Actually, there are very metal-poorstars (subdwarfs[?]) that lie just below the mainsequence although they are fusing hydrogen, thus marking the lower edge of the mainsequence's fuzzyness due to chemical compositon.
Stars usually enter and leave the mainsequence from above when they are born or they are starting to die, respectively.
In the lower-left is where white dwarfs are found, and above the mainsequence are the red giants and supergiants.
A star on the mainsequence is one that is generating light and heat by the conversion of hydrogen to helium by nuclear fusion in its core.
A star's position and length of stay on the mainsequence depend critically on mass.
The least massive, hydrogen-burning stars, the red dwarfs, sit to the lower right and may remain on the mainsequence for hundreds of billions of years.
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