[HN Gopher] Astronomers may have spotted the smallest possible s...
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Astronomers may have spotted the smallest possible stars
Author : pseudolus
Score : 68 points
Date : 2024-11-07 13:39 UTC (5 days ago)
(HTM) web link (www.science.org)
(TXT) w3m dump (www.science.org)
| veunes wrote:
| Saturn-size brown dwarfs? Who would've thought!
| jmclnx wrote:
| Not sure what is meant by size ? IIRC, Saturn is kind of
| "fluffy" and could float on liquid water.
|
| So a Brown Dwarf with the radius of Saturn, why not ? But its
| mass would probably be much greater than Saturn, but compressed
| down to Saturn's size :)
|
| I thought I read somewhere a Brown Dwarf with 13x the mass of
| Jupiter would be about the same size as Jupiter.
| astrobe_ wrote:
| I remember that before the Shoemaker-Jupiter collision [1] of
| '95 there were rumors that it could "ignite" Jupiter giving
| birth to a star. It was probably on the same level as the
| rumors that the CERN accelerator could create a black hole,
| though.
|
| [1] https://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9
| jessriedel wrote:
| > But distinguishing between smaller brown dwarfs and planets
| requires looking at how they form. Even though they can end up
| about as massive as 10 Jupiters, planets always arise around a
| star, from its surrounding disk of gas and dust.
|
| > In contrast, stars, including brown dwarfs, form on their own
| within giant collapsing clouds of gas.
|
| Is this really the standard terminology? It's not how I remember
| it, and it doesn't make much sense. There are tons of binary star
| systems, and while a some are three-body capture events, aren't
| most formed from the same gas cloud? (I.e., not "on their own".)
| Likewise, rogue planets (i.e., not bound to a star) can be formed
| in a stellar system and be ejected, but can't they also form on
| their own, e.g., a dust cloud with less than enough total mass to
| form a star? Surely you wouldn't call a sub-Jupiter-mass body a
| "brown dwarf" just because it formed in isolation?
| verzali wrote:
| I think its getting at the difference in formation (i.e.
| gravitational collapse vs accretion). I'm not sure you always
| need a parent star to form planets by accretion though, perhaps
| they could emerge from dust clouds spontaneously?
|
| As I remember it there is supposed to be some gap in size
| between the objects produced by these two different methods, so
| that nothing produced by accretion can be larger than something
| formed by collapse. But since that gap doesn't seem to exist in
| practice it really looks like we are missing something.
| JSteph22 wrote:
| As the article clearly points out several times by referencing
| the potential "upending of star formation theory" and the list
| of "brown dwarfs that shouldn't exist", current star and
| planetary formation theories are severely lacking, so you're
| right to question definitive statements such as stars forming
| "on their own".
| jessriedel wrote:
| I am questioning whether the article is accurately describing
| the scientific nomenclature. The author of course wants to
| say things are being "upended", but I suspect the author just
| didn't understand, or is inaccurately describing, the
| previous state of affairs.
| Jerrrrrrry wrote:
| The nomenclature has shifted as we have found stars are
| make our "average white star," less average, less white,
| and less of a star.
|
| The classification nomenclature is definitely in need of a
| refactoring.
| delta_p_delta_x wrote:
| There's a continuum from tiny planetesimal to a huge star of
| 50+ solar masses, and sometimes the distinction is blurred.
|
| Many red dwarf stars are hardly larger than Jupiter despite
| being more than 80 times as massive (this is commonly cited as
| the lower limit for protium fusion, which is the definition of
| a star[1]).
|
| > Surely you wouldn't call a sub-Jupiter-mass body a "brown
| dwarf" just because it formed in isolation?
|
| In my view, the criteria for what gets classified as brown
| dwarf stars isn't the circumstances of their formation, but
| only their mass and hence the nature of fusion (if any) in
| their interior. So if a gas cloud collapsed into a single sub-
| Jupiter-mass body, it is a planet. The article says a
| 7-Jupiter-mass star could be a brown dwarf, and I believe the
| lower limit is unclear because there _could_ be deuterium
| /tritium fusion at such low masses, and even at higher masses
| there could be no fusion at all[2].
