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Astronomers may have spotted the smallest possible stars

76 points by pseudolus 2y ago

veunes 2y ago
Saturn-size brown dwarfs? Who would’ve thought!
- jmclnx 2y ago
  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_ 2y ago
    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 https://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9
  - veunes 2y ago
    Yep, a 'Saturn-sized' brown dwarf would have Saturn's radius but vastly more mass
jessriedel 2y ago
> 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 2y ago
  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 2y ago
  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 2y ago
    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 2y ago
      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 2y ago
  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_brown_dwarfs https://coolcosmos.ipac.caltech.edu/page/low_mass_stars_brow... [2]: https://iopscience.iop.org/article/10.1088/0004-637X/770/2/120 https://iopscience.iop.org/article/10.1088/0004-637X/770/2/1...
aurelien 2y ago
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. 403 Forbidden ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Refresh (390 sec) Enable JavaScript and cookies to continue
numbol 2y ago
Just for pedantic remark, brown dwarfs is not stars, but they are substellar objects and formed in the similar fation (and this is why question about edge case of very large exoplanted vs very small brown dwarf is interestring)https://en.wikipedia.org/wiki/Substellar_object https://en.wikipedia.org/wiki/Substellar_object definition relies on sustained hydrogen fusion

76 points by pseudolus 2y ago

veunes 2y ago

Saturn-size brown dwarfs? Who would’ve thought!

jmclnx 2y ago
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_ 2y ago
  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 https://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9
- veunes 2y ago
  Yep, a 'Saturn-sized' brown dwarf would have Saturn's radius but vastly more mass

jessriedel 2y ago

> 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 2y ago
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 2y ago
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 2y ago
  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 2y ago
    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 2y ago
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_brown_dwarfs https://coolcosmos.ipac.caltech.edu/page/low_mass_stars_brow... [2]: https://iopscience.iop.org/article/10.1088/0004-637X/770/2/120 https://iopscience.iop.org/article/10.1088/0004-637X/770/2/1...

aurelien 2y ago

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. 403 Forbidden ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Refresh (390 sec) Enable JavaScript and cookies to continue

numbol 2y ago

Just for pedantic remark, brown dwarfs is not stars, but they are substellar objects and formed in the similar fation (and this is why question about edge case of very large exoplanted vs very small brown dwarf is interestring)https://en.wikipedia.org/wiki/Substellar_object https://en.wikipedia.org/wiki/Substellar_object definition relies on sustained hydrogen fusion