There are various modes of Cepheids like single modes: fundamental (F), first overtone (1O), and likewise. And then we have double modes: F/1O, 1O/2O etc. triple modes: 1O/2O/3O etc. ...
How do scientists know there are about 300 billion stars in a galaxy and there are about 100 billion galaxies?
Just to think about this is mind boggling. But how do scientist get these numbers? What technology/system/theory do they they use?