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Are brown dwarfs which don't sustain any fusion considered stars?

Are brown dwarfs which don't sustain any fusion considered stars?



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I was reading about the coolest stars, and was surprised to find about stars with a surface temperature lower than a candle, like 2MASS 0939-2448 A/B, of about 100 Celsius like CFBDSIR 1458+10B and even below 0 Celsius temperature where the water freezes like WISE J0855-0714. Then I thought, they might be stars where fusion only happens very deep in the core, and then I found out this:

Brown dwarf

In addition, many brown dwarfs undergo no fusion; those at the low end of the mass range (under 13 MJ) are never hot enough to fuse even deuterium, and even those at the high end of the mass range (over 60 MJ) cool quickly enough that after 10 million years they no longer undergo fusion.

Are these brown dwarfs which don't sustain any type of fusion considered stars? If so, what's the reason?


Brown dwarfs are technically stars, but not in your typical sense, as they cannot start or sustain nuclear fusion. Think about it this way: stars are formed in clumps inside giant gas clouds called nebulae, along with planets, but what if one of these clumps is too big to become a planet and too small to become red dwarfs. These are brown dwarfs. Some consider them to be stars, others consider them to be planets, and some neither. So, opinions vary.


Despite the name, brown dwarfs are not very brown. These objects, with masses ranging from 12 times that of Jupiter up to half the mass of the sun, emit light on their own … just usually not very much. The largest and youngest ones are quite hot, giving off a steady glow of warm light. From a distance, those stars would look indistinguishable from their stellar cousins, the red dwarfs. The smallest and oldest ones, by contrast, are barely visible, emitting radiation firmly in the infrared part of the spectrum. You wouldn't even be able to pick them out without the help of night-vision goggles.

For the most part, though, brown dwarfs sit somewhere in the middle and glow mildly, with dim magenta hues. This makes them rather unique in the galactic cast of characters.

But decidedly unlike the stars, brown dwarfs don't glow from the heat of nuclear fires raging in their hearts. Instead, their light and heat are simply leftovers from their initial formation. The objects were birthed from collapsing clouds of gas and dust (just like the stars, only less of it), and that gravitational collapse released a tremendous amount of energy. But the energy got trapped in the infalling material, locked inside for tens of millions of years, though the heat slowly radiates away into space in the form of lukewarm light.

As this heat escapes, the brown dwarf continues to dim, sliding from fiery red to mottled magenta to invisible infrared. The greater the mass at the object's birth, the more heat it can trap and the longer it can mimic a proper star. But the ultimate fate is the same for every brown dwarf, regardless of its pedigree.


When is a star not a star?

The line that separates stars from brown dwarfs may soon be clearer thanks to new work led by Carnegie's Serge Dieterich. Published by the Astrophysical Journal, his team's findings demonstrate that brown dwarfs can be more massive than astronomers previously thought.

To shine bright, stars need the energy derived from the fusion of hydrogen atoms deep in their interiors. If too small, hydrogen fusion can't occur, so the object cools, darkens, and turns into something called a brown dwarf.

Many researchers are trying to determine the mass, temperature, and brightness of objects on both sides of this divide.

"Understanding the boundary that separates stars from brown dwarfs will improve our understanding of how both form and evolve, as well as whether or not they could possibly host habitable planets," Dieterich explained.

Dieterich and colleagues -- including Carnegie's Alycia Weinberger, Alan Boss, Jonathan Gagné, Tri Astraatmadja, and Maggie Thompson -- demonstrated that brown dwarfs can be more massive than astronomers thought.

The latest theoretical models predict that the boundary separating stars from brown dwarfs occurs in objects that are between 70 to 73 times the mass of Jupiter, or about 7 percent the mass of our Sun, but the results from Dieterich and team question this prediction.

Dieterich's team observed two brown dwarfs, called Epsilon Indi B and Epsilon Indi C, that are part of a system that also includes a star of medium luminosity -- Epsilon Indi A. The two brown dwarfs are much too faint to be stars, but their masses are respectively 75 and 70 times that of Jupiter, according to the researchers' findings.

The team accomplished these measurements using data from two long-term studies -- the Carnegie Astrometric Planet Search at the Carnegie Las Campanas Observatory and the Cerro Tololo Inter-American Observatory Parallax Investigation run by the Research Consortium of Nearby Stars -- which allowed them to detect the minute motions of the two brown dwarfs against the background of more-distant stars.

To the team's surprise, their findings put Episilon Indi B and C in what was previously considered the stellar realm, even though we know from other observations that they are not stars.

