How Can a Star Survive a Supernova and Become a Hypervelocity Runaway?

By Launie Wellman — Festus, Missouri

In 2017 astronomers identified a group of unusually fast-moving stars that appear to originate from the Large Magellanic Cloud (LMC). These “hypervelocity” stars travel so quickly that the Milky Way’s gravity cannot hold them; over time they will escape into intergalactic space. That raised an intriguing question: if these stars were launched when their binary companions exploded as supernovae, how did the survivors avoid being obliterated by the blast?

Two Ways To Make A Hypervelocity Star

A well-known mechanism is a tidal breakup by the supermassive black hole at the Galactic Center, Sagittarius A*. When a tight binary passes close to that black hole, one star can be captured while the other is flung outward at very high speed — sometimes thousands of miles per second. The Sun, by comparison, orbits the Galaxy at roughly 140 miles (220 km) per second.

However, the clustering of several hypervelocity candidates toward the constellations Leo and Sextans suggested an alternate origin. A 2017 study proposed that these particular runaways could have been expelled from binaries inside the LMC, a satellite galaxy moving at roughly 250 miles (400 km) per second relative to the Milky Way. If a star in a tight binary explodes as a supernova, the surviving companion is released at about its pre-explosion orbital velocity; added to the LMC’s systemic motion, that kick can produce the extreme speeds we observe from Earth.

Why a Companion Usually Survives

It might seem counterintuitive that a companion could survive a nearby supernova, but there are several reasons this is common:

• Most of the energy escapes: The supernova blast expands rapidly into space. While it delivers intense radiation and a shock wave, the impulse at the companion’s surface typically strips only outer layers rather than unbinding the whole star.
• Self-gravity holds the star together: A star’s own gravity binds its mass. Even if a fraction of the envelope is peeled away, the bulk remains intact and continues as a bound star.
• Binary survivals are observed: Many systems survive and later appear as X-ray binaries, where a normal star orbits a compact remnant (a neutron star or black hole). The compact object can then strip material from the companion via tides and accretion, producing bright X-ray emission.

Depending on proximity and explosion energy, a surviving companion may lose a modest fraction of its mass, gain a high peculiar velocity, and show other telltale signs: surface contamination by supernova-processed elements, a stripped outer envelope (making it hotter and bluer), or fast rotation induced by the pre-explosion orbital motion.

What Observations Can Tell Us

To confirm a supernova-ejection origin, astronomers look for a combination of clues: the star’s trajectory pointing back toward the LMC, an unusually high space velocity consistent with the added LMC motion, and spectroscopic fingerprints such as anomalous surface abundances or a stripped-envelope appearance. Discoveries of many X-ray binaries and post-supernova survivors in the Milky Way show that complete destruction of a companion is not the default outcome.

Expert Note: While the black-hole slingshot and supernova-ejection are both viable channels for producing hypervelocity stars, the clustering toward Leo and Sextans and the LMC’s high systemic speed make the supernova-in-binary scenario especially plausible for that group.

Monica Valluri, Research Professor of Astronomy, University of Michigan, Ann Arbor, Michigan

Originally appeared in Astronomy Magazine.