Stars with at least half the mass of the Sun can also begin to generate energy through the fusion of helium at their core, whereas more-massive stars can fuse heavier elements along a series of concentric shells.
22.
The consequence of this result is that if one imagined breaking the sphere up into a series of concentric shells, each shell would collapse only subsequent to the shells interior to it, and no shells cross during collapse.
23.
During the Hellenistic period this was largely replaced by a more " scientific " model as imagined by Greek philosophers, according to which the earth was a sphere at the centre of concentric shells of celestial spheres containing the sun, moon, stars and planets.
24.
The new machine, which the Los Alamos National Laboratories and the Defense and Energy departments helped build, required perfecting a system of concentric shells to isolate the measuring device from any vibration, sound, light, temperature change or even seismic activity that could disturb its calculations.
25.
Adell and Baltzer, of L . S Starrett Co ., patented a design with concentric shells and pins in cam slots as the triggering mechanism, and Adell and Starrett received a 1907 patent for a design using a sliding block actuated by a tapered bore in the body to hold back the hammer.
26.
The service chiefs insisted on a yield of between 10-12 kt for two reasons : firstly, to minimise usage of scarce and expensive fissile material; and secondly, to minimise the risk of predetonation, a phenomenon then little understood, and the primary reason for using a composite core of concentric shells of plutonium and U-235.