"Taken together, our results mean that the existing models need to be revised," Dieterich concluded. "We showed that the heaviest brown dwarfs and the lightest stars may only have slight differences in mass. But despite this, they are destined for different lives -- one racing to dim and cool, the other shining for billions of years."

An improved definition of the dividing line between stars and brown dwarfs could also help astronomers determine how many of each exist in our own galaxy, added Weinberger.

"We are interested in whether stars and brown dwarfs always exist in the same proportion to each other in star-forming regions, which could help us understand the overall habitability of our galaxy," she said.

This work was supported by the National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship Program.


Contents

Brown dwarfs, a term coined by Jill Tarter in 1975, were originally called black dwarfs, a classification for dark substellar objects floating freely in space which were too low in mass to sustain stable hydrogen fusion (the term black dwarf currently refers to a white dwarf that has cooled down so that it no longer emits heat or light).

Early theories concerning the nature of the lowest mass stars and the hydrogen burning limit suggested that objects with a mass less than 0.07 solar masses for Population I objects or objects with a mass less than 0.09 solar masses for Population II objects would never go through normal stellar evolution and would become a completely degenerate star (Kumar 1963). The role of deuterium-burning down to 0.012 solar masses and the impact of dust formation in the cool outer atmospheres of brown dwarfs was understood by the late eighties. They would however be hard to find in the sky, as they would emit almost no light. Their strongest emissions would be in the infrared (IR) spectrum, and ground-based IR detectors were too imprecise for a few decades after that to firmly identify any brown dwarfs.

Since those earlier times, numerous searches involving various methods have been conducted to find these objects. Some of those methods included multi-color imaging surveys around field stars, imaging surveys for faint companions to main sequence dwarfs and white dwarfs, surveys of young star clusters and radial velocity monitoring for close companions.

For many years, efforts to discover brown dwarfs were frustrating and searches to find them seemed fruitless. In 1988, however, University of California at Los Angeles professors Eric Becklin and Ben Zuckerman identified a faint companion to GD 165 in an infrared search of white dwarfs. The spectrum of GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf star. It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs known at that time. GD 165B remained unique for almost a decade until the advent of the Two Micron All Sky Survey (2MASS) when Davy Kirkpatrick, out of the California Institute of Technology, and others discovered many objects with similar colors and spectral features.

Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs". While the discovery of the coolest dwarf was highly significant at the time it was debated whether GD 165B would be classified as a brown dwarf or simply a very low mass star since observational it is very difficult to distinguish between the two.

Interestingly, soon after the discovery of GD 165B other brown dwarf candidates were reported. Most failed to live up to their candidacy however, and with further checks for substellar nature, such as the lithium test, many turned out to be stellar objects and not true brown dwarfs. When young (up to a gigayear old) brown dwarfs can have temperatures and luminosities similar to some stars, so other distinguishing characteristics are necessary, such as the presence of lithium. Stars will burn lithium in a little over 100 Myr, at most, while most brown dwarfs will never acquire high enough core temperatures to do so. Thus, the detection of lithium in the atmosphere of a candidate object ensures its status as a brown dwarf.

In 1995 the study of brown dwarfs changed dramatically with the discovery of three incontrovertible substellar objects, some of which were identified by the presence of the 6708 Li line. The most notable of these objects was Gliese 229B which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in gas giant atmospheres and the atmosphere of Saturn's moon, Titan. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs known as "T dwarfs" for which Gl 229B is the prototype.

Since 1995, when the first brown dwarf was confirmed, hundreds have been identified. Brown dwarfs close to Earth include Epsilon Indi Ba and Bb, a pair of dwarfs around 12 light-years from Sun.


thank you. why is it called a star when it won't sustain fusion? 100 jupiter masses is what percent of sun's..

thank you. why is it called a star when it won't sustain fusion? 100 jupiter masses is what percent of sun's..

Brown dwarfs or pseudo stars of thirteen solar masses or more certainly can sustain deuterium fusion. They don't light up as the regular stars do though. But the fusion can go on in their cores nevertheless.

excerpt
A brown dwarf is a pseudostar a body of gas not massive enough for the gravitational pressure in its core to ignite the hydrogen-fusion reaction that powers true stars. The name "brown dwarf" is a play on the name of the smallest class of true stars, "red dwarf," but while red dwarfs are actually red, brown dwarfs are not brown, but purple or magenta. Objects ranging in mass between 13 and 75 times the mass of Jupiter—between 1.2% and 7% the mass of the Sun—are generally considered brown dwarfs. Clear rules for distinguishing large planets from brown dwarfs, however, are lacking. Some astronomers consider objects down to seven or eight Jupiter masses to be brown dwarfs, while others reserve this term for objects heavy enough to initiate deuterium fusion in their cores, that is, objects of 13 Jupiter masses or more. (Deuterium is a relatively uncommon form of hydrogen that has both a neutron and a proton in its nucleus deuterium fusion is a minor reaction in true stars and persists for only a few million years even in brown dwarfs.) In 2001, an international committee declared that objects heavier than 13 Jupiter masses should be labeled brown dwarfs regardless of whether they orbit true stars, while objects below this threshold should be labeled as planets if they are orbiting true stars and as sub-brown dwarfs if they are not.


