3 ms·

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

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...

pfdietz 2y ago
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 2y ago
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.

pfdietz 2y ago

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 2y ago

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.