Quick Answer
The danger would come from mass and distance, not from the object's small size. A close neutron-star flyby could strongly perturb Earth's orbit and create enormous tidal forces. If it were an active pulsar or magnetar, radiation could add another hazard.
A neutron star is only about city-sized but can contain more mass than the Sun. A close pass would be an extraordinary gravitational event.
What changes first
The danger would come from mass and distance, not from the object's small size. A close neutron-star flyby could strongly perturb Earth's orbit and create enormous tidal forces. If it were an active pulsar or magnetar, radiation could add another hazard.
The physical or biological constraint
The most important limit is that the hypothetical change cannot be treated as magic. Real systems are coupled: changing one property affects other processes, and the exact outcome depends on how the change is introduced and what remains unchanged. That distinction matters when separating established physics from speculation.
Established scienceThe scenario should be treated as a thought experiment. Where the consequences depend on unknown parameters, there is no single precise outcome.
Second-order effects
Once the immediate effect occurs, other systems respond. Populations, infrastructure, markets, ecosystems or orbits can shift because the original change alters the conditions under which they operate. These secondary effects are often larger than the first obvious consequence.
What would humans do?
People would adapt through technology, new rules and changes in behavior wherever adaptation remained physically possible. The speed of that response would depend on whether the event was sudden or gradual and whether the necessary resources were available.
The long-term picture
Over longer timescales, feedbacks would matter more than the initial shock. Ecosystems could reorganize, engineering systems could be redesigned, and evolutionary or geological processes could gradually produce a very different environment.
The bottom line
A note on our approach: Every article on WhatIfLab separates what current science establishes from what remains genuinely speculative. Where we cite a figure or finding, it reflects published, mainstream research at the time of writing — not a prediction dressed up as fact.