Stars can easily fuse atoms to give of heat and radiation. But at Wikipedia it said that only sub-iron atoms give of energy when fused and take energy when split, and post-iron atoms is the exact opposite. So, if enough heavy elements got together could a "reverse star" be made in which it gives of light and heat through means of fission? How long would such a star last, and why aren't they common, if they even exist?
I think it's an interesting question. The trick would be a sustained fission reaction, faster than half life, but slower than a chain reaction. A chain reaction could hardly be considered "star like" - it would just explode.
Lets say you had a planet sized object, maybe 100 parts Iron, basically inert, to 1 part Uranium , which would generate heat through half life decay and some neutrons would spread from uranium to uranium - many wouldn't. And as the object got hotter, more and more uranium would drift towards the center. You'd have gradual heating, and the object, in time, would probobly begin to glow. I'm guessing it's not possible using anything close to pure uranium, as the neutrons released in a uranium rich ball of matter would do 1 of 2 things - either blow the thing apart, or increase the rate of reaction and cause a chain reaction. But i think, with the proper mix of uranium and inert elements, and the uranium spread out, not all in the center, you probobly could get something sort of star-like. It would have much less available energy so it wouldn't burn nearly as long as a star. At least, that's my thought experiment answer to this question.
The problem with nothing but heavy, neutron rich elements is that, I don't see any way that wouldn't cascade and burn or blow up quickly. The nuclear reaction is driven by neutrons hitting the nucleus with potential energy and as the reaction happens, more free Neutrons are produced which speeds up the reaction significantly. A ball of of Uranium many miles across would undergo fission and explode. A ball heavy enough to not explode - say, jupiter sized, it would heat up and burn quickly and then settle down and cool off slowly. - perhaps similarish to a star going Nova then settling into a white dwarf.
If you have a material inside that slows the reaction, absorbs and re-admits Neutrons you could probobly generate a sustained burn - similar to what happens inside a nuclear plant while maintaining enough gravity to keep the object in one piece and over time, That would heat up and glow like a star in time and last for a while. Planet sized, even moon sized would probobly be more than sufficient, but you'd need the material to slow the reaction and keep the Uranium spread out. That's my take at least.
It is very unlikely with the normal fission process for most of the elements. They can be divided into 2 groups: slow reaction and fast reactions.
The elements with slow reactions do not generate enough energy in a short enough time to be able to heat sufficiently to provide light.
The elements with fast reactions would disappear before enough accumulated to form a large body.
Also a fusion chain reaction is not likely since the combination of material to maintain a self sustained reaction for thousands of years is very remote.