8

The one from 2014 is still the record holder I believe - in the sense that it is reasonably convincing that the unseen companion of the pulsar PSR 2227-0137 is consistent with being a white dwarf with a surface temperature below 3000 K. It is worth considering why such objects might be difficult to find. (1) It is only the highest mass white dwarfs that have ...


7

The answer is of order 1 million years to cool from a standard end of He burning temperature of just over $10^8$ K to the top end of the white dwarf temperature range you give in your question. The details would depend exactly on the mass and composition of the white dwarf and there are also some theoretical uncertainties in neutrino cooling rates. The ...


7

More massive stars have a more massive core and produce more massive white dwarfs. The relationship between the initial mass of the main sequence star and the final mass of the white dwarf is monotonic, but not linear. The Sun is expected to produce a white dwarf with a mass of around $0.5 M_\odot$, whilst a $8M_\odot$ star is expected to produce a carbon/...


6

WD 1856b is not more massive than the star it orbits. The radius of WD 1856b is much larger than its star because its star is a white dwarf; but WD 1856b is much less massive. That gives the star a diameter of a bit larger than Earth while the planet's size is about that of Jupiter. The star, WD 1856+534 is about 1/2 the mass of our Sun or about 500 Jupiter ...


5

This is a classic question in physical eschatology, seeing what happens if we extrapolate current understanding of astrophysics forward. The classic papers are (Dyson 1979) and (Adams & Laughlin 1997). Obviously, over very long timescales white dwarfs cool down, crystallize. and become "black dwarfs". This is fairly well-established from ...


5

There is no great explosion in the formation of a white dwarf. The outer layers of the star are blown off into space, forming a "planetary nebula" and the dense carbon-oxygen core remains. The core is pretty hot (100,000 degrees) Hot enough to radiate X-rays, but not the more energetic gamma rays. The Earth (assuming that it is still in an orbit) ...


3

Core collapse electron capture supernovae are expected to leave a low-mass, low speed neutron star remnant (e.g. Gessner & Janka 2018).


3

A neutron star. Heger et. al. (2002) state [There is] a narrow range, ∼ 9−10 M⊙, where degenerate oxygen-neon cores are formed that either collapse due to electron capture (Barkat et al. 1974; Miyaji et al. 1980; Nomoto 1984; Habets 1986; Miyaji & Nomoto 1987; Nomoto 1987; Nomoto & Hashimoto 1988) and make a neutron star Such relatively small stars ...


3

The issue is not whether the dense core can fuse, but what fusion processes can occur on its surface. Remember that novas happen when gas accumulate on the surface of white dwarf stars: the high temperature and density is enough to trigger runaway fusion. Hence this method, where you to magically make a dense core appear inside a gas giant or somehow drop it ...


2

Let us consider a 1.4 solar mass neutron star, which, if not spinning would have a radius of about 10km. Ignoring relativistic corrections (which will be significant) the angular velocity $\omega$ of a circular orbit just above that surface is given by $$r\omega^2 = GM/r^2$$ so $$\omega = \sqrt{\frac{GM}{r^3}}$$ and putting in values, we find $\omega = 13.6 ...


2

Yes it would. It is that way because the effective temperature is defined to be $(L/4\pi \sigma R^2)^{0.25}$. The radius of a neutron star is about 10 km $(1.4\times 10^{-5}R_\odot)$. They are born with surface temperatures of around $10^8$ K. The coldest white dwarfs have effective temperatures of about 3000 K. The luminosity ratio is $$ \frac{L_{\rm NS}}{...


2

Yes, you are correct in so far as what the Chandrasekhar limit is for a white dwarf supported by ideal electron degeneracy pressure and using a Newtonian equation of hydrostatic equilibrium. Neither of these latter assumptions is true for real (or hypothetical) white dwarfs of any composition. First, there is a negative correction to the ideal gas pressure ...


1

The "vary wildly" point is not supported by the accepted stellar evolution models that always end up with some carbon-12/oxygen-16 mixture at the degenerate core stage near the Chandrasekhar limit. Yes, the Chandrasekhar limit will be different for white dwarfs with different composition. You are right for mercury and hydrogen "white dwarf&...


1

Additionally, others haven't said it, is that a neutron star is also much smaller than a white dwarf: White dwarfs are Venus- and Earth-sized (e.g. about 7000 mi / 11000 km in diameter) while neutron stars (and stellar black holes) have the size of the Martian satellites Phobos and Deimos (e.g. about 10 mi / 16 km in diameter). The neutron star (or maybe a ...


1

A white dwarf is less than 1.44 solar masses, and is held up by electron degeneracy pressure, the rule in quantum mechanics that says electrons strongly resist being squeezed together. They're made of highly compressed but still more or less normal matter, mainly carbon and oxygen. Dispite their mass they're only about as big as Earth, meaning one teaspoon ...


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