deletions | additions       diff --git a/In_order_to_understand_how__.tex b/In_order_to_understand_how__.tex  index c679977..8adad12 100644  --- a/In_order_to_understand_how__.tex  +++ b/In_order_to_understand_how__.tex 
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In order to understand how a galaxies luminosity changes, you have to realize that there are new stars forming continuously in a galaxy. This means that while the luminosity of a group of stars the same age will increase when the first stars leave the main sequence, and decrease after that, the galaxy as a whole can not be modeled this way. This is because while some of themost luminous  giants are done finish  shell burning, new massive giants are replacing them at some other place in the galaxy. So, in general, the luminosity of the whole galaxy will decrease and increase with time. This is important because the mass will stay the same at all times, but the luminosity will increase and decrease depending on the stellar population at the time.