🧩 The basic idea: gravity waves and a cosmic chirp
Neutron stars are the crushed cores of exploded stars. They pack more mass than the Sun into a ball about the size of a city.
Sometimes two neutron stars orbit each other. As they circle, they send out gravitational waves—tiny ripples in space itself. Making those ripples costs energy, so the stars slowly spiral closer together. This creates a rising pitch signal often called a chirp, like a siren that climbs higher and higher.
For much of the inspiral, it is common to treat the stars as simple points. But near the end, they are so close that their size, shape, and even “star-fluid” behavior starts to matter.
💧 Not just gravity: the stars can become unstable
When the stars get very close, tides stretch them—similar to how the Moon pulls on Earth’s oceans, but far stronger. The key message here is that the system can hit a hydrodynamic instability.
Think of two slightly squishy balls spinning around each other. At first they glide in a stable dance. But when they get too close, the shapes they are forced into can no longer stay steady. It is like a shopping cart wheel that suddenly starts to wobble: once the wobble begins, the situation changes quickly.
This instability can happen even in normal Newton-style gravity (no fancy relativity needed), especially if neutron star matter is stiff, meaning it does not compress easily.
🚀 What happens at the end: a sudden plunge
The work follows the inspiral from a slow, step-by-step shrink into a rapid final fall.
Here is what the results suggest near the finish:
- A stability limit is reached before the stars fully touch.
- Just before contact, the inward speed can reach about 10% of the sideways orbital speed.
- After the stability limit is crossed, the rest of the story can take about one more orbit—very fast on cosmic timescales.
This faster plunge changes the last cycles of the gravitational-wave chirp. Instead of a long, smooth ending, the signal can speed up and then stop more abruptly as the stars collide and merge.
🎛️ Spins, squishiness, and relativity: what makes it better or worse
Not all neutron star pairs behave the same. The outcome depends on a few ingredients.
How squishy the stars are: Stiffer stars are more likely to hit the instability earlier. More compressible stars can stay stable closer to contact.
How fast they spin: If the stars are spinning, that can change the timing and the exact shape of the gravitational-wave signal. Fast spins can also affect the wave phase over many cycles.
Relativity: Extra relativity effects can shift where the instability happens and may make the final inward plunge even faster. The big takeaway is that near the end, fluid effects and relativity can both matter, and together they can make the last moments more head-on than a gentle spiral.
🔁 Will one star feed the other first? Probably not
You might imagine one neutron star spilling material onto its partner like water pouring into a bowl. That idea would create a very special gravitational-wave pattern, sometimes described as a reversed chirp.
But the analysis finds that stable mass transfer is extremely unlikely for typical neutron star pairs. In most realistic cases, the stars either touch or become unstable and plunge before any calm, long-lasting transfer can happen. Stable transfer seems possible only in unusual situations, such as when one star is very low-mass and conditions keep the system strongly locked in step.
So the most likely ending is still: rapid final plunge, contact, and merger.
Source Paper’s Authors: D. Lai, F. A. Rasio, S. L. Shapiro
PDF: https://arxiv.org/pdf/astro-ph/9304027v1