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Proxima Centauri Might Not Be the Closest Star To Earth
StartsWithABang writes The Alpha Centauri system consists of three stars, including Proxima Centauri, the closest star to Earth. But while main-sequence, hydrogen-burning stars are easy to find due to their visible light output, brown dwarfs — which only fuse the small amounts of deuterium they're born with — often emit no visible light at all, and can only be seen in the infrared. In 2013, WISE discovered a binary pair of brown dwarfs just 6.5 light years away, making them the third-closest star system to Earth, and leaving open the possibility that there may yet be brown dwarfs closer to us than any star, a question that it will take the James Webb Space Telescope to answer.
Proxima Centauri Might Not Be the Closest Star To Earth
Put another way, Sol Might Be the Closest Star To Earth
Jupiter is not a brown dwarf. It is not massive enough to even burn deuterium. No fusion == no star
Is the quality "star" one of mass, or energy output? Where between white and brown do you draw the line?
The no-man's land between planet and star is a question of interior nuclear fusion: planets don't do it, stars do. But a brown dwarf is an object that does it - a little bit, early in life. Fusion of (normal) hydrogen into helium is pretty hard to do, but fusion of deuterium - hydrogen's heavier cousin - is easier. Brown dwarfs have enough central heat/pressure from gravitational contraction that they fuse all their deuterium away, quickly, and then fade into the night. It's thought that this no-man's land mass range is from about 13 to 80 Jupiter masses.
Nope -- regardless of star status, Jupiter is farther away from the Earth than Sol.
Sheesh, it's just semantics. Definitions are for communication, if they call them brown dwarfs then you know what they're talking about. The IAU's considered an object with a mass capable of fusing deuterium a brown dwarf, which is 13 Jupiter masses. Don't like it? Too bad, as long as it's qualified with "brown dwarf" then you know what they're referring to. So, the term "closest start to Earth" is another issue of semantics. In the context of this article, it means, "closest object outside of our own solar system with a mass over 13 Jupiters." Now, if they start handing out medals and big prize money to stars for being the closest to Earth, then go ahead and debate it, otherwise who cares?
Those are both post-main-sequence stars. Neutron stars happen post-supernova when their mass is too low for their gravitational collapse to compress them beneath the Schwarzschild Radius (which would make them black holes). The Scwarzschild Radius is the spherical radius resultant of calculations dependent upon mass; the distance from the center of a sphere to surface that the mass would need to be compressed into in order to become a black hole. I'm not sure whether the gravitational acceleration during collapse is countered by continued energy output, pressures resulting from electrons being pushed into higher energy levels such that the Pauli Exclusion Principle isn't violated, or some combination of both. Neutron stars are neat because they're basically enormous Bose-Einstein condensates.
Your question about white dwarfs is spot-on though because those stars are pretty much what #49246567 describes. A star fuses enough hydrogen that the mass of its outer layers drops off, inner reactions then push that layer outward to form a red giant, the red giant sheds those outer layers, and a white dwarf is that hot, deuterium-rich star left behind. That star then cools and dims over time.
Hydrogen is fused into heavier deuterium that sinks to lower layers in a star
Stars do not produce deuterium. Any environment that can fuse normal hydrogen can easily burn up any deuterium that is present. Nearly all the deuterium in the universe is believed to be left over from the Big Bang. More info here.
There's a continuum of sorts between gas giant planets and dwarf stars, with a few notable points where you could draw the distinction. They all come from the same general start - a cloud of interstellar gas collapses into a spherical object. Depending on how big it is, you can get different objects.
First you have gas giants, no fusion at all. This would be your Jupiter and Saturn type planets. Jupiter is actually about as big, volume-wise, as a gas giant can get. Add more mass, and it starts getting denser rather than bigger.
At 13 Jupiter masses, you have enough gravitational pressure to fuse deuterium. This is what most astronomers define as a brown dwarf star, but others, and apparently you, consider it to still be a planet. Previous terminology included "substar", which I would not be opposed to. Deuterium isn't particularly common, so these objects glow very dimly, as far as stars go.
At 65 Jupiter masses, you can start fusing lithium as well. This is one way to distinguish brown dwarfs from other stars - red dwarfs and yellow dwarfs, like our sun, consume their starting lithium very quickly, and so the presence of lithium spectra indicates a brown dwarf.
At around 80 Jupiter masses, it starts fusing hydrogen, becoming a red dwarf, like Proxima Centauri. Still very dim, but at this point it's undeniably a star.
At around 750 Jupiter masses, the star develops a more complex internal structure, and becomes a yellow dwarf, such as Sol.
So where do you draw the line? Anywhere you want, but most astronomers settled on the simplest one: if it's undergoing fusion, it's a star, if it isn't, it's a planet.
Not all stars that were large enough to do core hydrogen fusion become red giants. Most red dwarf stars won't because they are fully convective, meaning that they will fuse almost all of their hydrogen in the "core" and not have hydrogen fusion in the shell (and no red dwarf has enough mass to do helium fusion). When low mass red dwarf stars run out of gas they simply pass directly to the white dwarf stage (burnt out core)
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No, or to be precise, not significantly. That calculation was done the other way around: what if all that dark matter was in the form of brown dwarfs, rogue planets, asteroids etc. Then it turns out you need such a huge number of such objects that they would definately be very very prominent in observations. So going back to the observational side of the question, given how hard it is to detect those things we get a very rough upper limit on their mass contribution. How close to that limit we actually are is something new observations try to answer and as in the case of the exoplanets it turns those things are way more comon than we thought initially. Still the upper limit on their total mass is such that they cannot account for more than a few percent of the dark matter.
IIRC they are called MACHOS as opposed to WIMPS which is the kind of exotic particle people are looking into now. Astronomers suck at acronyms...