Say the size of the Sun. I know Hydrogen will form a sun, but what about rocks?
Say you smash a load of planets together, so that there mass is similar to that of the Sun, will some kind of weird star form?
Or will it just sit there?
It depends on the overall chemical composition of the planets you’re starting with. Gas giants like Jupiter and Saturn are mostly hydrogen and helium anyhow, so if you piled enough of those together you’d just get a garden-variety star.
Rocky planets like Earth are trickier. In rough terms, a star is just a ball of matter that has compressed itself (via its own gravity) to a sufficient density that the nuclei inside it are smashed together via nuclear fusion. When nuclei lighter than nickel get smashed together, they release energy that helps the star hold itself up against its own gravity. However, with heavier nuclei, there is (a) more gravity to overcome and (b) less energy released, so the star can’t support itself for as long by burning the same amount (the same mass) of fuel. If you piled a bunch of Earths together, you might get something that would shine for a brief amoung of time, but not for nearly as long as a conventional star.
You might also be interested in this exceedingly silly and fascinating thread from a few years back.
Rock is not going to fuse easily, if at all, so there won’t be any fusion producing pressure in the center, and the rock-star will start off smaller than the Sun, due to its higher density, so gravity will be more relevant. It might be able to support itself for a little while from heat produced in its coalescence, but once that’s gone, it’ll collapse into a neutron star.
Thank you so much for that link. Somehow I must have missed that thread the first time around. 
Depends on the size you want and what you want.
Earth probably glowed pretty nicely from gravitational energy heat for 10 million years give or take. It took in the range of billion before just the surface cooled down to a solid.
Surface area goes up as the square, volume as the cubed. Make something significantly more massive than the earth, its going to be glowing hot for a 100 million to a billion years without too much trouble. And thats ignoring any possible minor fusion processes that might be going on.
Yeah, and you can only get so massive before neutron star collapse. Whats that mass limit if its the density of rock/iron ?
I think it’d probably marry and divorce a few starlets in Hollywood before dying of a cocaine overdose. You know, like all other rock stars.
I understand your point, but I disagree. The OP contemplates a coming together of rock, which I would understand to be stony asteroids, moons, etc.
The major difference between Jupiter and, say, Proxima Centauri is size – Proxima is big enough to begin hydrogen fusion, while Jupiter is not, but they are comprised o the same list of constituent elements, with hydrogen being far and away the largest fraction. We can presume that any truly random coming together of space debris will, by the law of averages, be predominantly hydrogen and helium, from their ubiquity. Even if the coalescence is of Oort or Kuiper belt material, iceballs and icy minor planets, the result will still be dominated by hydrogen and helium.
On the other hand, a coalescence of terrestrial planets, the colission of a few million Earths or Venuses, would be largely iron and nickel, the cores so composed being the greater part, and, with appropriate admixtures of heavier elements, the dense matter that would coalesce at the core of the astronomical body so formed.
As against this, the OP supposes that rocky bodies coalesce. Not random material dominated by hydrogen, not tettrestrial planets dominated by nickel-iron cores, but 10^n stony asteroids, satellites, minor planets, etc.
And rocks are predominantly oxides, carbonates, sulphates, etc., of light metals and of silicon: calcium, magnesium, aluminum, and the like.
Such a coalescence would eventually melt, and then boil, under gravitational pressure, producing a gaseous body radiating in the infrared. It would not fuse; correct so far. A Sun-sized mass of rock would be a gaseous ball with oxygen the largest of a wide spectrum of constituents. The silicon will concentrate at the core, with a shell of the most massive light metals surrounding it, etc.
But keep piling it on, and it will continue to increase heat and pressure. Eventually either the core or the innermost shell will achieve the proper pressure/temperature level for fusion to begin. Now, remember we are talking improbabilities here – but that’s what the OP asks. We are concentrating rocks, and we would have to do this to the level of the most massive stars to achieve fusion. But without limit to the accretion of rocks, sooner or later achieve it we will.
And at that point the melted-rock cocentration we’re supposing would ignite, in stellar-fusion terms, and follow the final stages of giant-star evolution, eventually going supernova and collapsing into a dense-matter state (take your pick on where it’ll stop).
Correct me where I’m wrong on this if I’ve made any mis-assumptions.
First of all, not everything will fuse at all (at least, not in a way that will release energy): The Earth and similar planets are mostly iron, which is the bottom of the energy curve, so if our “rock” is mostly iron, we’re completely out of luck on the fusion question. Second, while things lighter than iron (silicon, say, or oxygen, the primary components of most “rock”) will fuse exothermically eventually, the OP said that we’ve got the mass of the Sun, which wouldn’t lead to pressures high enough to ignite them: The Sun isn’t massive enough to fuse anything above helium. Even if there’s trace hydrogen in the rocks, that wouldn’t fuse, either, since you need a purity of about 90% hydrogen or more to really get anywhere with hydrogen fusion.
Well the OP did say the size of the Sun ;), so I’m going to say that if you gathered together a truly honking amount of silicate rock, you would eventually get something like a white dwarf or the core of a giant star, where the pressure and termperature is enough for oxygen to undergo fusion reactions. It’s hard to say just how such an object would evolve because there’s apparently no way one could form naturally; in giant stars the pressure of overlayers of lighter elements makes a difference. I wouldn’t even know where to begin taking into account the effect of other elements in the nuclear reactions.
I specified that the “rock” I was postulating was largely silicates, carbonates, oxides, etc. of light metals. These elements will fuse given sufficient temperature and pressure. I completely agree that “rocks” with Earth’s composition would end up with an iron-nickel core that would nevr fuse. But I took ‘rock’ to have its everyday meaning - where I go outside and pick up a rock and it is limestone, sandstone, granite, feldspar, orthoclase, etc. And these, or more precisely their constituent elements’ nuclei, will fuse.
Ah, I see your objection. It looked like you were saying “rock as opposed to hydrogen”, which didn’t make much sense, rather than “rock as opposed to iron”. I still maintain that it would reach the neutron star stage before conditions got to where oxygen could fuse.
See Woodstock, 1969.
As Lumpy notes, the closest real-life analog to the OP’s pile o’ Earths is probably the core of a massive star (above eight solar masses or so) at the end of its life. At this stage, such a star will still have lighter elements like helium & oxygen that are being burned in its outer layers; but in the core, you’ll have neon and silicon being fused as well. The heavier elements aren’t fused for long before the star goes supernova; Table 1 in this review; says that a star with a mass twenty times that of the Sun will be turning oxygen into silicon during roughly the last year of its life, and turning silicon into iron only in the last couple of weeks before the supernova. So on the face of it, you might think that you could get a pile o’ Earths to last for a day or two before collapsing in a Horrendous Space Kablooie; it would just look like the core of a massive star if you stripped away the outer layers.
That said, comparing our pile o’ Earths to a massive star’s core would be like trying to figure out the MPG of a Prius from our knowledge of steam locomotives. If I had to guess, I’d say that your new star wouldn’t last more than a couple of days before it went supernova — and as Chronos asserts, it might not have a stable stage at all. If you ever raise the funds to run this experiment, I’m sure that a fair number of astrophysicists would get a betting pool going on how long it would last — and there’d probably be at least one taker for any plausible scenario you could name.
Actually, the spot I’ll take in that betting pool would be “stays ‘stable’ for a while from the heat of accretion, but no fusion”. I can’t say off the top of my head how long it’d stay hot enough for that without calculations that I probably won’t bother to do, but I’d guesstimate something of order a year or so.