What is lowest possible mass for neutron stars and white dwarfs?

[raxo2222]

There are maximum mass for these stars.

But what are known lightest objects (neutron star/white dwarf) and theoretically how light these can be?

Edited June 10, 2017 by raxo2222

[Gaarst]

For a neutron star it's the Chandrasekhar mass limit, about 1.3 solar masses, below which the stellar remnant doesn't contain enough mass to overcome electron degeneracy pressure and therefore cannot collapse to a neutron star. You then get a white dwarf.

For white dwarfs, there is no lower limit as long as your initial body is massive enough to fuse hydrogen. Below a certain mass, the star would not inflate like the Sun will (because He fusion will never take place), but you'd end up with a white dwarf anyway.

Edited June 10, 2017 by Gaarst
Clarity

[tater]

Well a neutron star will start with a star above 1.4 solar masses.

The smallest main sequence stars are M class (red), and there has been one observed not much bigger than Jupiter.

[magnemoe]

An issue with the smaller red dwarfs is that none has burned up their hydrogen yet. 
 

[raxo2222]

What if a lot of antimatter was dumped on neutron star (or its mass was removed something else)? Would it puff up faster and faster and then explode due to gravity being weaker?

 

Edited June 10, 2017 by raxo2222

[Scotius]

20 minutes ago, raxo2222 said:

What if a lot of antimatter was dumped on neutron star (or its mass was removed something else)? Would it puff up faster and faster and then explode due to gravity being weaker?

 

This...is not how gravitation works. Antimatter =\= antigravity. Anti-star will generate as much pull as a normal star of equal mass.

[raxo2222]

7 minutes ago, Scotius said:

This...is not how gravitation works. Antimatter =\= antigravity. Anti-star will generate as much pull as a normal star of equal mass.

I know, but antimatter would turn matter to energy on contact resulting in some mass converted to energy

[Scotius]

Good luck finding anything for antimatter to annihilate inside a neutron star

[sevenperforce]

26 minutes ago, Scotius said:

Good luck finding anything for antimatter to annihilate inside a neutron star

Antimatter would readily annihilate on contact with a neutron star.

2 hours ago, tater said:

Well a neutron star will start with a star above 1.4 solar masses.

The smallest main sequence stars are M class (red), and there has been one observed not much bigger than Jupiter.

The smallest main sequence stars are not much larger than Jupiter in terms of radius, but they are many, many times more massive.

However, I don't think the OP's question has really been answered. A neutron star progenitor must be larger than 1.4 solar masses, but the actual mass of the degenerate neutron remnant from a 1.4-solar-mass supernova will be much lower than 1.4 solar masses. Depending on the type of supernova, the degenerate remnant may have the majority of the progenitor's mass or it may retain only the barest fraction of it. 

So the real question is this: if you took the smallest known neutron star and gradually removed material from it (via antimatter annihilation, as suggested above, or via co-neutron-star collision, or any other hypothetical mechanism), at what point would the neutron star be so small that its gravity would no longer be able to overcome neutron-degeneracy pressure? That's a very good question, one which we don't yet have a good answer to. We do not have a robust equation of state for neutron-degenerate plasma, so it is hard to say.

[magnemoe]

32 minutes ago, Scotius said:

Good luck finding anything for antimatter to annihilate inside a neutron star

Neutrons I guess, Surface has normal matter anyway. 
I guess the surface layer of normal but degenerated matter would be thicker on an small neutron star. 
Remove enough mass and it would become an white dwarf. 

[tater]

5 minutes ago, sevenperforce said:

The smallest main sequence stars are not much larger than Jupiter in terms of radius, but they are many, many times more massive.

Yes, about 100X Jupiter's mass for the smallest actually observed. Two "many"s for 2 orders of magnitude? OK. We're still talking on the oder of 0.1 M_sol.

Presumably, approaching 0.1 solar mass (certainly in the same order of magnitude, since the Sun might become a WD of what, 0.6 M_sol), we might see a white dwarf. 

Edited June 10, 2017 by tater

[YNM]

What really exist are :

- Boundaries where electron degeneracy occurs (normal lump of matter vs. degenerate)

- Boundaries between white dwarf and neutron star mass

- Boundaries between neutron star and black hole mass

There's no upper limit on how large a black hole can be and no lower limit on how small a white dwarf can be.

 

 

 

... Or is it ?

Actually, if Brown Dwarf were to be considered (and proven) degenerate, they'd be the lowest mass white dwarfes. There's an additional limit then between a planet and a brown dwarf...

Oh wait a second...

Edited June 11, 2017 by YNM

[kerbiloid]

What if leave a rocky planet (much lighter than Jupiter) for a long time?

Of course, initially it will be just staying unchanged.
But surely quantum effects, tunneling, etc. will move it to some lowest energy state where a tunneling is no more possible.
Won't its core become a kinda the same like white dwarf?

[MythicalHeFF]

I think there actually is a lower mass limit for a white dwarf. The less massive they get, the bigger they get, and the lower their density becomes. So I'd say that after a certain point, the density would get low enough that it would no longer be held up by electron degeneracy, but instead by simple gas pressure like a gas giant or brown dwarf is. 

[magnemoe]

2 hours ago, Jack Joseph Kerman said:

I think there actually is a lower mass limit for a white dwarf. The less massive they get, the bigger they get, and the lower their density becomes. So I'd say that after a certain point, the density would get low enough that it would no longer be held up by electron degeneracy, but instead by simple gas pressure like a gas giant or brown dwarf is. 

