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Two Unequal Shots Sent a Star Flying

Original: "A pair of unequal jets imparted the kick velocity to the neutron star of Cassiopeia A"
· Dmitry Shishkin, Muhammad Akashi, Noam Soker
arXiv:2609.16138v1 · 2026-09-14 · CC BY 4.0 · 1 min
High Energy
One ring is big, the other small. They were carved by two jets of different strength, and the star went flying off to one side.

When a star explodes, it can fling out narrow jets of matter. The supernova remnant Cassiopeia A shows two rings: a large northern one and a small southern one. Think of them as two cannon shots, except one shot is 40 times more powerful than the other.

Researchers built a computer model of the explosion. First the star blew up evenly in all directions, and four seconds later two jets struck in opposite directions. The northern jet was far stronger than the southern one. The model reproduced the same rings seen in telescope images. The mismatch between the jets gave the neutron star a shove, and now it is traveling at 400 kilometers per second, which is Moscow to St. Petersburg in a single second.

The rings were once thought to come from rising bubbles of nickel. Now it is clear: they were carved by two unequal jets.

This approach can be applied to other remnants of exploded stars. If rings and fast-moving stars turn up often, then unequal jets must be a common feature of explosions.

Whole graph →

The neutron star in Cassiopeia A moves at about 400 km/s. In one second it covers the distance from Moscow to St. Petersburg.

\Delta p_j = p_{\rm up} - p_{\rm down}
The momentum transferred to the neutron star through the asymmetry of the jets
v_{\rm kick} = \frac{\Delta p_j}{M_{\rm NS}}
The kick velocity obtained from conservation of momentum
f_{\rm st} = \frac{1}{\rho} \sqrt{|\vec{\nabla}P \cdot \vec{\nabla}\rho|} \, \mathrm{sgn}(\vec{\nabla}P \cdot \vec{\nabla}\rho)
A quantity that determines the growth of the instability at the boundary of accelerated layers
Scientists
Enrico FermiFritz Zwicky
Concepts
cassiopeia a kilonova magnetorotational supernova supernova jets neutron star spin crab nebula core collapse