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Imagine a gigantic cube 5 times the size of the sun and 5 times as massive ramming its way Into the sun, would the sun just explode or fade out as its smashed by the ram?

*please assume the ram object is impervious to damage

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    $\begingroup$ impervious to damage - now that's a neat trick. $\endgroup$ – AndreiROM Dec 21 '16 at 17:06
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    $\begingroup$ Well, it has to be pretty well impervious in order to remain a cube, instead of contracting to a sphere under the influence of its gravity. $\endgroup$ – jamesqf Dec 21 '16 at 18:18
  • $\begingroup$ What cube all I see is 4 Andromeda galaxies... $\endgroup$ – user6760 Dec 22 '16 at 4:33
  • $\begingroup$ @jamesqf: I think you mean collapsing into a black hole. $\endgroup$ – Joe Bloggs Dec 22 '16 at 9:59
  • $\begingroup$ @Joe Bloggs: No, I don't. 5 solar masses isn't going to collapse into a black hole, at least not directly. Internal heat from gravitational contraction will keep it going a while, and depending on composition, it might start fusion reactions and become a star. $\endgroup$ – jamesqf Dec 23 '16 at 19:15
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I'm assuming that by '5 times the size of the sun' you mean 5 times the effective volume, not 'with a width 5 times the diameter of the sun'. I'm also assuming that the ram is moving implacably through the solar system perpendicular to the plane of the ecliptic and has some motive force that will carry it on that path after collision rather than stopping.

With that out of the way:

Your cube is the same density as the sun! Hooray! Not that it really matters. What matters here is mass:

Your cube will start to affect the sun long before it reaches it, pulling the sun, the planets and basically everything in the solar system out of their positions. Before this cube gets to the sun (depending on its exact velocity) we can imagine that the planets will be sucked inwards and upwards towards the combined centre of mass of the sun/cube system, resulting in them potentially being:

A: Thrown out of the solar system

B: Crashing into the cube

C: Crashing into each other

D: Miraculously still orbiting something resembling the centre of the solar system, just in much crazier orbits

Needless to say even the approach of the cube has destroyed the solar system as you know it.

Then it gets close to the sun.

The first thing to note is that you said the ram is impervious to damage. I'm going to take that to mean it reflects all energy that hits it. This is hilarious for a few reasons:

1: The cube will appear very bright and will get brighter as it approaches

2: The reflected radiation from the cube will hit the sun again, meaning that as the cube approaches the sun the sun's own photosphere will start to be blown off by its own solar wind. The sun will also start to oh-so-slowly move away from the cube, propelled by it's own power (stellar fission rates behave weirdly with regards to temperature, but as a general rule more energy in an area leads to higher rate of fusion.

3: The solar wind reflected from the cube will start to push on the planets from an unusual direction, adding yet another vector to their orbits and playing merry hell with their magnetospheres. Expect some utterly astounding aurora.

Then the cube hits the sun. Again: the exact effects differ depending on the speed. Either it will be a relatively slow impact (below the escape velocity of the sun from the cube) with the sun spreading evenly across the face of the cube before 'running' off the edges or it will be a spectacularly energetic affair that causes the sun to 'splash' parts of itself away from the cube and into the interstellar void. Both of these will make the sun temporarily brighter as the sheer amounts of energy involved (and a lot of photons trapped in the sun's vast bulk) are liberated in the form of heat and light.

In either case the sun will lose the density it requires to maintain a stable fusion reaction. It will either do this because it's just been smeared into a thin cloud of hot gas by a high velocity impact, or it will do it because it's core has been replaced with an inert block and it is now a 'sea' of stellar elements (hydrogen, lithium, oxygen, carbon, a few heavy metals because it's a few generations old) surrounding the cube. In either case it goes 'dark' again, radiating only infrared.

And your cube continues on its way to who knows where.

So the answer to your question is: It depends. How fast is it going?

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    $\begingroup$ Now THAT'S an answer. $\endgroup$ – wyldstallyns Dec 21 '16 at 17:27
  • $\begingroup$ A large fraction of the Sun will be gravitationally captured by the five times solar mass cube. However, as you said, everything depends on the cube's velocity. A high-speed cube will spray the Sun every which way. A slow one will carry away most of the Sun's mass. None of this takes away from your good answer. Nicely done! Plus one from me. $\endgroup$ – a4android Dec 22 '16 at 8:03

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