Do supernova explosions produce lumps of uranium or atoms of uranium?
The nuclear reactions do not produce enough energy to balance the force of gravity.Stars of all masses spend the majority of their lives fusing hydrogen nuclei into helium nuclei: we call this stage the main sequence. When all of the hydrogen in the central regions of a star is converted into helium, the star will begin to "burn" helium into carbon. However, the helium in the stellar core will eventually run out as well; so in order to survive, a star must be hot enough to fuse progressively heavier elements, as the lighter ones become exhausted one by one. Stars heavier than about 5 times the mass of the Sun can do this with no problem: they burn hydrogen, and then helium, and then carbon, oxygen, silicon, and so on... until they attempt to fuse iron. . . .
from http://curious.astro.cornell.edu/supernovae.php [Broken]. . . . The lack of radiation pressure generated by the iron-fusing core causes the outer layers to fall towards the centre of the star. This implosion happens very, very quickly: it takes about 15 seconds to complete. During the collapse, the nuclei in the outer parts of the star are pushed very close together, so close that elements heavier than iron are formed.
What happens next depends on the mass of the star. Stars with masses between about 5 and 8 times the mass of our Sun form neutron stars during the implosion: the nuclei in the central regions are pushed close enough together to form a very dense neutron core. The outer layers bounce off this core, and a catastrophic explosion ensues: this is the visible part of the supernova.