I need some ideas for a physics or astronomy related speech?
That isn't much to go on. What is the target audience? What is your knowledge level on the subjects? Is there a specific or general area you want to focus on or are supposed to be focusing on? How in depth do you want to go? Etc...
Drakkith, those are all important questions any speaker needs to answer.
.net: One more: How much time have you been given? You will need to be sure you fall within the time limits allowed for your speech.
The easiest subjects to speak about are those that you know best.
Separate names with a comma.