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B First Direct Measurement of Brown Dwarf Mass Limit: 6.7% Sun

  1. May 15, 2017 #1
    by Ken Croswell

    New observations indicate that objects born with a mass just 6.7 per cent that of the Sun can shine for trillions of years rather than fizzle out as failed stars known as brown dwarfs.

    Link: New Scientist
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  3. May 15, 2017 #2
    Why call them "failed star"? They succeeded in avoiding the hot fusion disaster. They are successfully storing hydrogen for future galactic development instead wasting it.
  4. May 16, 2017 #3


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    What do you mean by this? Storing implies retrieval. How's that ever going to happen?
  5. May 16, 2017 #4


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    They could collide with another star, but that is a very unlikely process.

    What is a "hot fusion disaster"?
  6. May 16, 2017 #5
    Is this not something we already knew? Brown dwarfs are objects that fuse deuterium and/or lithium and range anywhere from 13 to 80 Jupiter masses. This 6.7% of the Sun's mass is just another way of saying 70.2 Jupiter masses. Which is slightly less massive than originally estimated. If the object is massive enough to begin hydrogen fusion then it has become a star. Brown dwarfs are not "failed stars" any more than planets are "failed stars." They simply lack the mass to begin the hydrogen fusion process. An object with just over 70 Jupiter masses that is fusing hydrogen is also known as a Red Dwarf star, and they do indeed fuse hydrogen at their core for well over a trillion years. Brown Dwarfs, on the other hand, will fuse all of their deuterium and/or lithium in less than a billion years. But we knew this as well. I'm struggling to discover any new revelations in this article.
    Last edited: May 16, 2017
  7. May 16, 2017 #6
    I take it the exact boundary was not something we already knew.
  8. May 16, 2017 #7


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    And now with a more precise estimate. That is the point.
  9. May 16, 2017 #8
    That is true. It is only estimated that somewhere between 13 and 14 Jupiter masses deuterium begins to fuse, and Brown Dwarf is only able to fuse lithium once it became greater than 65 Jupiter masses. The upper end for a Brown dwarf was estimated to be between 75 and 80 Jupiter masses before hydrogen fusion began. According to the article, these new observations suggest hydrogen fusion can began with only 70.2 Jupiter masses (6.7% of the sun's mass).

    The mass of the object is really immaterial. It is the actual processes that is occurring that makes the difference. Regardless of its mass, if an object is not fusing hydrogen, the object cannot be a star. Regardless of its mass, if an object is not fusing deuterium then the object cannot be a Brown dwarf. The mass range is just our best estimate as to the ranges of these various processes.
  10. May 16, 2017 #9
    I do not need to know. If no one can retrieve the hydrogen then the brown dwarf succeed in avoiding retrieval too. I just wanted to know why it is labeled "failed star". Why is proton fusion considered a success?
  11. May 16, 2017 #10


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    It's just a figure of speech used to describe the fact that they are just under the mass limit necessary to be stars.
  12. May 16, 2017 #11
    What is the central density of a 70 Jupiter mass brown dwarf?
  13. May 16, 2017 #12
    will depend on elements it is made of and the temperature.
  14. May 17, 2017 #13
    Is it a property of brown dwarfs, shared with planets and white dwarfs, that their interior is degenerate and its density depends weakly on temperature?
  15. May 17, 2017 #14
    The new work is noteworthy for two reasons:

    1. It's the first DIRECT MEASUREMENT of this number. Previous work over the past 50+ years has relied on theoretical calculations. In contrast, the new work measures actual masses of 37 red and brown dwarfs--enough to see the boundary between the two.

    2. The number comes in slightly below previous (theoretical) estimates, although the discrepancy is probably not that great, considering the uncertainties in both the theoretical calculations and the new observations.

    Bottom line: It's nice to finally have an actual MEASUREMENT of this number.
  16. May 17, 2017 #15
    Yes. but gas law is still there adding pressure. Someone should calculate for pure hydrogen.

    When stars collapse from gas clouds they are hot enough to stop contracting for awhile. Should be true for brown dwarfs and planets too. The gas law still applies in degenerate matter but it is tiny.

    If you are O.K. with estimates than your 70 Jupiter dwarf has around 70 Jupiter dwarf density.
    Last edited: May 17, 2017
  17. May 19, 2017 #16
    How so?
    Heat compressed ice. At warming to about 0 degrees, the pressure drops to 6 mbar for a wide range of initial pressures of compressed ice.
    While most other condensed substances do increase pressure on heating, it is not clear that the pressure should be a linear sum of ideal gas pressure to the condensed matter pressure.
  18. May 19, 2017 #17
    Can ice exist with degenerate matter?

    0 degrees is not likely to exist.
  19. May 21, 2017 #18
    Do brown dwarfs contain any oxygen nuclei?
  20. May 21, 2017 #19


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    Everything apart from the very first stars does. Why do you ask?
  21. May 21, 2017 #20
    The very old ones would still have some oxygen from inter-stellar gas.
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