# Can a neutron star ever be less than about 1.44 solar masses (the Chandrasekhar limit)? Why not?

I learned about the Chandrasekhar limit as being the UPPER limit, in terms of mass, for a white dwarf...

But, I have never heard of a neutron star being BELOW that mass, so I have wondered, recently, if that is also a LOWER limit for neutron stars...

I suppose that the squished-together protons and electrons that the neutron star is made of might lose their 'degeneracy', if the gravitational potential is (or becomes) low enough, and the star might 'poof-out' (or proof back out) to a white dwarf...

Has this ever been known to happen?

Can a neutron star experience Hawking radiation and actually lose mass, like a black hole?

If black holes can, theoretically at least, exist at very small masses as long as they are small enough voluminously (compacted within their Swarzschild or Kerr radii), then why can't neutron stars?

• NS-BH binaries can cause the NS component to lose mass to below the Chandrasekhar limit. See this Physics SE question. – fasterthanlight May 24 at 22:55
• You need an event horizon for Hawking radiation. OTOH, hot neutron stars radiate EM like any hot body does, and they can lose energy via electromagnetic interactions with plasma in the neighbourhood, but of course that energy is pretty small relative to their rest mass. – PM 2Ring May 25 at 1:45

A number of neutron stars in binary systems have measured masses below $$1.44 M_\odot$$ (e.g. a pulsar of mass $$1.251 \pm 0.021 M_\odot$$, McKee et al. 2020). I think the current lowest mass contender is $$1.174 \pm 0.004 M_\odot$$ (Martinez et al. (2015). See the plot below with a pictorial representation of the current state of neutron star mass measurements from Horvath et al. (2020). Conveniently, the vertical line is I think at about 1.4 solar masses.
The Chandrasekhar mass for a ball of iron at the centre of a core collapse supernova is more like $$1.15 M_\odot$$ because ionised iron has 2.15 mass units per electron, rather than the 2 of carbon or oxygen, and because the electron energy required to neutronise iron is much lower than for carbon and oxygen.
The theoretical lower limit to the neutron star mass is about $$0.1-0.2M_\odot$$, but none have been observed below $$1M_\odot$$ and there isn't any known astrophysical mechanism to produce them. A much fuller answer to this last part, which I won't cut and paste here can be found at Physics SE. In brief, the lower limit arises because the adiabatic index of material in the interior falls below 4/3 (because of inverse beta decay, the formation of neutron-rich heavy nuclei and the disappearance of some free neutrons) and the material is too compressible to form a stable star with a negative binding energy.