Elon Musk and singer Grimes welcome baby named Y

Their daughter is named Exa Dark Siderael Musk and known as Y

Grimes and Elon Musk, who separated last September, now appear to be back together. AP Photo

Technology entrepreneur Elon Musk and Canadian singer Grimes have welcomed their second child, a girl named Exa Dark Sideræl Musk and known as Y.

The baby was born via a surrogate, said Grimes, born Claire Elise Boucher, in an interview with Vanity Fair. Y is the younger sibling of X Æ A-12, 2, pronounced "X AI Archangel" and known as X for short.

In the interview, Grimes was guarded about details of the new arrival. The fact the couple had welcomed a second child was not revealed deliberately but because the interviewer, Devin Gordon, heard the sound of a baby crying while speaking to the singer at home.

Grimes confirmed that the baby is known as Y, but her full name is Exa Dark Sideræl Musk. Gordon confirms that Sideræl, an "elven spelling of sidereal", is pronounced "sigh-deer-ee-el" after receiving a voice note from the singer, who said Dark was chosen because of its association with "the unknown. People fear it but truly it’s the absence of photons. Dark matter is the beautiful mystery of our universe."

Exa, the interview explains, "is a reference to the supercomputing term exaFLOPS (the ability to perform 1 quintillion floating-point operations per second)".

Of the name, Grimes said: “I was fighting for Odysseus Musk ... a girl named Odysseus is my dream.” She revealed that they also call the baby Sailor Mars, which is a tribute to the manga series Sailor Moon.

Letter nicknames are clearly a family favourite — the singer refers to Musk as E and says that she often goes by C.

In the interview, the singer confirms that, unlike X Æ A-12, she doesn't intend to publicly show baby Y's face.

"Whatever is going on with family stuff, I just feel like kids need to stay out of it, and X is just out there. I mean, I think E is really seeing him as a protege and bringing him to everything and stuff … X is out there. His situation is like that. But, yeah, I don’t know."

She later says: "The best situation here is me training the girl and Musk training the boy."

In September, Musk confirmed that he had separated from Grimes and was co-parenting with her. However, they now appear to be back together.

"There’s no real word for it,” she told Vanity Fair. “I would probably refer to him as my boyfriend, but we’re very fluid. We live in separate houses. We’re best friends. We see each other all the time ... We just have our own thing going on, and I don’t expect other people to understand it.”