In physics, negative mass is mass with negative properties. Negative star mass can be created by imploded stars, like Questar M-17. (TAS:"Beyond the Farthest Star")
- In speculative (real world) physics, negative mass is mass that repulses all other mass. Instead, in "Beyond the Farthest Star", negative matter actually had extremely strong gravity. So clearly, in the Star Trek universe, negative mass is something completely different from what we currently call negative mass. Which is actually a good thing, since negative matter is a mathematical construction, and most likely not a physical reality.