What is hypotonic, isotonic and hypertonic solution?

An isotonic environment is when the concentration of solutes and solvent (water) are the same. 

When a cell is hypertonic, it shrinks because the concentration of solvent/solutes are unequal. If the inside of the cell has less solutes and more solvent, the solvent inside (water) will diffuse out the cell because of the concept of "going doing the concentration gradient". Anything will travel from a high concentration to a low concentration. In the case of hypertonic, water will move out the cell and causes it to shrink. 

Hypotonic is when the cell is enlarged by water moving inside. So a hypotonic cell will look like it's big and expanded. Water goes where there is less concentration of it. You can also think about it from another perspective. Water always go where there is more solutes. So if the solute concentration like sodium or sugar or ect. is greater inside a cell or a piece of potato, then water will go there (since if there is a high concentration of solutes, then there is low concentration of solvent). 

Most smaller stars don't have a radiative zone.