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Hot answers tagged white-dwarf

38

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 Chandrasekhar limit ($\sim1.4$ solar masses). The unstable object resulting from the collision could not be supported by electron degeneracy pressure; when the temperature ...

20

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 white dwarf that has cooled down to the extent that it no longer emits any radiation in the visible part of the spectrum. But this is an unworkable theoretical ...

17

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 relative wind speed. An analytic approximation to the accretion rate, known as Bondi-Hoyle accretion, goes as the inverse cube of the relative speed. In its ...

12

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 to a surface temperature of 7140 K, cooling approximately 500 more K ... takes around 0.3 billion years, but the next two steps of around 500 K ... take first 0....

11

Short answer: The Sun will lose about half of its mass on the way to becoming a white dwarf. Most of this mass loss will occur in the last few million years of its life, during the Asymptotic Giant Branch (AGB) phase. At the same time the orbital radius of the Earth around the Sun will grow by a factor of two (as will the outer planets). Unfortunately for ...

11

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 the case of white dwarfs that are born at the upper end of the "natural mass range" for white dwarfs and then accrete more mass as part of a binary system. An ...

10

Nobody really knows how type Ia supernovae detonate (or deflagrate) - there are a number of possibilities. The "vanilla" possibility is not what you state in your question, it is that the white dwarf accretes sufficient mass that it approaches the Chandrasekhar limit and becomes dense enough in its core to commence carbon burning. However, the emerging ...

10

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 protons near the surface, but not in sufficient numbers to be degenerate. Let us assume though that you were able to build a hydrogen white dwarf that had ...

10

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 example. The radius can be estimated by combining the luminosity of the white dwarf with its temperature estimated from spectroscopy. The mass can then be ...

9

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 answers about how long it takes to become one. Different people come up with different thresholds for becoming one. I would argue that 3000 K is too hot to ...

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

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, since they are much stronger. Proton degeneracy is weaker than electron degeneracy due to the far greater mass of the proton compared to the electron. 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/...

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 ...

6

This question has two parts: Surface Temperatures A very useful diagram which shows surface temperatures, and also gives you the temperature of any star you can observe is the Herzsprung-Russell Diagram, this one from le.ac.uk. As you can see, the yellow of our own sun places it in the 4.5 kKelvin to 6 kKelvin, as noted in the question. This temperature ...

6

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 of asymptotic giant branch stars that do not quite get hot enough to initiate carbon fusion. They have initial central temperatures of $\sim 10^{8}$K, that ...

6

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 significant mass, almost all mass lost by Sirius A as a red giant and later AGB will escape into space. Sirius B may become a recurrent nova due to some ...

6

I'll address the three sub-questions individually, as a way of fully answering the title question. While there are indeed three planets orbiting PSR B1257+12, note that it's a pulsar, the compact remnant of an energetic event involving the system's progenitor. However, that event would likely have destroyed any planets that originally orbited the star, ...

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

The smallest, precisely measured mass for a neutron star is now $1.174 \pm 0.004 M_{\odot}$ - Martinez et al. (2015). The theoretical lower limit is more like $0.1M_{\odot}$, but there are no obvious formation channels to produce such an object. See https://physics.stackexchange.com/questions/143166/what-is-the-theoretical-lower-mass-limit-for-a-...

5

No, the two limits are not the same - there is some range of masses that both white dwarfs (WDs) and neutron stars (NSs) can have. The Chandrasekhar mass limit suggests that WDs cannot be more massive than about $1.4\,M_\odot$. However, this is true for non-rotating WDs. Rapidly rotating WDs might be as massive as $2\,M_\odot$. Accretion in a binary ...

5

Intro for the uninformed: A standard candle is an important concept in astronomy, helping to map out distances in the Universe. Since the observed flux $F$ of a light source decreases with distance $r$ by a known factor ($r^2$), if we know its intrinsic luminosity L, we can calculate the distance. For large distances, where bright sources are needed, we ...

5

The material at the surface of a white dwarf is not degenerate. The "visible" surface is defined as where the optical depth exceeds some threshold and this will occur at a low enough density that even at a few hundred kelvin, the ratio of the Fermi energy to the thermal energy is too low for significant degeneracy. In addition, at these temperatures, the ...

5

Fun question. Without doubt it would be very violent and spectacular, but not much is known about stellar collisions and only a few have ever been observed. Most stellar collisions happen due to tight orbits where the stars spiral in towards each other or perhaps, 3 or more body chaotic star systems with unstable orbits that lead to an impact. Space ...

5

Certainly a red-dwarf star can have enough energy for a planet around it to be in the goldilocks zone. There are some difficulties with red-dwarf stars and Earth like planets. The planet would need to be very close to the star and as a result, tidally locked. The orbital period would be quite short, so there would be no seasons and one side of the ...

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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 ionised carbon, oxygen or helium; $\mu_e = 1$ for hydrogen, $\mu_e= 56/26$ for iron (56)). This assumes a white dwarf star of uniform, ionised composition and an equation of ...

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 ...

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