As somebody has said, stars are easy to build, just put enough bananas somewhere in space and you’ll get a star.
But things are a bit more complicated. If you don’t have enough bananas, you will get a planet or a planetoid like Pluto, or even just a space rock.
On the other hand, as it is well known, if you have too many bananas you are going to create a black hole immediately. No sunshine there!
Moreover, not only the size, composition matters as well. If you have only – let’s say enough rocks for thousand Earths, you’ll never get a star either, because there is no fusion material there, to give the pile a certain fire?
Things are more complicated. There are fission materials even in bananas. And every star is not only a thermonuclear reactor where the fusion is going on, every star is a combined fusion-fission reactor. For a deuterium atom at least, it may decay before it is fussed. But there is a plenty of heavier elements in an average star, which also decay and contribute a little to the energy output.
If there are uranium and thorium inside Earth, they are in our Sun as well. The whole fission chain in fact. The majority of the energy of our Sun comes from the fusion, though.
Now, smaller objects like the Earth are almost exclusively fission powered from inside. Are smaller objects like Mars or the Moon cold inside? Not because they have exhausted their fuel, but because their ratio between surface and volume is such, that the produced heat escapes more easily. Still it is quite hot in the center of the Moon. Only the cold crust is relatively (and absolutely!) thicker in the Moon’s case. Fires of radioactivity are still burning there. Why would they have stopped, it’s not their time yet.
The crusts of objects like Earth or Venus are thinner for purely geometric reasons – surface/volume ratio. If you put a thousand Earths worth of granite in one place, you would get a Jupiter of molten lava. The crust would be red hot, a lava ocean!
Why? The bigger a granite planet, the more watts of power it produces by fission. When it is big enough, 1000 or more degrees Celsius hot surface is needed to emit this heat further to the cold space.
These kind of objects are likely among those “hot jupiters” around many stars. Particularly those not so near to its star, but still very hot. They are in fact small fission stars.
But for the smaller fission starts like Venus or Earth, they are hot several kilometers deep, and all the way to the center. To reach the hot spots, you must dig through rocks on Earth, or dive through tens of kilometers of gases on Venus.