious dark matter, *whatever* it is, went through the Big Bang too, but doesn't participate much in nuclear reactions. And, whatever it is, there seems to be about 5x more of it than of all the ordinary matter (visible and in-) put together. Exciting times to do physics, hence the thumbs up...)
I recall the phrase for remembering the sequence of star types to be "Oh Be A Fine Girl, Kiss Me Right Now, Sweetheart" , but that only works up to "Kiss Me" for the sequence shown in the article, which as LTY instead of RNS after OBAFGKM. Maybe the phrase should be changed to "Oh Be A Fine Girl, Kiss Me Lots This Year"?