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.

Last night I had a sad dream. Crying as I talked.

Oh, red eyes.

Summer Star, why are you lost.

You are searching for we who disappeared.

That´s why I have sad dreams.

Trivia

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