Making fardh ghusal after sehri

Q: As the month of Ramadhaan approaches, I would like to ask the following:

1. If a person goes to sleep and upon waking up for sehri, he notices he is in need for a compulsory ghusal due to a dream he had. Can that person eat sehri, START HIS FAST and thereafter have a ghusal before fajr?

2. Or should he have a ghusal before sehri? (What if there is a fear of missing sehri?)

3. If a person does have a ghusal AFTER sehri before fajr, ONCE HE STARTED HIS FAST, will his fast be accepted/valid?

Studying astronomy

Q: "Is astrological study forbidden for us." I mean can we study it just to gain some new knowledge, not meant to use it for fortune telling... Just for getting knowledge about the effect of heavenly bodies on our natural behaviour.