Makes sense, in this case it would simply end up as an large brown dwarf. 
Fusion stop but the core will not degenerate as the pressure is not high enough. 
However as small red dwarfs live a long time none have probably reached this stage yet

[YNM]

10 hours ago, Jack Joseph Kerman said:

I think there actually is a lower mass limit for a white dwarf. The less massive they get, the bigger they get, and the lower their density becomes. So I'd say that after a certain point, the density would get low enough that it would no longer be held up by electron degeneracy, but instead by simple gas pressure like a gas giant or brown dwarf is. 

 

7 hours ago, magnemoe said:

Makes sense, in this case it would simply end up as an large brown dwarf. 
Fusion stop but the core will not degenerate as the pressure is not high enough. 
However as small red dwarfs live a long time none have probably reached this stage yet

Actually, the case is somehwat reversed.

A very low mass red dwarf have no degeneracy anywhere in them - photons (and energy) released from nuclear reaction results in excess radiation pressure, preventing a degeneracy from occuring.

In the largest brown dwarfs, no fusion occurs beyond lithium burning - so there're no photons, no excess radiation pressure, hence there should be degeneracy.

Inferrentially, this might mean that all objects that is classified as a brown dwarf have some degeneracy in it.

Wonder whether this could be made into one criterion of differentiating between brown dwarfs and planemo/giant planets.

Edited June 11, 2017 by YNM

[sevenperforce]

20 minutes ago, YNM said:

Inferrentially, this might mean that all objects that is classified as a brown dwarf have some degeneracy in it.

Wonder whether this could be made into one criterion of differentiating between brown dwarfs and planemo/giant planets.

There's already a simple discriminant between brown dwarves and giant planets. The former have fused in their lifetimes; the latter never have and never will.

[YNM]

3 minutes ago, sevenperforce said:

There's already a simple discriminant between brown dwarves and giant planets. The former have fused in their lifetimes; the latter never have and never will.

So all brown dwarf passes the lithium depletion test then ?

[MythicalHeFF]

3 hours ago, YNM said:

So all brown dwarf passes the lithium depletion test then ?

No, only more massive brown dwarfs can or have fused lithium. Less massive ones fuse deuterium instead of lithium or actual hydrogen. It's the latter that distinguishes brown dwarfs from true stars.

[Adstri]

5 hours ago, YNM said:

 

Actually, the case is somehwat reversed.

A very low mass red dwarf have no degeneracy anywhere in them - photons (and energy) released from nuclear reaction results in excess radiation pressure, preventing a degeneracy from occuring.

In the largest brown dwarfs, no fusion occurs beyond lithium burning - so there're no photons, no excess radiation pressure, hence there should be degeneracy.

Inferrentially, this might mean that all objects that is classified as a brown dwarf have some degeneracy in it.

Wonder whether this could be made into one criterion of differentiating between brown dwarfs and planemo/giant planets.

Are you saying that brown dwarfs can be classified as white dwarfs? I thought that a white dwarf was the remaining core of a low mass star that has puffed out its outer layers. (Sorry if I misinterpreted that statement)

[Hannu2]

On 10.6.2017 at 8:49 PM, raxo2222 said:

What if a lot of antimatter was dumped on neutron star (or its mass was removed something else)? Would it puff up faster and faster and then explode due to gravity being weaker?

 

You can see the mass versus density graph in figure 2 in this article:

http://cds.cern.ch/record/435428/files/0004317.pdf

If you begin from low mass at lower left corner of the graph and add mass you go towards first maximum at 1E9 g/cm^3. If you add mass after that, next stable state is on right side of minimum at about 1E15 g/cm^2. Materal will change phase from baryonic matter held by electron degeneration pressure to neutron star held by neutron degeneracy pressure. Such a phenomena is possible and is called Type 1a supernova. If you continue to add mass you climb on curve until there are next maximum at near 1E16 g/cm^2. There are no known stable states after that and material collapses to black hole.

If you begin from heavy neutron star near the black hole transition and decrease the mass, you go towards minimum at 1E14 g/cm^3. Then there seems to be a phase transition from neutron star material to normal state near the lower left corner. I have never seen any hypotheses what kind of nuclei would form in such transition and would there be enough energy to overcome gravitational binding so that whole "star" would explode. It is not interesting for scientists because there are known natural processes which could lead mass leak from a neutron star and it is absolutely unexpected to ever detect such a phenomena.

Neutron stars are made from quarks. So, ordinary antimatter (antiprotons and antineutrons) would annihilate it, but it would be quite challenging to produce couple of sun masses of antimatter to make an experiment.

[YNM]

17 hours ago, Adstriduum said:

Are you saying that brown dwarfs can be classified as white dwarfs? I thought that a white dwarf was the remaining core of a low mass star that has puffed out its outer layers. (Sorry if I misinterpreted that statement)

Theoretically, white dwarf are only "white" because they were very hot at start - once they cool down (after stupendously long time), they'll be cool and faint. The only thing that distinguishes them would purely be the temperature, atmosphere thickness, and mass.

Edited June 12, 2017 by YNM

[sevenperforce]

But brown dwarves are a very very different thing from white ones.

[Adstri]

2 hours ago, YNM said:

Theoretically, white dwarf are only "white" because they were very hot at start - once they cool down (after stupendously long time), they'll be cool and faint. The only thing that distinguishes them would purely be the temperature, atmosphere thickness, and mass.

Well I know that, I was just asking about the brown dwarf thing. Also, I think they would start out as blue rather than white and slowly fade to red.