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Researchers Discover a Star's Minimum Possible Mass 112

paulmac84 writes "Stars that don't have enough mass never shine, dying billions of years before their bigger counterparts. But astronomers have never been able to measure the exact mass limit, because the lightest stars that do shine can be simply too faint to detect. Now, new images show for the first time how big a star must be to avoid impending doom. The long-awaited new images finally lay this question to rest, say the authors. The dimmest stars were measured as being 8.3% of the Sun's mass. All protostars that are smaller than this are headed for life as a brown dwarf."
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Researchers Discover a Star's Minimum Possible Mass

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  • by Aidski ( 875851 ) on Thursday August 17, 2006 @06:45PM (#15931253)
    ...Unless newer technology finds dimmer stars, and they have to lower the minimum again.
    • Re: (Score:2, Insightful)

      I have a strange suspicion that after I go RTFA, I'll be able to come back and say RTFA!
    • by gardyloo ( 512791 ) on Thursday August 17, 2006 @07:20PM (#15931414)
      Cute. Though people could just go ahead and read the article. To wit:
      Although the telescope would have been able to detect fainter stars, none could be found- so it appears that they simply don't exist. "We checked the instruments over and over again" said Professor Richer "but we don't see any stars fainter than this".
      • by denim ( 225087 ) *
        Of course they don't see stars fainter than the minimum they can see. That seems kind of circular.

        And when they say "shine", what do they mean? In what spectrum? To what brightness? Another way to ask this is, what makes a brown dwarf "brown"?
        • by jZnat ( 793348 ) *
          No, what they're saying is that their telescope could detect fainter stars, but it didn't find any.
          • by denim ( 225087 ) *
            Which only proves that it didn't find any, not that such don't exist. Radiation from such could, I expect, be blocked by various other objects or phenomenon. And would probably be hard to see in the first place.
            • by qeveren ( 318805 )
              True, the old 'absence of proof is not proof of absence' thing almost always applies in science. But as stars go, the lower the mass, the more common they are. It's very unlikely that they'd not see at least one lower-than-this-limit star with their instruments if they were out there; there'd be enough of them that they couldn't all coincidentally be hidden.
              • by denim ( 225087 ) *
                Agreed, which says to me that they only believe they'd be able to detect such objects, not that they actually can. That's my theory. IIRC, Jupiter puts out more energy than it gets from the Sun. Is it not smaller than their lower limit? What's the difference between an object like Jupiter and a brown dwarf or star? As I said earlier, what makes a brown dwarf "brown"? What spectrum range defines a star?

                What I'm saying is that if it puts out energy, why is it not a star? Counter proposal: is a black ho
                • by denim ( 225087 ) *
                  BTW, can we now consider our Earth/moon system to be a double planet? The moon is a gravity-rounded rock, so by the new definition, it's a planet. What shall we call it now? That follows for all such moons in our system, such as those around Jupiter, Saturn, Neptune, and Uranus. Whew, we've got a lot of "planets" now! They really killed themselves once they declared Charon as a planet, IMHO. Opens up the field too far.
                  • I don't think so, because the barycentre of the Earth-Moon system is inside the Earth (as opposed to, in the case of Pluto and Charon, somewhere in space between them).
                • by Squiffy ( 242681 )
                  So you're arguing science without understanding it? Shame on you.
                  • by denim ( 225087 ) *
                    What am I saying I don't understand?
                    • by Squiffy ( 242681 )

                      What's the difference between an object like Jupiter and a brown dwarf or star? As I said earlier, what makes a brown dwarf "brown"? What spectrum range defines a star? What I'm saying is that if it puts out energy, why is it not a star? Counter proposal: is a black hole putting out energy? Is it a subset of "star"? What sort of energy need a "star" emit? How much of this energy? How often? Is a faded old white dwarf still a star? Define your terms, sirs.

                      These are things you don't understand, by your own

                • IIRC, Jupiter puts out more energy than it gets from the Sun.

