The structure of graduate education in the US differs somewhat from that in Germany, one consideration is the candidate's current formal level of education - in the North American educational system, graduate school entrance usually requires a Bachelor's degree, whereas most German PhD programmes actually require a completed Master's degree, unless the programme has implemented a fast track option. Same is the case with UK. Many German universities have very attractive PhD astrophysics programmes. Currently there are a total of 37 research institutes or university departments active in astronomy and astrophysics in Germany and most of them offer the possibility to pursue a doctorate. Some of the best programmes in this field are located in US by way of Stanford, Harvard, MIT which provide generous financial assistance. These well-regarded universities from around the world have shown strength in producing research related to a number of topics in the field of space science. You should choose a University based on its scope, your needs as well as the financial packages available.

Attaching a link for the top programmes in the world

https://www.topuniversities.com/university-rankings/university-subject-rankings/2015/physics-astronomy

wish you the best!