Brown Dwarfs: “Over-Achieving Jupiters” not “Failed Stars”

Why is the term “failed star” synonymous with brown dwarfs? On the one hand, brown dwarfs lack the mass to sustain nuclear fusion in their cores. On the other hand, who said brown dwarfs were trying to be stars? Who ever said that becoming a star was the pinnacle of stellar living? Perhaps brown dwarfs are perfectly happy the way they are. In a world of equality and political correctness, brown dwarfs could be viewed as “over-achieving Jupiters”, or gas supergiants

Brown dwarfs are often considered to be the bridge between planets and stars, they are too massive to be considered to be a planet (as they have convective interiors with no layered differentiation of chemicals with depth), and yet they are too small to be a star (they cannot fuse hydrogen in their cores, although some brown dwarf classes may fuse lithium and deuterium). That said, brown dwarfs do occupy the lower right-hand corner of the Hertzsprung-Russell diagram, so they are still classified in stellar terms. Although “brown dwarf star” is probably a little too generous.

Brown dwarfs are also technically not “brown”, they are a kind of dirty orange (with a hexadecimal colour of #EB4B25) as astronomers don’t recognize brown as a colour.

So “brown” dwarfs aren’t really brown and they are suffering an identity crisis between being a star and a planet. In fact, before brown dwarfs became brown dwarfs, there were suggestions to call these strange planetary/stellar bodies substars or planetars (can you sense the confusion?).

Compared with our Sun, brown dwarfs are pretty small (0.01-0.08 Solar masses) but compared with a gas giant such as Jupiter, they are huge (13-80 Jupiter masses). However, brown dwarfs don’t expand much larger than the radius of Jupiter (making it hard to distinguish between a brown dwarf and a gas giant exoplanet).

Therefore, why don’t we be a little more “glass half-full” when describing brown dwarfs. Although brown dwarfs undoubtedly have star-like qualities, they have strong planet-like qualities too. So in the traditional superlative descriptions of some astronomical objects (i.e. supermassive black hole), why not emphasise the brown dwarf’s strong planetary points. Rather than “brown dwarf”, what about “gas supergiant” and rather than “failed star”, why not “over-achieving Jupiter”?

Special thanks to Adam Zuckerman for the entertaining conversation we had when discussing the pros and cons of Are Brown Dwarfs More Common Than We Thought?


New Cutoff for Star Sizes

By: John Bochanski December 23, 2013 13

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Astronomers have found a gap between "real" and "failed" stars.

What does the smallest star look like? This question is deceptively difficult to answer. Stars spend most of their lives fusing hydrogen in their cores, a prime time of life called the “main sequence.” As you go down the scale of stellar sizes on this sequence, stars become dimmer, cooler, and less massive. But determining the absolute properties of the smallest stars — their mass, radius, temperature, and overall light output — is challenging for at least three big reasons.

This diagram shows the relation between size (compared with the Sun) and temperature (kelvin) for stars and brown dwarfs. As astronomers suspected, there's a clear gap between where stars end and brown dwarfs begin. Click here for zoom.

P. Marenfeld & NOAO / AURA / NSF

December 23, 2013 at 5:16 pm

Interesting. If the surface temperature of a contracting cloud of gas exceeds 2,100 K, it can be said, &ldquoWe have ignition.&rdquo

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December 26, 2013 at 8:51 am

And if it never reaches 2100 K it sputters for a while on impulse (Lithium) power.

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December 27, 2013 at 1:30 pm

Both the article and the above comments are cool.
I note that age is as big a factor on brown dwarf temperatures as mass (and much more so than radius). Thus there are brown dwarfs that fill the gap in the diagram and even crawl up the stellar sequence when they are young enough. The "lithium test" is needed to distinguish those from true stars. These authors are not looking at young regions (just very nearby) and so are not bothered by them. The result is valid and cool because it is the stars which have a stable and mostly age-independent radius-mass-age relation it is that which comes to a "cool" end at the bottom of the main sequence and has been measured in this paper.

