Arachnid Nebula

From Memory Alpha, the free Star Trek reference.

The Arachnid Nebula is an interstellar dust cloud located far from the planet Vulcan. The Arachnid Nebula was featured on the front cover of "The Cosmos A to Z", an astronomy book by Laura Danly that claimed the nebula was 6.5 billion kilometers in diameter. The book which was an inspiration to Captain Jonathan Archer in his childhood.

In 2151, the NX class starship Enterprise and a Vulcan civilian transport vessel, the Vahklas, completed a survey of the nebula. The crew of the vessels found that it was actually more than 8 billion kilometers in diameter, not 6.5 billion. They also noted over 20 million cubic meters of disodium and traces of ethylchlorate in the nebula. Enterprise intended to send back the new information to help revise the astronomy books. (ENT: "Fusion")