The menstrual cycle is from Day 1 of bleeding to Day 1 of the next time of bleeding. Although the average cycle is 28 days, it is normal to have a cycle that is shorter or longer. Girls usually start having menstrual periods between the ages of 11 and 14.
In nuclear physics, nuclear fusion is a nuclear reaction in which two or more atomic nuclei come very close and then collide at a very high speed and join to form a new nucleus. During this process, matter is not conserved because some of the matter of the fusing nuclei is converted to photons (energy).
As time passes the fusion process causes He to accumulate in the core. The increase in the number of He nuclei begins to interfere with the H nuclei collisions and causes a reduction in the rate of H fusion (sometimes called helium poisoning). This reduces the thermal pressure, and the star will begin to contract.