Galileo discovered the sun is the center of the solar system, he improved the telescope, he proved that the moon has craters on it, he claimed Venus has phases, and he observed sun spots on the sun.
I believe that at the time of Galileo and other pioneers in astronomy there was no formal astronomy study; he (and others) must have learned most of what they knew by themselves.
Galileo Galilei
Modern astronomy.
Astronomy
In astronomy, he used a telescope.
Yes
astronomy and philosophy.
yes.
From 1609 till his death.
Some of Galileo's most significant work was in the field of kinematics, identifying that the total ...by him (or to be written by him) were forbidden, though this latter part was not enforced. ... Galileo Galilei - major areas of study ...
Galileo
Astronomy/physics