Who was Galileo's student and correspondent on heliocentrism?

Galileo's student and correspondent on heliocentrism was Benedetto Castelli.

Castelli, born Antonio Castelli in Brescia in 1578, became a Benedictine monk in 1595 and adopted the religious name Benedetto. He studied at the University of Padua, where Galileo was an important academic influence. Castelli later became a mathematician and professor, succeeding Galileo at the University of Pisa and eventually teaching at Rome's Sapienza University.

Their relationship was both personal and intellectual. Castelli supported Galileo's astronomical work, including studies of sunspots, and participated in discussions of Copernican theory. In December 1613, Grand Duchess Christina of Tuscany questioned Castelli about whether belief in a moving Earth conflicted with Scripture. Galileo responded with a detailed letter to Castelli arguing that biblical passages should not be read as scientific textbooks. That letter circulated widely and became an important document in the Galileo affair.

Castelli is sometimes remembered mainly as Galileo's disciple, but he was also an accomplished mathematician and hydraulic engineer. His students included Evangelista Torricelli and Bonaventura Cavalieri.