An enormous nebula that Jonathan Archer had known since childhood from the cover of an astronomy book, charted by Enterprise in 2151 with the help of a ship of emotional Vulcans.
The Arachnid Nebula appears in the Enterprise episode Fusion. It measures billions of kilometres across, far larger than Earth's astronomers had estimated.
For Archer the visit is personal. A picture of the nebula was on the cover of an astronomy book he read as a boy, and seeing it in person is one of the ambitions he brought with him into space.
While charting it, Enterprise meets the Vahklas, a civilian Vulcan ship whose crew are trying to live with their emotions instead of suppressing them. The two crews work on the survey together. One of the Vulcans, Tolaris, presses T'Pol into a mind meld that she tries to break off, with lasting effects on her health.