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Context For those interested

A Class 3 Kardashev Civilization has sent a small recon fleet (~1 million ships) to lay siege to system. Sometime after arrival, an extraordinarily dangerous entity is discovered on one of the worlds in the system; posing such a grave threat that it must be immediately destroyed with extreme prejudice. Such is the nature of the threat that your in system ships must be considered a total loss and sending more ships would only add to the losses. So the entire planet, and system for that matter because you can't be too safe, must be destroyed. Naturally, you call in a Super Nova Strike (really a Type 1a) and have the nearest white dwarf jumped into the solar atmosphere (specifically on/in the photosphere, upper convective zone) of the host star.

When I say jumped, I mean the White dwarf does not need to traverse the intervening space. It effectively winks into existence at its destination.

This is the way the civilization handles the situation because reasons, so please no answers containing "they should have done XYZ instead". And the recon fleet lacks the firepower to effectively deal with the threat.

The Meat of the Question

The host star has a mass roughly equal to the sun and the white dwarf is dangerously close to its Chandrasekhar limit, only needing a few hours to accrete the mass needed to push it over the edge and trigger a Super Nova. In the intervening hours before annihilation what happens to the planets in the system now that the host star has effectively doubled its mass? What would those on the surface of the planet experience?

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  • $\begingroup$ Nova and super nova are two different things, but you seem to be discussing both? $\endgroup$ – Ville Niemi Mar 20 '17 at 6:08
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    $\begingroup$ I really can't see the arrangement going Supernova. In effect you are merging a star, with a white dwarf. Granted most of the fuel in the white dwarf would be gone. But assuming a main sequence star, the host star would have plenty of fuel left. The result would be a really large main sequence star, which has access to hotter burning fusion cycles. The star would get hotter, release more photons, which stabilizes the gravitational collapse. In effect you get an older, blue giant. $\endgroup$ – Aron Mar 20 '17 at 7:14
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    $\begingroup$ @Aron It would not stabilize the white dwarf since no fusion would happen inside it, just on its surface. His idea is that since the dwarf is "dangerously close to its Chandrasekhar limit" it would ingest enough material to become a black hole and trigger a super nova. $\endgroup$ – Ville Niemi Mar 20 '17 at 7:47
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    $\begingroup$ @VilleNiemi Yes. But the material that is ingested is hot. My point is that there is a complex interplay between the main sequence star heating the white dwarf faster, or the white dwarf collapsing faster. Remember we also have potential energy in the mix, where the ingested material would gain linear and orbital kinetic energy, which would delay the collapse. My money is on the main sequence star. $\endgroup$ – Aron Mar 20 '17 at 7:52
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    $\begingroup$ @Aron My money would be on recurrent nova. The heat and compression would be sufficient for fusion. I think the likely result would be recurrent nova, but apparently the reaction could be stable and result in your model (more or less). $\endgroup$ – Ville Niemi Mar 20 '17 at 8:15
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The orbits of the planets will be affected since the central force is doubled but the current velocity of a body is unchanged. So they will move towards the sun.

It takes the Earth 2 months (6/π actually) to move a distance along its orbit equal to 1AU. So in a few hours its motion will not be a significant portion of that.

It will start moving inward at a fraction of the rate of a free-falling body, in addition to its original speed. That's of the same order of magnitude, since that’s how orbits work.

The people on the planets would not notice the orbit changing or the sun getting closer, in such a short time.

They might notice that the tides are bigger than expected, but the tides from the sun are a minor component and the tides are actually a resonating system of basin gyres, not the simple bulge you get on a water world. So again, a few hours is not enough time for things to move around much.

You can pretty much ignore it.

