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Comment by jl6 | Hacker News Reader

> Gas physics suggests you shouldn't be able to make objects with the mass of Jupiter on their own

This is new to me. I always imagined that on the way to star formation the coalescing gas would start as a small central mass and grow larger and larger, passing through a Jupiter-mass stage and eventually igniting. I also imagined that the process could involve several small central masses which would variously end up as binary/ternary/++ star systems and all manner of brown dwarves/planets/debris in orbit around the center. And I also imagined that this process would stop at whatever point the gas supply ran out.

So in my mind, a pair of Jupiters would be a failed binary star system that didn’t quite have enough mass to achieve ignition as a pair of stars.

But maybe I’m missing a gas-physics reason why it couldn’t happen like this.

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Stars are formed when clouds of gas and dust collapse into themselves.

Either the cloud is big and dense enough to collapse into a star, or it will remain as a cloud.

Our Jupiter was created when, during the collapse, other forces pulled part of the collapsing cloud away from the sun, into a stable orbit.

> Stars form when the dust and gas clouds in a nebula cool, progressively fragment and eventually collapse under their own gravity. The smallest stars are about 80 Jupiter masses, below which the core is not dense enough to fuse hydrogen, but smaller objects can coalesce through the same process, including dimly glowing brown dwarfs – sometimes called failed stars – and, below about 13 Jupiter masses, planetary-mass objects. But theoretical predications suggest that the lower boundary for an object forming through a star-like gravitational collapse is about three to seven Jupiter masses.

https://www.theguardian.com/science/2023/oct/02/jumbos-jupit...

Don't quote me on this but I think the limiting factor is that gravitational pull needs to be stronger than the expansion of the universe.

The expansion of the universe isn't really related to the formation of individual stars.

But it sounded cool.

That it did

> Either the cloud is big and dense enough to collapse into a star, or it will remain as a cloud.

Why is this? Why can’t a small molecular cloud coalesce into small objects?

Gas clouds are made of light molecules that a plant would need a large mass to retain, perhaps. Consider how Earth's gravity isn't strong enough to retain He.

When I first read this I was wondering who He was, I guess you mean helium

But Jupiter’s gravity is strong enough to retain helium and hydrogen.

After the planet has formed, yes. You need something Jupiter mass to already exist as a dense planet to retain those gases that way though. Before that forms the gases disperse too easily so you never get to the Jupiter mass planet stage.

It's only if the mass of the cloud in it's dispersed state is significantly greater than that of Jupiter that it's gravity is strong enough to prevent it's constituents maintaining dispersal due to their own heat.

> Earth's gravity isn't strong enough to retain He.

Escape velocity is dependant on temperature, so Earth gravity can't retain He at Earths temperature - A colder Earth perhaps could.

Light molecules? You mean like, photons?

I kid

I suspect it may not be able to cool quickly enough. When a cloud collapses the gravitational potential energy goes into heat, and that heat has to be radiated for the collapse to continue. If the gas is too cold or too low in density it may not (I am guessing) be able to do this fast enough before the cloud is either disrupted or accretes too much additional gas.

“An interstellar cloud of gas will remain in hydrostatic equilibrium as long as the kinetic energy of the gas pressure is in balance with the potential energy of the internal gravitational force.“

https://en.wikipedia.org/wiki/Star_formation#Cloud_collapse

That's a fascinating thought actually. Why does a mass of gas always have to materialize into a solar system? Why not into one or more planets, if the amount is too low? Or also the other way around: what if there is so much gas that it's too much to fit into a single star, what would it materialize into then? A twin star solar system, or something else?

There is no "too much". The result would simply be a larger star.

There is a theorized “Eddington Limit” that stars can only grow to a certain mass before the power of their fusion reactions counterbalance gravity and they blow off additional matter instead of absorbing it. It’s approximately a few hundred solar masses.

How does the Eddington Limit relate to black hole formation ? Does it means that accretion is slow until the star's matter has finished fusing ?

Black holes form when there is insufficient force to counter accretion and subsequent compression by gravity. There are a few forces that act as a counter:

- thermally generated pressure as a result of exothermic reactions (like nuclear fusion),

- Coulomb pressure (atomic nuclei opposing each other), or

- quantum-mechanical degeneracy pressure (keeps white and brown dwarfs and neutron stars from collapsing)

If you have a really big gas cloud I think you just get multiple stars, not particularly in a "solar system". There are some telescope images of stellar nurseries where you can see several young stars in the same gas cloud.

You can see the Pleiades with the naked eye.

Surprise so many uncertainties of the age and even distance of them https://en.wikipedia.org/wiki/Pleiades

Most of the distance measurements are within a percent of each other, except for a single survey with is 10% off all the others. That is not such a large uncertainty as to be of significant note.

It would be interesting to hear more. My only (completely uneducated) guess is that if you don’t have enough gas to form a star, the density isn’t sufficient to begin the process at all.

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