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December 27, 2013 at 1:39 pm

(Doesn't that sound like a nice title for a Christmas story?) This is an important finding. It makes sense that as a star's mass increases its volume and radius would also increase, because there would be greater outward pressure from faster hydrogen fusion. A brown dwarf, without fusion, would have no force to keep it from collapsing under the increasing force of gravity. Do I have that right? Just for fun I looked for 2MASS J0513&ndash1403 on a star chart. It should be between Kappa and Mu Leporis. I wonder how bright it is, and how big of a telescope one would need to see it?

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December 28, 2013 at 7:57 pm

Not sure about that, Gibor. The chart is confusing because temperature is graphed from hot to cold, whereas star formation proceeds from cold to hot. If it is massive enough, nuclear fusion begins, and the temperature suddenly jumps. to at least 2,100 K. Brown dwarfs never get to that point, so they never get hotter than about 1,800 K, and when they age, they cool and move to the right in the diagram. There can be no dwarfs in the gap, so no test is needed.

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December 28, 2013 at 9:15 pm

This is Sergio Dieterich, lead author of this study. Gibor Basri is absolutely right (as he usually is!) on his comments about radius and youth. If you look at Figure 11b of the paper, there is actually what seems to be a higher radius sequence above the main sequence that is likely formed by the few young objects in the survey.
Peter, stellar formation actually evolves mostly from hotter to cooler temperatures. This is because what dominates the temperature budget at that phase is the release of gravitational energy during pre-main sequence contraction.
dieterich at astro dot gsu dot edu

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December 28, 2013 at 9:25 pm

Anthony Barreiro, note that there is a typo in the press release. 2MA 0513-1403 should be 2MA 0523-1403. It is only 10 minutes, so the general constellation position you got is still right. We used the 0.9m (36 inch) telescope at Cerro Tololo Inter-American observatory in Chile (pictures at http://www.recons.org) to observe it. It is an extremely faint 21.05 magnitude in V (green light), which required 20 minute exposures with a CCD. Even with the most powerful telescopes, I don't think you would be able to see it with an eye piece because you would not have the benefit of long integration times. If you would like a finder chart, I can send you one. Drop me a note at dieterich at astro dot gsu dot edu

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December 29, 2013 at 11:21 am

Thanks for answering these questions directly Sergio Dieterich, and for the work you and your fellow professionals are reporting in this paper. Finding the limits of natural phenomena is always important, and this work will no doubt help astronomers improve stellar modeling. I enjoyed reading your paper. But no doubt like Anthony, who wanted to see the littlest star for himself, I was a bit disappointed to read that 2MA 0523-1403 is so faint. I was hoping that this star would be within the range of the Gaia mission, so that its characteristics could be nailed down even tighter. Also, I was wondering, since in general the lighter the stellar class, the more examples exist, can we expect that this trend continues all the way down to this bottom y&rsquoall have discovered? Will there be many more stars like this one?

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December 29, 2013 at 5:09 pm

Bruce Mayfield,
Because the magnitude scale is logarithmic, sometimes we forget just how faint things can get. Increasing the magnitude by 1.0 is equivalent to making an object 2.5 times fainter, and every subsequent increase of 1 unit makes THE PREVIOUS VALUE 2.5 times fainter. It is generally accepted that under dark clear skies the human eye can just reach 6th magnitude. 2MA0523 is 15 magnitudes fainter than that. Doing the math, that means about ONE MILLION times fainter than what can be seen by the naked eye. You are right that GAIA will not reach these faint objects (the GAIA cutoff is roughly 20th magnitude), but there is really nothing that GAIA can do that cannot also be done from the ground for individual stars. GAIA wins in the fact that it can do billions of stars, and not in detailed studies of a single star. For instance, we were able to get a lot of color information necessary for determining temperature from the ground. That is something GAIA would not be able to do even if the target was brighter. Right now the consensus is that the trend of more stars with fainter luminosities peaks at slightly more massive stars, and that there is a rather sharp fall-off after that. So while we expect to find a few more nearby stars like 2ma0523, they are certainly not a lot of them. We know form the WISE survey that stars appear to outnumber brown dwarfs by about 6 to 1, and it could be that a similar ratio is also representative for very low mass stars. We don't really know the shape of the drop-off yet. Several groups, including ours, are working on it.


Are brown dwarfs more like stars or planets?

Although brown dwarfs have some similarities to both planets and stars they don’t quite fit either category. They’re believed to form through the same process as other stars, made when gas and dust coalesce, but without the mass to sustain prolonged hydrogen fusion at the core.

After formation, these objects are too small to be considered a star (we refer to them in terms of Jupiter masses, not solar masses) and have a surface temperature less than 2,500 degrees Celsius (4,532 degrees Fahrenheit). They fit somewhere between our current definitions of stars and planets suggesting that perhaps these two classes are not as clear cut as we first thought.

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