Star Fissure is an anomaly of the age of Riven (specifically the Temple Island) that appears as a rift between two separate systems. Beneath it is a star field. It eventually leads to Earth, which is the home of The Stranger.
Its origin is unknown, as nobody wrote it into the age purposefully. Gehn considered that it was actually an unexpected by-product of the changes Catherine and Atrus wrote into the Age during their escape, and it was not written on purpose, as they wouldn't want the world (and him) destroyed. Catherine's visions tell that it was born out of the will of the Maker to serve some greater purpose.
A great column of wind was formed when first appeared. Despite what it might seem, there is no vacuum as in space. theorized that the fabric of Riven has been breached in a way that permits matter to be hospitably exchanged between two discreet but overlapping spaces - much like a Link. The star field is a gentle space hospitable to life and breathable atmosphere. Atrus explained it as a flowing river when he fell into.
Atrus jumped into the Star Fissure with The last Myst Linking Book. While falling he linked to the age, leaving the linking book falling freely into the Star Fissure, and fell to Earth. The Myst linking book was found by The Stranger.
While Gehn is trapped on Riven he conducted many experiments, considering navigating the stars, and possibly find the Myst Book. observes the rotation of the stars in the fissure, and discovers that the dark cloud-like formations migrate in a cyclical path through the starfield  He had Rivenese "criminals" thrown into it for punishment, to observe their fate. Although they survived the fall, Gehn was not able to see where they ended up to. He sealed it with an iron construct and mounted ta telescope to observe it. As the natives were too afraid to approach the vortex, Gehn had to urge them to proceed for the construction. The Moiety had found the combination that opened the viewport, considering opening the Fissure.
The Stranger opens the Star Fissure to signal Atrus. It begins to slowly consume the objects very close to it. The Riven linking book is then dropped into it, followed shortly by The Stranger falling into it. Atrus knows that the Fissure will safely guide The Stranger to where he came from. It helped contribute to the imminent destruction of the age of Riven, and the age can no longer be safely visited.
The last piece of possible evidence known about the Star Fissure is contained in Uru. In the beginning, at the cleft, the telescope from Riven is seen buried in the ground, as well as a wahrk skeleton; the gigantic stone Moiety dagger from Riven can be found inside the volcano's crater nearby the cleft. This suggests that, as the telescope and dagger (and presumably a wahrk) fell into the Star Fissure, it does lead to Earth, specifically to The Cleft, which physically resembles the fissure in shape. This could also shed some light on the identity of The Stranger, as he would have found the Myst linking book near the Cleft.
When an Explorer returns all four of the Bahro pillars, the Star Fissure will form on their Relto. Unlike the Star Fissure seen in Riven, it does not consume objects close to it.