In his commentary on Matthew, Aquinas focuses the journey of the Magi as the “first fruits of the Gentiles, because they were the first Gentiles to come to Christ.” By recognizing that their wisdom originated in Christ they came to worship Jesus. Because they were Gentiles coming to Christ he gives two reasons why it was a star that guided them:
For Aquinas, the story of the Magi, was the story of Gentiles being brought in to Christ because of the promises of Abraham. Since Abraham was to look to the skies and see that his descendants would be more numerous than the stars it was right for God to use a star to guide the magi. And because the Gentiles came to know God by his created things then it was also right for God to use a star to reveal Christ.
Taken from Aquinas’ Commentary on the Gospel of St. Matthew