Natsuhiboshi

is the title of the song that both Sumaru and Natsuhi sang in Hoshigakure Arc. It is a lullaby Natsuhi sang to her son when he was a baby. Sumaru usually sings the lullaby whenever he remembers his deceased mother.

Rōmaji
Natsuhiboshi, naze akai? Yuube kanashii yume wo mita. Naite hanashita. Akai me yo.

Natsuhiboshi, naze mayou? Kieta warashi wo sagashiteru Dakara kanashii yume wo miru

Natsuhiboshi, naze akai? Yuube kanashii yume wo mita. Naite hanashita. Akai me yo

Natsuhiboshi, naze mayou? Kieta warashi wo sagashiteru Dakara kanashii yumewo miru

English
Summer Star, Why are you so red. Because, I had a sad dream last night. My eyes are red from the tears I've shed. Swollen as I cried.

Summer Star, Why've you lost your way? I'm searching for a child whose gone afar. He can't be found, though I've searched all day. My sad dreams come once more.

Trivia

 * may refer to the planet Mars which is also known as the 'star in the summer'.