Do Astronomers need a PhD in Physics to be well paid?

You would normally only get a PhD in one particular subject, not multiple PhDs, so to be an astronomer you'll need a PhD in astronomy.  

What degrees you'll need before doing the PhD vary depending on where you are.  In some places you'll do a bachelor's degree and a master's degree, in others you can go straight from a bachelor's to a PhD, while in some you'll do an integrated master's degree and never get a bachelor's.  These degrees might often be in physics, astronomy, 'physics and astronomy', or even in mathematics, depending on the conventions of the country and university or college - there may be no difference in the modules taken, or there could be a lot.  

If you want to be an astronomer, it's a good idea to take astronomy modules within whatever the degree is.  It's possible to get a degree named physics or mathematics without doing these, but an astronomy degree will contain a lot of physics and mathematics.  You'll want to check that you can do astronomy modules before you start a degree course.