Transcript: Conceptually we can divide the evolution of stars into three rough stages: the early stages or pre-main sequence stages of evolution, the main sequence itself when hydrogen is converted into helium, and post pain sequence stages which vary depending on the mass of a star. To take the example of the Sun, the Sun has spent roughly thirty million years reaching the main sequence, will spend nine billion years in total on the main sequence, followed by about a billion years as a red giant, and then a large or essentially infinite amount of time as a cooling white dwarf. What this implies for censuses of stars outside the main sequence is that the visibility of a star depends roughly on the amount of time that it spends in each evolutionary stage. So if a star like the Sun spends less than one percent of its time reaching the main sequence and about ten percent of its time as a red giant, then if we surveyed a hundred stars like the Sun at different stages of their evolution we’d find only about one on its way to the main sequence, ninety percent we’d find on the main sequence, and about ten percent we’d find as red giants. This is not the whole truth however because the different evolutionary stages have different luminosities, and short-lived but luminous stages of stellar evolution are much more visible than long-lived or dim stages of stellar evolution. Thus the stellar catalogs are over represented in the short-lived luminous stages such as red giants and of course supernovae.