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).
That is where there is a sufficient force to fuse them. In order for nuclear fusion to occur, two nuclei must be pushed together against the forces that push them apart. These forces are very large, so it takes an incredible amount of pressure to accomplish this. Because stars are so massive, the necessary pressure is encountered in their core due to gravity.