It is well explained in Dennis T Hall
When voltage is applied to the fluorescent lamp, here's what happens:
∙ The starter (which is simply a timed switch) allows current to flow through the filaments at the ends of the tube.
∙ The current causes the starter's contacts to heat up and open, thus interrupting the flow of current. The tube lights.
∙ Since the lighted fluorescent tube has a low resistance, the ballast now serves as a current limiter.
When you turn on a fluorescent tube, the starter is a closed switch. The filaments at the ends of the tube are heated by electricity, and they create a cloud of electrons inside the tube. The fluorescent starter is a time-delay switch that opens after a second or two. When it opens, the voltage across the tube allows a stream of electrons to flow across the tube and ionize the mercury vapor.
Without the starter, a steady stream of electrons is never created between the two filaments, and the lamp flickers. Without the ballast, the arc is a short circuit between the filaments, and this short circuit contains a lot of current. The current either vaporizes the filaments or causes the bulb to explode.