                  This is possibly true, although the difference in energy being received and the observed temperatures are within a reasonable tolerance (~ 10%?); whether this is because Jupiter has some extra energy due to radioactive decay, or continued gravitational compression, or whether it's just a measurement error is not clear.

                  Is it not smaller than their lower limit?

                  Yes. It's not expected that Jupiter is fusing any significant amount of deuterium or n

                  • by denim ( 225087 ) *
                    Good answer.

                    So the definition of a star is an object which emits radiation due to fusion, or which did at one time?
                    • Yeah, that's about it.

                      If the object isn't heavy enough to undergo fusion by itself, it's not considered to be a true star...but objects which were heavy enough to be a star, and then went into the red supergiant phase or go nova, the left-over stellar core (a "white dwarf" or "neutron star", respectively) will still be considered a star until they cool below 1000K or so.
        • by qurk ( 87195 )
          I didn't mean to treat you bad Didn't know just what I had But, honey, now I do And don't it make my brown eyes Don't it make my brown eyes Don't it make my brown eyes blue
        • what makes a brown dwarf "brown"?
          Two possible reasons:
          1. It's scared shitless of larger stars.
          2. Suntan lotion.
          Also, the politically-correct term for them is "dwarves of color", er, "short stars of color", uh, "stars of color of diminutive ...", ah, ah, "vertically challenged stars of color".
          Yeah, that's it, "vertically challenged stars of color".
          • by denim ( 225087 ) *
            Can "dwarves of color" be tossed?
          • by skam240 ( 789197 )
            Everyone will have to forgive me because this is completely off topic but this is really annoying the piss out of me.

            Your signature The "USAPATRIOT" Act has nothing to do with patriotism, so I pronounce it "the you sap at riot act" to avoid confusion. literally makes absolutely no sense and is annoying. While I am certainly not a fan of the PATRIOT act suggesting that its acronym means or should read something completely unintelligible is neither funny or interesting.

            • Your signature The "USAPATRIOT" Act has nothing to do with patriotism, so I pronounce it "the you sap at riot act" to avoid confusion. literally makes absolutely no sense and is annoying.

              Just like the USAPATRIOT Act, which makes no sense (for its purported purpose -- stopping terrorism), and is annoying to people who value their freedom.
              The problem is that pronouncing it "the patriot act" or "the you ess eh (Canadian pronunciation of "eh") patriot act" lends it an air of legitimacy, like it's actually a

    • Re: (Score:1, Redundant)

      by helioquake ( 841463 )
      ...Unless newer technology finds dimmer stars, and they have to lower the minimum again.

      There may be some truth in this comment. Could it be 8.2% instead?

      Whenever you read an article like this, we should pay attention to the error bar. Is it 8.3% +/- 1, 5, or 50%?
    • by Lave ( 958216 ) * on Thursday August 17, 2006 @07:26PM (#15931450)
      Sorry to be pedantitc but FTFA:

      Although the telescope would have been able to detect fainter stars, none could be found- so it appears that they simply don't exist. "We checked the instruments over and over again" said Professor Richer "but we don't see any stars fainter than this".

      So they could have detected much dimmer stars but didn't - so assuming a big enough sample, they discovered the minimum mass to initiate fusion. Pretty impressive.

      So finally a Definitive Answer! Until someone bothers to look at a larger sample set,, finds dimmer stars, and they have to lower the minimum again.

      • by 4D6963 ( 933028 )

        So finally a Definitive Answer! Until someone bothers to look at a larger sample set,, finds dimmer stars, and they have to lower the minimum again.

        I'd rather say, finally a definitive answer, period. The reason why stars cannot be smaller than that limit is theorical, and the observation allows us to find that limit. A larger sample set will not lower the minimum, ever. It will only reduce the margin of error. The dimmest star you could ever find is comprised in the margin of error we got.

    • by cswiger2005 ( 905744 ) <cswiger@mac.com> on Thursday August 17, 2006 @07:26PM (#15931451) Homepage
      If there were dimmer stars present there, the Hubble's main camera would have been sensitive enough to have seen them...they're pretty sure of this because they were able to notice some very dim white dwarfs (a white dwarf is the remenant stellar core of a bigger star which went nova; they are very hot [initally] but also very tiny), which are dimmer than the smallest M-class stars still in the main sequence.