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    $\begingroup$ By using online simulators (like Super Planet Crash stefanom.org/spc ) you can see indeed that adding a massive body is disruptive for a planetary system, but hardly on few hours time scales. $\endgroup$ – L.Dutch - Reinstate Monica Mar 20 '17 at 7:17
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    $\begingroup$ For comparison, Earth moves at about 30 km/s in its orbit around the Sun (in a Sun-centric reference frame). In three hours, it moves 324,000 km. Even if Earth fell directly toward the Sun (toward nadir), which it wouldn't, remember that the orbital radius of the Earth around the Sun is ~150,000,000 km. In three hours it would move from, say, 150,000,000 km to 149,676,000 km. Even if you allow for 24 hours, that's still only about 2.6M km, going from 150,000,000 km or thereabouts to 147,408,000 km. To match the distance changes in Earth's normal orbit, you need about 5M km. Totally negligible. $\endgroup$ – user Mar 20 '17 at 7:24
  • $\begingroup$ @MichaelKjörling Please make that "heliocentric reference frame" not "Sun-centric etc etc". It may mean the something, but it will feel more technically correct. $\endgroup$ – a4android Mar 20 '17 at 7:55
  • $\begingroup$ @a4android (1) I ran out of space in the comment. (2) It is too late to edit the comment. (3) Particularly here on Worldbuilding, I generally try to favor understandability without subject field expertise over using technical jargon when technical jargon isn't actually required. $\endgroup$ – user Mar 20 '17 at 7:59
  • $\begingroup$ @MichaelKjörling In which case, you are exonerated. I was just being pedantic. Clarity is to be preferred to technicality any day of the week. Good point about the changes to the Earth's orbit. $\endgroup$ – a4android Mar 20 '17 at 8:03
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The immediate effects will be limited

Discussed in JDLugoz's answer, and other comments. The distances on a solar system scale are too large for there to be immediate effects on orbits that are noticeable.

You will not get a black hole

First off, passing the Chandrasekhar limit will not directly turn a white dwarf into a black hole. There is an intermediate stage of a neutron star. The white dwarf is made of electron degenerate matter, while the neutron star is made of denser neutron degenerate matter.

Even if you dropped the white dwarf into the core of the main sequence star, so that all the matter merged right away, you still probably won't get a black hole. The upper limit for a neutron star is about 2-3 solar masses; you have ~2.44 and a good portion of that will be blown off by a supernova. I'm going to stick with 'probably' here, since stellar dynamics are poorly understood at best.

This reaction will take too long

According to your Wikipedia link, the consensus is that as a white dwarf reaches the limit of the mass that can be supported by electron degenerate matter, the increasing pressure and temperature in the core causes convection to start for ~1000 years and a re-ignition of carbon fusion. This causes an expanding front of carbon fusion with oxygen fusion following behind which propagates through the star.

The first thing the solar system will notice are neutrinos

Assuming that you added matter quickly enough to the white dwarf to blow right through the convection stage (I don't know if that is possible), the carbon and oxygen burning phases will run their course in a matter of seconds. In both of those processes, energy losses due to neutrinos become significant due to proton-proton side reactions. For carbon fusion they are equal to the thermal energy produced by carbon fusion, for oxygen I could get not exact comparison.

Neutrinos, while very weakly interacting with matter, would be relevant on supernova scales, as discussed in xkcd. There is no hard-science tag to motivate me to do full calculations, but the initiation of carbon and oxygen fusion should provide heavy neutrino flux through the solar system. Wikipedia suggests a release of 1e44 J of thermal energy, coupled with a similar release of neutrinos. This isn't that far off the 1e46 J of neutrinos in a 10 sec burst from a collapsing Type II supernova.

Not much warning all in all

Unless those neutrinos add up to something significant there isn't much other warning that the solar system is about to be crushed with a shock wave.

The change in the skies, as viewed from the planet, will be evident as soon as the second star shows up. The convective temperature increase of the dwarf will make it brighter and brighter over the hours until explosion.

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So, depending on how close the white dwarf is dropped next to the primary star, there might be one or more smaller novae before the full supernova. That would probably be enough on its own to flash-heat the planet quite a bit, but the ejecta of the nova itself probably wouldn't have enough time to reach the planets before the supernova itself. And once the supernova goes off, the eject from that will be moving almost an order of magnitude faster, so it's likely to overtake the ejecta from the initial nova(e) before any potential impact.

I do have to wonder, though, why they don't just drop the white dwarf directly on the planet in question. Or drop two stars: a red dwarf directly onto the planet to make sure it goes immediately, and then the white dwarf for a Supernova strike to sterilize the system and surrounding area.

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  • $\begingroup$ The white dwarf is dropped directly on the surface of the host star, and would begin to fall into it. Due to the nature of the threat it is feared that failing to wipe it out in a single strike would allow it to escape. Their reasoning is why use two stars to accomplish what one could do just as easily. $\endgroup$ – Joe Kissling Mar 20 '17 at 14:38

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