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.

Comments

Are Brown Dwarfs Stars?

[jfdavis668]

I don't think brown dwarfs count as stars.

Re:Are Brown Dwarfs Stars?

[stox]

Is the quality "star" one of mass, or energy output? Where between white and brown do you draw the line?

Re:Are Brown Dwarfs Stars?

[Anonymous Coward]

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.

Re:Are Brown Dwarfs Stars?

[Sperbels]

What about white dwarfs and neutron stars?

Re:Are Brown Dwarfs Stars?

[duck_rifted]

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.

Re:Are Brown Dwarfs Stars?

[ShanghaiBill]

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.

Re:Are Brown Dwarfs Stars?

[gman003]

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.

Re:Are Brown Dwarfs Stars?

[rossdee]

herve villechaize was a star

Re:Are Brown Dwarfs Stars?

[ShanghaiBill]

Okay, so it must be helium fusion going on in dwarfs then?

In stars like our sun, the main process is the proton-proton chain. Deuterium is produced, but it is then almost immediately fused with an additional proton into He3, which then undergoes additional fusion to form He4.

Stars bigger than 1.3 solar masses use the Carbon-Nitrogen-Oxygen Cycle to fuse hydrogen into helium in a reaction catalyzed by carbon.

White dwarfs have little material left to fuse. They are mostly carbon and oxygen, but they are not big enough or hot enough to fuse the carbon or oxygen into heavier elements. So they slowly cool off and die.

 

Re:Are Brown Dwarfs Stars?

[Neil Boekend]

The decay from white dwarf to black dwarf is slow. In fact it's so incredibly slow that the universe isn't old enough to have them in it yet.
Estimates for the time it takes are 10^15 to 10^37 years, depending on factors like pronton decay and WIMP existence. The universe is "only" 13.8*10^9 years old.

This makes their indetectability practically moot.
Theoretically it's still fun to think about. The only ways to detect them with current physics would be occlusion (low detection chance) and gravitational influence (low detection range).

Re:Are Brown Dwarfs Stars?

[WCMI92]

White dwarf and neutron stars are the remnants of stars that had core fusion during the main sequence.

Brown Dwarf stars never had core hydrogen fusion and thus are failed stars. The smallest thing you can truly call a star is a Red Dwarf because they at least have core hydrogen fusion.

Re:Are Brown Dwarfs Stars?

[WCMI92]

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.

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)

Re:Are Brown Dwarfs Stars?

[tnk1]

Jupiter is producing more heat due to convection bringing core heat to the surface, it's not doing fusion. Like on Earth, core heat is due to the extreme pressures and radioactive materials heating the core to high temperatures. None of that pressure is enough to fuse nuclei together, however.

Brown dwarfs will actually do fusion, but only a little of it, and only for a relatively short time since they only have enough mass to fuse heavy hydrogen nuclei, which is the easiest thing that you can fuse. The problem is that there isn't much of the heavy versions of hydrogen, and so you're not going to burn for very long, nor will you burn brightly.

 

Intriguing headline..

[devon_halley]

Surely the Sun is the closest star to Earth, right?

Re:Intriguing headline..

[rubycodez]

No, Linsey Lohan is, so drugged up she's rarely vertical

Re:Intriguing headline..

[rudy_wayne]

As long as Uranus doesn't get any closer to me, I'm OK with it.

Place your bets...

[billybob2001]

Proxima Centauri Might Not Be the Closest Star To Earth

Put another way, Sol Might Be the Closest Star To Earth

...or perhaps Jupiter is?

Re:Place your bets...

[erice]

Proxima Centauri Might Not Be the Closest Star To Earth

Put another way, Sol Might Be the Closest Star To Earth

...or perhaps Jupiter is?

Jupiter is not a brown dwarf. It is not massive enough to even burn deuterium. No fusion == no star

Re:Place your bets...

[by (1706743)]

...or perhaps Jupiter is?

Nope -- regardless of star status, Jupiter is farther away from the Earth than Sol.

Re:Place your bets...

[Anonymous Coward]

Ok, deuterium, fine - no need to get heavy.

Them are no stars...

[140Mandak262Jamuna]

Come on brown dwarfs are not really stars. It is a con job to call them stars. It is like these New Jersey cruise sales boilerrooms selling "Masala Cruise with Bollywood stars" and then on board you see one guy who played the corpses in a murder mystery starring the Amir Khan and another who was the fourth thug beaten by Rajnikant enthiran the robot.

Re:Them are no stars...

[NoKaOi]

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?

Re:Them are no stars...

[techno-vampire]

Sheesh, it's just semantics.

I don't think that word means what you think it does.

childish language

[caviare]

Wow, did you know that a binary system consists of literally two stars! And a pair of stars is like two stars also, awesome! So a binary pair must be like two doubly awesome stars. Party on dudes!

Semantics

[e065c8515d206cb0e190]

If it's not the closest, are we going to rename it?

Re:Semantics

[starless]

If it's not the closest, are we going to rename it?

Proximish Centauri??

Re:Semantics

[Roger W Moore]

If it's not the closest, are we going to rename it?

No, because it will presumably still be the nearest in the constellation of Centaurus. It's 'Proxima Centauri' not 'Proxima'

Re:Dark Matter?

[Anonymous Coward]

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...