      Basicly, this observation is in reasonably close accordance with the theories about stellar fusion; basicly, an potential star needs to have about ten or fifteen times Jupiter's mass before deuterium fusion is possible, and about 70 times Jupiter's mass before normal hydrogen fusion happens (according to the models).

      Jupiter weighs 1.899 * 10^29kg; Sol weighs 1.989 * 10^32 kg (or about 1050 times what Jupiter weighs).
      8.4% of Sol's mass is 1.65 * 10^30, or 87 times what Jupiter weighs.
      • Thanks for the numbers, that was exactly what I was asking myself, how far is Jupiter weight off from the limit. Now, what's the difference between a brown dwarf and a massive Planet? Just the way the became into existence or is every massive object close to a star considered as a planet and if it is by its own a brown dwarf?
        • A planet is something which does not have enough mass to sustain a fusion reaction.

          Unless a star is nearby, planets are effectively invisible at stellar distances since they radiate no light of their own. Even with a star nearby, it's easier to notice the gravitational wobble of the planet shifting the star's orbit and causing doppler changes to the star's spectrum than to observe the planet directly via reflected light.

          A "brown dwarf", sometimes referred to as a T-class star is something that emits enough
      • by DeanAsh ( 531960 )
        Sol's mass is 1.98892 × 10^30 kilograms, not 10^32 (courtesy of Google). Likewise, Jupiter masses 1.8987 × 10^27 kilograms (not 10^29)
  • Um... yay? (Score:4, Interesting)

    by Capt'n Hector ( 650760 ) on Thursday August 17, 2006 @06:46PM (#15931256)
    This is a simple math/physics problem. I'm not quite sure what the grand point of it is though (kinda like the pluto(!)=planet debate). Maybe you can graph the distribution of star masses, and then see how much "dark matter" there is on the tail end of brown dwarfs.
    • Re:Um... yay? (Score:5, Insightful)

      by ZorbaTHut ( 126196 ) on Thursday August 17, 2006 @07:52PM (#15931600) Homepage
      Many years ago people believed that heavy things fell faster than light things. They didn't bother testing this theory because they knew it to be true. Then, one day, someone tested that theory and found it was false.

      Perhaps it is a simple problem to answer mathematically. And now we've tested it. We have actual data. Does the data match up with the mathematical answer? Maybe, maybe not, I don't pretend to know. But I imagine people out there do - so either we've got another point of verification that our models are good, or it's time to figure out what's wrong with them.

      Either way, this is what's called Science.
      • by rthille ( 8526 )
        Now that's crazy talk. Everyone knows that the most important knowledge is passed down from generation to generation in a verbal tradition or a book that's (mis)copied by scribes. There's no need for 'fact checking'. Jeeze, kids these days...
      • by G-funk ( 22712 )
        I've never seen any mathematical proof that heavy things don't fall faster than light things. F=Gm/d^2.
        • But to work out speed from that you then have to consider F = ma, so when you work out acceleration the mass cancels out (i.e. acceleration is independent of mass). Since s = ut + 0.5a(t^2), this gives a speed also independent of mass.
          Note that there's a lot of other factors such as air resistance which are important too, though.
      • by ars ( 79600 )
        "Many years ago people believed that heavy things fell faster than light things. They didn't bother testing this theory because they knew it to be true. Then, one day, someone tested that theory and found it was false."

        Bit of a problem in your argument there - heavy thing DO fall faster then light things!

        It's just that on earth, the earth is so much larger then the falling item, that if the faller is a little bigger or a little smaller it's not much noticable.

        OTOH if you get some nice large objects - say th
        • Bit of a problem in your argument there - heavy thing DO fall faster then light things!

          Actually, if you go to a decent science museum, they should have an exhibit where they show something like a feather and lead shot being dropped in a vaccuum...really and truely, they fall at exactly the same speed and hit the ground at the same time.

