<p>The answer to your question is both yes and no, depending on the circumstances.</p>

<p>Two white dwarfs colliding would likely yield a <a href="https://en.wikipedia.org/wiki/Type_Ia_supernova#Double_degenerate_progenitors" rel="nofollow noreferrer">Type Ia supernova</a>, assuming the combined mass exceeded the <a href="https://en.wikipedia.org/wiki/Chandrasekhar_Limit" rel="nofollow noreferrer">Chandrasekhar limit</a> (<span class="math-container">$\sim1.4$</span> solar masses). The unstable object resulting from the collision could not be supported by electron degeneracy pressure; when the temperature rises rapidly due to the collision, there is nothing to stop it (compare this to thermal pressure in a "normal" star, which can make the star expand or contract to compensate for changes in temperature).</p>

<p>The rise in temperature triggers fusion, which then increases the temperature, which triggers more fusion . . . and so on and so on, in a <em>very</em> short time span. This is the same process, more or less, as in an accretion-powered type Ia supernova. The resulting explosion destroys the object, ejecting matter into space.</p>

<p>As with normal Type Ia supernovae, this sort of collision would likely happen in a binary system, with two white dwarfs in close orbits losing energy to gravitational waves and spiraling in (the odds of two unrelated white dwarfs colliding is very, very low). I'm not sure how luminous this event would be in the gravitational wave spectrum; likely fainter than a neutron star-neutron star inspiral but still strong. The speeds of the components could be quite fast, meaning that a lot of energy would be released in the collision.</p>

<p>Things are slightly more complicated if the combined mass is less than the Chandrasekhar limit. An example system that is predicted to undergo a collision is <a href="https://en.wikipedia.org/wiki/SDSS_J0106-1000" rel="nofollow noreferrer">SDSS J010657.39-100003.3</a>. The total mass of the two white dwarfs is <span class="math-container">$\sim0.6$</span> solar masses, certainly below the limit. In roughly 37 million years (<a href="https://academic.oup.com/mnrasl/article/413/1/L101/1747379" rel="nofollow noreferrer">Kilic et al. (2011)</a>), they will collide and form a <a href="https://en.wikipedia.org/wiki/Subdwarf" rel="nofollow noreferrer">subdwarf</a> - fusing helium, interestingly enough (compare this to the carbon/oxygen fusion in Type Ia supernova, which in those circumstances is unstable). Similar scenarios could play out in other systems.</p>

<p>This sort of a star would not be a "normal" star in the sense that it wouldn't truly be on the main sequence - just a bit below it. It would be an O- or B- type subdwarf, less luminous than main sequence stars of the same spectral type. I saw a couple of articles on these objects claiming that they would be stars just like the Sun - it's not true! Given that white dwarfs are highly depleted in hydrogen - remember, they are the degenerate cores of old stars - there can be virtually no hydrogen burning, even in a shell. Core helium burning is the only option.</p>

<p>On a final note, one explanation for the formation of <a href="https://en.wikipedia.org/wiki/R_Coronae_Borealis_variable" rel="nofollow noreferrer">R Coronae Borealis variables</a> is the collision of two white dwarfs. I don't think we have solid evidence yet for preferring this over other models (e.g. something involving hydrogen loss from the outermost shell), but if it <em>is</em> the correct pathway, then the answer to your question could be a definitive yes. Note that this would require intermediate-mass white dwarf progenitors.</p>

