Paul uses a question for emphasis. You can translate this as a statement. Alternate translation: "But, I say the Jews certainly have heard the message about Christ" (See: [[rc://en/ta/man/translate/figs-rquestion]] and [[rc://en/ta/man/translate/figs-quotations]])
Both of these statements mean basically the same thing and Paul uses them for emphasis. The word "their" refers to the sun, moon, and stars. Here they are described as human messengers that tell people about God. This refers to how their existence shows God's power and glory. You can make explicit that Paul is quoting Scripture here. Alternate translation: "As the Scriptures record, 'The sun, moon, and the stars are proof of God's power and glory, and everyone in the world sees them and knows the truth about God.'" (See: [[rc://en/ta/man/translate/figs-parallelism]] and [[rc://en/ta/man/translate/figs-personification]] and [[rc://en/ta/man/translate/figs-explicit]])