          If you plug in F=M1a and F = gM1M2/r^2, you discover that the gravitational attaction of a heavier object to the earth exactly counterbalances the weight of the heavier obje

          • by ars ( 79600 )
            "Actually, if you go to a decent science museum, they should have an exhibit where they show something like a feather and lead shot being dropped in a vaccuum...really and truely, they fall at exactly the same speed and hit the ground at the same time."

            No, not "really and truely", only "approximately". I guess you missed where I wrote the difference on earth is not noticable. But just because you can't see it doesn't mean it's not there.

            You have an error in your equation: you forgot about the acceleration o
            • You have an error in your equation: you forgot about the acceleration of the EARTH toward the test object!

              No, actually, I haven't.

              Notice that both objects are being released at the same time; the motion of the Earth towards them is not different for one object versus the other, presuming it was significant at all (which it is not; the Earth weighs about 10^26 times what a 5kg lead pellet weighs). When you drop two objects from the same height, at the same time, in a vaccuum, they will hit the ground at ex

              • by ars ( 79600 )
                "Notice that both objects are being released at the same time; the motion of the Earth towards them is not different for one object versus the other"

                You seem to have forgotten we are talking about how fast objects fall, not comparing them. Go back and read the post that started this.

                "presuming it was significant at all (which it is not; the Earth weighs about 10^26 times what a 5kg lead pellet weighs)."

                WOW! you don't say, I have never heard of this before, the earth is heavy? The fact I said this at least 3
                • You seem to have forgotten we are talking about how fast objects fall, not comparing them.

                  How do you measure velocity, eh? Measure the distance, divide by the elapsed time?

                  Well, in the specific example I gave, and in the historic example involving a leaning tower in Italy, the distance and the time are exactly the same for the two objects...so the the velocity ("how fast"), is also the same.

                  Go back and read the post that started this.

                  The post by ZorbaTHut, hmm? Modded +5 because it had a good point, wh

          • by Teancum ( 67324 )
            While I think this whole thread is utter BS, I would point out that when you drop a much more dense object (generally considered "heavy") compared to a substantially less dense object like a feather in an atmosphere, the heavier and more dense object usually drops much faster. Visibly so.

            Or the thing to point out is that atmospheric drag is a big deal. As for the miniscule amount that a slightly more massive object would have on gravitaional attraction, it isn't worth worrying about. And if you do the ma
            • I would point out that when you drop a much more dense object (generally considered "heavy") compared to a substantially less dense object like a feather in an atmosphere, the heavier and more dense object usually drops much faster. Visibly so.

              Of course-- that is so intuitively true that nobody [1] questioned it for the 2000-odd years between Aristotle & Gallileo.

              On the other hand, you can take two pieces of paper, which weigh the same, and have the same density, and crumple one into a tight ball, and

              • by Teancum ( 67324 )
                OK, think more like a black hole with Jupiter's mass. The point I'm trying to make is that the mass of the object is immaterial.

                This BTW is a serious problem with other scientific investigations, where some "theories" are offered, but realisticly eliminating extra factors that influence the results is often difficult or impossible to do. The whole issue of global warming, for instance, has so many variables that the one often used conclusion, that human-caused pollution is directly causing global warming
    • Re:Um... yay? (Score:4, Interesting)

      by Artifakt ( 700173 ) on Thursday August 17, 2006 @07:58PM (#15931633)
      It's not quite simple. It's admittedly very simple in the abstract, for a model star where you're only looking at what combinations of temperature and core density allow standard stellar fusion at a break even rate (All normal stars run at break even, in the short enough run, in the sense that the total energy produced is equal to the light pressure keeping interior layers from collapsing, plus the light emmitted to space). Physicists such as Hoyle and Gamow pretty much wrote the math for this at least forty years ago, and much of this was known well before the US designed the "Super" in 1949-50, where it turned out to be applicable (although some of it was so classified then that even the best professional Astrophysicists couldn't assume they had seen nearly all the relevant literature).