|
| I think the article was trying to make the distinction between
| gravitational collapse and accretion, but honestly, accretion
| can also sometimes go runaway and produce a body that is about
| brown-dwarf mass (i.e. 13-80 Jupiter masses).
|
| [1]:
| https://coolcosmos.ipac.caltech.edu/page/low_mass_stars_brow...
|
| [2]:
| https://iopscience.iop.org/article/10.1088/0004-637X/770/2/1...
| pfdietz wrote:
| Not deuterium/tritium fusion, but rather deuterium/protium
| fusion: D + p --> 3He + photon. There is no tritium. The star
| is not hot enough for the 3He to then fuse with itself, as it
| does in our Sun or to some extent in red dwarf stars. In the
| center of our Sun, the Dp reaction goes so fast that a D
| nucleus would be consumed in about 1 second.
|
| A lot of sources just say "deuterium burning", and people
| assume this is DD. But at low temperature, the reaction rate
| is strongly affected by barrier penetration, and this is
| strongly influenced by the reduced mass of the two nucleus
| system.
|
| The 3He3He reaction is slow in red dwarfs, and I understand
| the concentration of 3He in them builds up to about 1% before
| plateauing. I don't think any red dwarf is yet old enough to
| reach that stage. Red dwarfs are fully convective; unlike our
| Sun their entire mass circulates through the core and becomes
| available to undergo nuclear fusion. This (and their low
| luminonsity) makes them very long lived, up to a trillion
| years, far longer than the universe has yet existed.
| Retric wrote:
| When no category really fits the best option is to just come
| up with a new name.
|
| Planets were initially defined by their motion in the nights
| sky. That's continued to this day by saying they must have
| cleared their orbit. So if a large mass formed alone it
| really doesn't fit the ancient or modern definition of a
| planet.
|
| Further, many of these things may eventually become stars as
| they attract enough mass. Calling something that turns into a
| star in a 10 billion years a planet until suddenly swapping
| to young stellar object when the conditions change, just
| doesn't fit IMO.
| goodcanadian wrote:
| _Is this really the standard terminology?_
|
| Well, it is close enough. The definition is that a brown dwarf
| forms like a star through gravitational collapse within a
| protostellar nebula, but it just wasn't massive enough to
| ignite. However, a planet forms in an accretion disk around a
| star that is forming out of the larger cloud. Of course, nature
| does not always perfectly align itself within our neat
| categories, and it is not really possible to distinguish
| between a brown dwarf and a supermassive planet that happened
| to be ejected from its star. Anyway, perhaps such a
| supermassive planet that is not ejected should really be
| considered a brown dwarf in a binary with the "parent" star.
| For gravitational collapse to occur in the first place,
| however, requires a minimum density which sort of puts a lower
| limit on the mass of a brown dwarf, so smaller objects are not
| really going to form independent of an accretion disk, but they
| may form in one and be ejected as you say. So yes, there is
| kind of a practical line between planet and brown dwarf, but it
| is a bit of a fuzzy line.
| munk-a wrote:
| Does this also hold for rogue planets? I was under the
| impression that some rogue planets could form outside of any
| star's accretion disk.
| dylan604 wrote:
| But they did form in some star's accretion disc, just not
| the one they are roguing their way through. In theory,
| Jupiter could have caused a planet to go rogue when it made
| it's way to the inner solar system.
| s1artibartfast wrote:
| My understanding is that smaller rouge plants do not form
| in isolation and are always the product of ejection.
|
| However, the categorization again is fuzzy, with
| definitional overlap between "planet", "sub-brown-dwarf",
| and "brown dwarfs"
|
| Planets cover spherical objects ranging from 0.001 Jupiter
| masses to stars at ~80 Jupiter masses. Brown dwarfs range
| from 3-80 Jupiter masses.
|
| Brown Dwarfs _are_ planets and can form outside of an
| accretion disk. Planets smaller than brown dwarfs are not
| thought to be able to form outside of accretion disks.
| aurelien wrote:
| Science.org is made without counsciouness in the way they force
| the reader to use javascript. But if you have really something to
| say, I would like to read it in Text WebBrowser only, I do not
| want to use crap web browser that are made to make you think you
| read something interesting. Pseudo journalism kill science.
|
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