CC BY-SA 3.0
2017-12-18T03:40:42.600Z
I'd add to that that a main sequence star below the Chandrasekhar limit needs to fuse Hydrogen to look like a star.  Helium fusion by this method would make a very unusual star I would think.
userLTK
CC BY-SA 3.0
2017-12-18T14:28:34.107Z
In the supernova situation, wouldn't the relative speed affect the outcome?
Martin Argerami
CC BY-SA 3.0
2017-12-18T14:32:41.527Z
I can't stay up that late; can someone DVR the collision for me?  :-)
Carl Witthoft
CC BY-SA 3.0
2017-12-18T15:19:21.433Z
@userLTK Very good point; edited. Given that white dwarfs contain no significant amounts of hydrogen, hydrogen fusion is not going to happen in a subdwarf like these.
HDE 226868
CC BY-SA 3.0
2017-12-18T15:20:17.730Z
I've clarified that a bit. We can expect the collision speeds to be quite high; these objects are the result of mergers in tight binary systems, implying high orbital speeds, high collision speeds, and high energies.
HDE 226868
CC BY-SA 3.0
2017-12-18T16:42:20.757Z
I'm not sure if the low mass WD merger form a stable He burning star.  Since the majority of the matter in the WD's is degenerate I'd assume a helium flash would be triggered and consume most of the He.  From there without a large blanket of hydrogen plasma on top to damp the energy release wouldn't the merger blow itself apart in some form of anomalous supernova?
Dan Is Fiddling By Firelight
CC BY-SA 3.0
2017-12-18T18:25:04.217Z
Interesting! There must be numerous examples of the remnants of both scenarios out there given the abundance of binaries. Surely the first (supernova 1a) situation must leave a remnant behind, also.
Jack R. Woods
CC BY-SA 3.0
2017-12-18T22:39:52.353Z
Adams & Laughlin https://arxiv.org/abs/astro-ph/9701131 look at brown dwarf collisions in the degenerate era, a slow process providing a handful of red dwarfs. I have done some simulations of the corresponding white dwarf process, which has a cross section about 100 times less: it is too rare to really deplete white dwarfs before the galaxy dynamically dissolves. But it will provide rare lightshows and some unusually bright stars for more than $10^{20}$ years.
Anders Sandberg
CC BY-SA 4.0
2017-12-18T02:16:02.940Z
2019-02-13T02:20:14.863Z
2019-02-13T02:20:14.863Z
HDE 226868
<p>Would the temperatures during such a collision be able to ignite nuclear fusion, bringing the dead star back to life? If so, would it only be able to fuse for a short time before running out of fuel, or would it turn into a full-fledged star?</p>

CC BY-SA 3.0
2017-12-18T14:51:05.800Z
Are white dwarfs no longer considered stars? First Pluto, now white dwarfs?
Todd Wilcox
CC BY-SA 3.0
2017-12-18T15:52:49.890Z
I initially thought it was a post from scifi.stackexchange.com :D
algiogia
CC BY-SA 3.0
2017-12-19T00:15:48.650Z
@ToddWilcox what, do you suggest _Pluto is not a star anymore_? You've destroyed my world-view there... next you'll tell me Jupiter is not made of ice cream...
leftaroundabout
CC BY-SA 3.0
2017-12-20T07:54:31.950Z
@ToddWilcox: "Are white dwarfs no longer considered stars?" has a delightful touch of the metaphysical about it - akin to "Are dead people no longer considered human?
Chappo Hasn't Forgotten
<star-formation><white-dwarf>
If two white dwarfs collided, would they become a star?
Gliese
# If two white dwarfs collided, would they become a star?

(Asked by: username_0 on 2017-12-18)

Would the temperatures during such a collision be able to ignite nuclear fusion, bringing the dead star back to life? If so, would it only be able to fuse for a short time before running out of fuel, or would it turn into a full-fledged star?

**Tags:** star-formation, white-dwarf
> **Comments:**
> **username_1**: Are white dwarfs no longer considered stars? First Pluto, now white dwarfs?  
> **username_2**: I initially thought it was a post from scifi.stackexchange.com :D  
> **username_3**: @ToddWilcox what, do you suggest _Pluto is not a star anymore_? You've destroyed my world-view there... next you'll tell me Jupiter is not made of ice cream...  
> **username_4**: @ToddWilcox: "Are white dwarfs no longer considered stars?" has a delightful touch of the metaphysical about it - akin to "Are dead people no longer considered human?  

---

## 1 answers

### Answer by username_5 on 2017-12-18. Score: 38 | Accepted answer

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](https://en.wikipedia.org/wiki/Type_Ia_supernova#Double_degenerate_progenitors), assuming the combined mass exceeded the [Chandrasekhar limit](https://en.wikipedia.org/wiki/Chandrasekhar_Limit) ($\sim1.4$ solar masses). The unstable object resulting from the collision could not be supported by electron degeneracy pressure; when the temperature rises rapidly due to the collision, there is nothing to stop it (compare this to thermal pressure in a "normal" star, which can make the star expand or contract to compensate for changes in temperature).

