Inelectronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit. Its essential component is a metal wire or strip that melts when too much current flows, which interrupts the circuit in which it is connected. Short circuit, overloading, mismatched loads or device failure are the prime reasons for excessive current.
A fuse consists of a metal strip or wire fuse element, of small cross-section compared to the circuit conductors, mounted between a pair of electrical terminals, and nclosed by a non-combustible housing. The fuse is arranged in series to carry all the current passing through the protected circuit. The resistance of the element generates heat due to the current flow. The size and construction of the element is (empirically) determined so that the heat produced for a normal current does not cause the element to attain a high temperature. If too high a current flows, the element rises to a higher temperature and either directly melts, or else melts a soldered joint within the fuse, opening the circuit.
Nuclear fusion is a high pressure, high temperature nuclear reaction in which light atoms are "fused" together to create heavy elements. It is the gravity of the sun which creates the temperature and pressures needed for fusion, and it is the outward-pushing fusion that prevents the star from collapsing in on itself. The sun begins with hydrogen and turns it into helium, which then may or may not be fused again to form heavier things. The heaviest fusion substance in the sun is iron, and as iron and even the lighter products like helium fill up the core, it signifies the slow death of the star. Fusion stops when there is not enough light fuel for fusion left.