Here is a young Luminous wearing his star shaped locket.
When he was born, many from all around the Light Realm came to offer gifts to the newborn child, among them was Lady Estrella, Luminous' aunt, who gave him this star locket that plays a soothing tune. As a child, Luminous wore that locket, and the music from the locket reminds him of his aunt. Luminous really loves this locket and can be seen wearing it around. At age 7, when Lady Estrella was away at the War of Light and Dark against Lord Akura, which Luminous was unaware of, he would hear the melody of his locket while he misses his aunt. After the war, where Estrella was going to the world in the distant cosmos to get her powers back, Luminous gave her his locket to always remember him when she is gone, and when she plays the melody of this locket, she would think of Luminous.