              Here's just some of what makes it more complex for the real world though, and I'm probably missing plenty of other complexifying factors:
              Spinning Star? What range of rotation rates occurs in low-mass stars, How much pressure does it relieve at the core at a minimum? (Is there any real occurance of a low mass star with absolutely no rotation?)
              Which Population (I or II). Low mass stars can be very old, as they burn their fuel so slowly. This affects how much of the heavier elements are found in their cores. Just where newer generation stars formed makes a big difference in how much of what heavier elements are in them, but there's not much of a difference theoretically possible for the first generation. Are their faint stars can we observe, but not get enough of a spectrum on to be confident of their composition?
                Are there any convection currents in low mass stars? Do such, as yet unproved, currents include the full range of modalities found in a star the size of our Sun, or fewer? (or maybe even something truly novel, completely different than in bigger stars?). We're not even real sure how typical current patterns within our Sun are for stars of its general type, last I looked.
                Can having a large, close companion star significantly reduce the minimum mass threshold, or would any such received radiation effects be trivial?

           
      • Re: (Score:3, Informative)

        by helioquake ( 841463 )
        I'd add chemical composition (metallicity, namely), too, to your list.
      • Spinning Star? What range of rotation rates occurs in low-mass stars, How much pressure does it relieve at the core at a minimum? (Is there any real occurance of a low mass star with absolutely no rotation?)

        The rotation of a star would presumably be a result of its original collapse. Conservation of angular momentum. Hence the really enormously fast rotation of extremely collapsed objects like pulsars.

        A low-mass star, then, would probably spin relatively slowly. Less mass means less angular momentum. Si

      • It's not quite simple. It's admittedly very simple in the abstract, for a model star...

        I bought a model star once. Probably a Revell kit. It was quite simple. Too simple. Two halves of a ball, with a page of assmbly instructions, three pages of instructions on the proper use of model glue, two pages of instructions on the proper application of model paint, and seven pages of disclaimers. All in 8 languages.

        I filled it with hydrogen and detonated it. Made a really nice star for a few milliseconds...
    • The Planets-vs-Plutons argument is really about taxonomy - how to label things, and how people feel about them. The reason that the precise definition of a planet's size matters is that if you set the bar too high, then Pluto is no longer called a planet, and everybody who grew up learning that we had 9 planets gets told we only have 8 and gets really grumpy, but if you set the bar too low, not only does Pluto get bumped to being Planet 10 (because Ceres got promoted), but there's about 50 other things whi
  • 8.3%? (Score:2, Funny)

    by Anonymous Coward
    Apparently, size does matter
  • by Anonymous Coward on Thursday August 17, 2006 @07:03PM (#15931339)
    ... that's 87 Jupiters [google.com].
    • +1. That's the first thing that came to my mind. So I guess the events of 2010 can't happen then. Punk ass Jupiter, thinks it's tough.
      • Re: (Score:3, Interesting)

        by Anonymous Coward
        I believe that 2010 should still be feasable. It's been a few years, but as I recall it the monoliths descended into Jupiter and used some exotic forces to compact it down to a scale where it was finally dense enough to ignite fusion. This article only speaks to how massive something must be for gravity to compact it that far; theoretically all you really need for a self-sustaining reaction is the proper density and pressure, however those might be achieved.
        • Yeah, but its not a one-time thing, you have to maintain it. So those mystical monoliths would have to hang out and continue exerting their magical force to make it work.
          • by Blnky ( 35330 )
            I thought about that too and realized it is still possible. The monoliths can maintain the reaction. After all, for each monolith, "...it's full of stars!" :P

          • by ceoyoyo ( 59147 )
            No you don't. If you took some portion of Jupiter's mass and turned it into neutron star stuff then fusion could occur in the rest of the regular gas. The trick is to get the pressure up, it doesn't matter how. You can do it with a certain mass of regular matter or you can use a bit of stable exotic matter with the regular matter packed around it.
            • by qeveren ( 318805 )
              First, neutron star material is NOT stable outside of the conditions of a neutron star. You can't 'take a small piece of it'... that piece would just explode, excessively violently, if it were unconstrained. Second, fusion at the surface of neutron degenerate matter takes place at an enormous rate due to gravitational compression (we're talking an escape velocity of half the speed of light at the surface). Jupiter'd likely just get disrupted, not continue to burn, star-like.
              • by ceoyoyo ( 59147 )
                True. I don't know what the minimum sustainable mass for a neutron star is, but remember, there won't be a few pounds of it floating around in space, it'll be in the core of Jupiter and there would be a decent ball of it.

