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38 votes
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If two white dwarfs collided, would they become a star?

The answer to your question is both yes and no, depending on the circumstances. Two white dwarfs colliding would likely yield a Type Ia supernova, assuming the combined mass exceeded the ...
HDE 226868's user avatar
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33 votes
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Why do white dwarfs cool down so slowly?

Since space is empty, heat can only be transmitted by radiation. Stefan-Boltzmann's radiation law states that the energy flow is proportional to the surface area, times the temperature to the fourth ...
Anders Sandberg's user avatar
23 votes

Why would rocky earth-like planets be rare around white dwarf stars?

Short answer Most planets of this type may have formed too close to their parent star (the progenitor of the white dwarf) to have avoided being swallowed when it became a red giant. Another ...
ProfRob's user avatar
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22 votes
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Why did Chandrasekhar use 2.5 for molecular weight in 1931?

The mean molecular mass per electron and its influence on the maximum mass of a star supported by electron degeneracy pressure is discussed in detail by Stoner (1932). It was in fact Stoner, who had ...
ProfRob's user avatar
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20 votes

How long does it take for a white dwarf to cool to a black dwarf?

I don't think there is an accepted definition of a "black dwarf" - it is not a term used in the scientific literature. A popular definition that appears to circulate on the internet is that it is a ...
ProfRob's user avatar
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18 votes

Is there a possibility that a white dwarf can turn into a neutron star or a black hole?

The answer is: to a neutron star - possibly; to a black hole, no. The process whereby a neutron star is formed is known as an accretion induced collapse and is being seriously debated, especially in ...
ProfRob's user avatar
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17 votes
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Will Sirius B start accreting from A and become a supernova type Ia?

Will Sirius B start accreting? Yes, it is doing so now. Sirius A will have a wind and some of that wind will be captured by the white dwarf. The effectiveness of wind capture is a strong function of ...
ProfRob's user avatar
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17 votes

Why do white dwarfs cool down so slowly?

One obvious reason is that the bulk of a white dwarf is much hotter than indicated by the surface temperature. About 99% of a white dwarf's mass is at temperatures about 100 times higher than the ...
ProfRob's user avatar
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15 votes
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How small stars help with planet formation

You need to add some numbers for a definitive answer. Low mass stars, say less than 0.9 times the mass of the Sun, do not make any contribution to enriching the interstellar medium. That is because ...
ProfRob's user avatar
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13 votes

Why did Chandrasekhar use 2.5 for molecular weight in 1931?

The mean molecular mass per electron is very close to 2.0 for lighter elements (ignoring hydrogen), but rises to around 2.5 for heavier elements. If you think that the interior of the stellar object ...
BowlOfRed's user avatar
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12 votes
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How long does it take for a white dwarf to cool to a black dwarf?

I think what you need is here on the Wikipedia. In section "Radiation and cooling," it says "The rate of cooling has been estimated ... After initially taking approximately 1.5 billion years to cool ...
Kornpob Bhirombhakdi's user avatar
10 votes

White Dwarf and Degenerate Matter

The density of white dwarfs is not hypothetical, it can be measured. The short answer is that the density is so high that a stable star can only be supported by electron degeneracy. Sirius B is an ...
ProfRob's user avatar
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10 votes
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Are white dwarf stars supported by proton degeneracy as well?

Straightforwardly no. For a start there are almost no free protons inside a white dwarf. They are all safely locked away in the nuclei of carbon and oxygen nuclei (which are bosonic). There are a few ...
ProfRob's user avatar
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10 votes
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Are white dwarfs made of carbon or electron degenerate matter?

Most white dwarfs consist of fully ionised carbon and oxygen atoms$^1$. Each carbon nucleus must be accompanied by 6 electrons and each oxygen nucleus by 8 electrons. The electrons are essentially a ...
ProfRob's user avatar
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10 votes

What are the most up to date, accepted, evolution stages of big stars that end in compact objects or a planetary nebula?

It is now known, through a detailed study of the cooling times of white dwarfs in open clusters of known age, and through modelling of their progenitors, that white dwarfs arise from single main ...
ProfRob's user avatar
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9 votes

How long does it take for a white dwarf to cool to a black dwarf?

I think the most important part of any answer is that, as Rob Jeffries said, "black dwarfs" aren't really a thing in the astronomical literature, and I suspect that's the reason that you get different ...
Mark Foskey's user avatar
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9 votes

Why do white dwarfs cool down so slowly?

Space is a thermos You say "Shouldn't this temperature difference [...] cool them down much faster?" And that's a common point of confusion, because space isn't cold (at least not the way we ...
Darth Pseudonym's user avatar
8 votes
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Which Chandrasekhar Limit do I use? 1.39 or 1.44?

The "classic" Chandrasekhar mass is given by $$ M_{\rm Ch} = 1.445 \left(\frac{\mu_e}{2}\right)^{-2}\ M_{\odot},$$ where $\mu_e$ is the number of mass units per electron ($\mu_e =2$ for ...
ProfRob's user avatar
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8 votes
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What is the coolest white dwarf known?

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 ...
ProfRob's user avatar
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8 votes
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How does the Chandrasekhar limit relate to the ignition of carbon in white dwarf stars?

The carbon ignites if the densities become high enough to initiate pycnonuclear reactions. These fusion reactions occur, even in thermally cold, crystalline carbon if the zeropoint oscillations are ...
ProfRob's user avatar
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8 votes
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What is the maximum radius of a pure iron-56 planet?

It won't ever collapse into a black hole, it will collapse to form a neutron star. As you add mass to your planet, initially it will get larger, perhaps till it reaches a few times the size of the ...
ProfRob's user avatar
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8 votes
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White dwarf supernova luminosity

Type Ia supernovae are not standard candles in the sense that all such supernovae have the same luminosity. This is also the case for other standard candles such as Cepheids. What we can say, is that ...
ProfRob's user avatar
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7 votes

Are white dwarf stars supported by proton degeneracy as well?

Proton degeneracy is not important, because its effect is much smaller -- much like nuclear particles in theory also are dictated by gravity, but the electromagnetic and nuclear forces are dominating, ...
nataliaeire's user avatar
7 votes
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Chemical reactions in white dwarf and carbon allotropes

White dwarfs are objects the size of the Earth, but with a mass more similar to the Sun. Typical internal densities are $10^{9}$ to $10^{11}$ kg/m$^{3}$. White dwarfs are born as the contracting core ...
ProfRob's user avatar
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7 votes
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Do more massive stars become larger or smaller white dwarfs?

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 ...
ProfRob's user avatar
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7 votes
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Exactly how long does it take for the exposed core of a star to cool from its starting temperature (several billion K) to ~50,000 K?

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 ...
ProfRob's user avatar
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7 votes

What determines the temperature of a white dwarf?

White dwarfs are "dead cinders" which are hot because they formed hot. Once it has formed, a white dwarf spends most of its time cooling down. The surface temperature of a white dwarf is ...
ProfRob's user avatar
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6 votes

Will Sirius B start accreting from A and become a supernova type Ia?

The distance between Sirius A and B is between 8 and 31.5 AU and even when Sirius A becomes a red giant it will be still above 6 AU. Such distance is too large and does not allow Sirius B to accrete ...
Vladislav Peichev's user avatar

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