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The Initial Mass Function (IMF) is a function that describes the distribution of mass mass'  of stars. Spiral galaxies are composed of all different types of stars ranging from massive stars, to low mass stars. The mass of a galaxy will always be constant as the galaxy ages, but the luminosity will change with age. This happens because as time progresses, stars run out of hydrogen in their core and turn off the Main Sequence and new stars are continually formed. If you analyze a small stellar group with the same age, the most massive stars deplete their hydrogen supply quickest and are the first stars to leave the Main Sequence. Once the stars turn off the main sequence, they begin shell burning which increases the luminosity of the star. This means, that when a star leaves the main sequence and enters the giant phase, it will be contributing more light than it did on the main sequence. Because it is contributing more light, the M/L ratio will decrease during this phase. Once that star has finished shell burning and its time as a giant, it essentially stops contributing light to the galaxy. The most massive stars will be the most massive giants and will contribute the most light. A giant 20 times the mass of the Sun will be more luminous than a giant that is the mass of the Sun. This means that as stellar population of the same age ages, the mass will always stay the same, but the luminosity will increase once the first stars become giants and continue to decrease. It will decrease because the stars that are no longer giants won't contribute to the luminosity of the system. Even though new stars are leaving the main sequence to replace the old giants, they will never be as bright because they're not massive enough.