                In the book I believe Clarke just said "heavier elements." The monoliths were sucking up Jupiter's high clouds and turning the material into heavy elements, which would have the same effect, you'd just need more of it.

                Whether it would work for the millions of years mentioned in the book,
      • by JDevers ( 83155 )
        Well, you have to remember that the obelisks are what caused that to happen. There is nothing explaining that they aren't 95% of the mass of the new sun and jupiter just provides the hydrogen (which would probably last plenty long enough to do what was needed on Europa).

        Plus, it was just a book =)
    • by Null Nihils ( 965047 ) on Thursday August 17, 2006 @07:33PM (#15931484) Journal
      How much is that in Libraries of Congress?
    • Mentioning that, can anyone explain to me whats the difference between the large gaseus planets like jupiter and small stars. Is there really any difference in their compesition or is it simply a matter of size?
      • Re: (Score:3, Informative)

        Mostly size, or rather, mass. If you dumped enough hydrogen into Jupiter, it would shine. You just need enough gravitational pressure at the core to sustain hydrogen fusion, which is what the article is discussing.

        That said, relative composition IS different; Jupiter has a dense rocky core of heavier elements (the same sort of stuff the inner planets are made of), surrounded by metallic, liquid, and gaseous hydrogen and some helium. The sun is almost completely made of hydrogen and helium, with a reside
    • by shemnon ( 77367 )
      Try as I might, I cannot get google to tell me how many black monoliths are needed to make Jupiter 87 times more massive than it is...
      • I seem to remember that the monoliths weren't making it bigger*, but denser. The idea being that Jupiter isn't a star because it doesn't have the neccesary internal pressure to initiate fusion. Instead of making it big enough that its own gravity would bring the pressure to the needed level, they just squished it. (Of course, the question then is what happens when they stop squishing it?)

        * That wouldn't be a great idea, since they were trying to help Europa's life. I bet 87 times bigger would be big enough
  • by Biff Stu ( 654099 ) on Thursday August 17, 2006 @07:29PM (#15931472)
    Didn't Karen Carpenter set the standard for the minimum mass of a star?

  • So what? (Score:3, Funny)

    by FlyByPC ( 841016 ) on Thursday August 17, 2006 @07:42PM (#15931532) Homepage
    In Hollywood, the minimum mass of stars has been on the decline for decades now...
  • by JanneM ( 7445 ) on Thursday August 17, 2006 @07:50PM (#15931592) Homepage
    Brown Dwarf? That's "colored star of alternaive height" to you, mister!
  • Researchers Discover a Star's Minimum Possible Mass
    You want the minimum mass for a star? Just weigh Calista Flockhart.
  • by tverbeek ( 457094 ) on Thursday August 17, 2006 @09:39PM (#15932032) Homepage
    So apparently Orson Welles - even at his heaviest - was still too "lightweight" to be a real star.

    Ironic.
  • by damburger ( 981828 ) on Friday August 18, 2006 @06:51AM (#15933552)
    8% of the Suns mass is still about 100 times the mass of Jupiter. So all that crap about turning Jupiter into a star in "2010" was a load of bollocks. Like, well, pretty much everything in that shite film.
    • by cr0sh ( 43134 )
      If I remember the movie correctly (I don't remember how closely it follows the book), didn't a bunch (like a large cloud encircling the planer) of monoliths descend into Jupiter's atmosphere before it turned into a star? If so, the mass to cause that to occur came from the monoliths. Since we don't know what the mass of a monolith is, it could be quite large...

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