The rise in temperature triggers fusion, which then increases the temperature, which triggers more fusion . . . and so on and so on, in a *very* short time span. This is the same process, more or less, as in an accretion-powered type Ia supernova. The resulting explosion destroys the object, ejecting matter into space.

As with normal Type Ia supernovae, this sort of collision would likely happen in a binary system, with two white dwarfs in close orbits losing energy to gravitational waves and spiraling in (the odds of two unrelated white dwarfs colliding is very, very low). I'm not sure how luminous this event would be in the gravitational wave spectrum; likely fainter than a neutron star-neutron star inspiral but still strong. The speeds of the components could be quite fast, meaning that a lot of energy would be released in the collision.

Things are slightly more complicated if the combined mass is less than the Chandrasekhar limit. An example system that is predicted to undergo a collision is [SDSS J010657.39-100003.3](https://en.wikipedia.org/wiki/SDSS_J0106-1000). The total mass of the two white dwarfs is $\sim0.6$ solar masses, certainly below the limit. In roughly 37 million years ([Kilic et al. (2011)](https://academic.oup.com/mnrasl/article/413/1/L101/1747379)), they will collide and form a [subdwarf](https://en.wikipedia.org/wiki/Subdwarf) - fusing helium, interestingly enough (compare this to the carbon/oxygen fusion in Type Ia supernova, which in those circumstances is unstable). Similar scenarios could play out in other systems.

This sort of a star would not be a "normal" star in the sense that it wouldn't truly be on the main sequence - just a bit below it. It would be an O- or B- type subdwarf, less luminous than main sequence stars of the same spectral type. I saw a couple of articles on these objects claiming that they would be stars just like the Sun - it's not true! Given that white dwarfs are highly depleted in hydrogen - remember, they are the degenerate cores of old stars - there can be virtually no hydrogen burning, even in a shell. Core helium burning is the only option.

On a final note, one explanation for the formation of [R Coronae Borealis variables](https://en.wikipedia.org/wiki/R_Coronae_Borealis_variable) is the collision of two white dwarfs. I don't think we have solid evidence yet for preferring this over other models (e.g. something involving hydrogen loss from the outermost shell), but if it *is* the correct pathway, then the answer to your question could be a definitive yes. Note that this would require intermediate-mass white dwarf progenitors.

> **Comments:**
> **username_6**: I'd add to that that a main sequence star below the Chandrasekhar limit needs to fuse Hydrogen to look like a star.  Helium fusion by this method would make a very unusual star I would think.  
> **username_7**: In the supernova situation, wouldn't the relative speed affect the outcome?  
> **username_8**: I can't stay up that late; can someone DVR the collision for me?  :-)  
> **username_5**: @userLTK Very good point; edited. Given that white dwarfs contain no significant amounts of hydrogen, hydrogen fusion is not going to happen in a subdwarf like these.  
> **username_5**: I've clarified that a bit. We can expect the collision speeds to be quite high; these objects are the result of mergers in tight binary systems, implying high orbital speeds, high collision speeds, and high energies.  
> **username_9**: I'm not sure if the low mass WD merger form a stable He burning star.  Since the majority of the matter in the WD's is degenerate I'd assume a helium flash would be triggered and consume most of the He.  From there without a large blanket of hydrogen plasma on top to damp the energy release wouldn't the merger blow itself apart in some form of anomalous supernova?  
> **username_10**: Interesting! There must be numerous examples of the remnants of both scenarios out there given the abundance of binaries. Surely the first (supernova 1a) situation must leave a remnant behind, also.  
> **username_11**: Adams & Laughlin https://arxiv.org/abs/astro-ph/9701131 look at brown dwarf collisions in the degenerate era, a slow process providing a handful of red dwarfs. I have done some simulations of the corresponding white dwarf process, which has a cross section about 100 times less: it is too rare to really deplete white dwarfs before the galaxy dynamically dissolves. But it will provide rare lightshows and some unusually bright stars for more than $10^{20}$ years.  

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