Nebula. Collapse. Protostar. Main Sequence. Red Giant. Planetary Nebula. White Dwarf.  This is the cycle of life for a star like our Sun.  By observing stars across the galaxy, we see snapshots of different points in a star’s life cycle.  It’s the same with people; If you went for a walk in a city and observed people for a day, you would see every single stage of a person’s life: Infant, child, adolescent, youth, adult, middle aged, senior.  How would you put them in order if you knew nothing about them? With people you might go up and ask them, but with stars we can...