Reconstructing a Five-Star Smashup
The “altercation” happened 50,000 years ago: The binary star Mel 34 was ejected from a young star cluster at a speed of 100,000 mph (46 km/s)—the result of a violent interaction that seemed buried in the cosmic past. But a group of astronomy detectives has now reconstructed part of the cluster’s history and identified a five-star smashup as the most likely cause for Mel 34’s high-speed departure [1]. This unlikely collision offers important information about the fate of young, massive stars.
The star cluster R136 is a grouping of around 60,000 stars in the Large Magellanic Cloud, a small galaxy 160,000 light-years from Earth. The cluster is about 2 million years old, which is fairly young as clusters go. “R136 is very special because it’s the youngest and the most massive star cluster in the local group of galaxies,” says Simon Portegies Zwart from Leiden University in the Netherlands. Previous studies of R136 have identified several dozen “runaway” stars that have been kicked out of the cluster. Runaways are common around clusters, but their origins are not always clear. R136 is young, so it’s a good place to study the process that produces runaways, Portegies Zwart says.
Recently, Portegies Zwart and his colleagues analyzed data on R136 taken by the Gaia spacecraft [2]. That analysis revealed 55 runaway stars, all with masses at least 10 times that of the Sun. Of these massive runaways, Mel 34 was the most recent to be ejected. “If we want to understand what happened in this cluster, then let’s start with the last object to escape,” Portegies Zwart says. He and his colleagues have now pieced together a probable history for this star system.
Mel 34 is made up of two very massive stars—one 139 solar masses and the other 127 solar masses—that orbit each other once every 154 days. Such a relatively tight orbit implies that this binary experienced several gravitational encounters in the past, in which it flew close to another star. Such an encounter steals energy from the binary’s orbital motion, causing the two stars to orbit more closely. The released energy gives a boost in linear speed to both the binary and the other star. If the boost is large enough, this mechanism can eject one or both of the objects from the cluster.
Portegies Zwart and colleagues looked for evidence of such an event for Mel 34. Considering conservation of energy and momentum, the other star would be streaming out of the cluster in the opposite direction, but the team searched and found nothing in the predicted region.
However, the researchers did find a 46-solar-mass star, called VFTS 590, that was streaming out at the right speed and distance to have interacted with Mel 34, but its direction implied missing momentum. The team surmised that Mel 34 and VFTS 590 interacted with a third body. The best candidate was Mel 39, a binary star that was only partially observed at the time. Portegies Zwart and colleagues performed preliminary calculations and predicted Mel 39’s total mass to be 220 solar masses. This mass estimate, along with other predicted properties, agreed with subsequent observations [3].
With this clue in hand, the researchers came up with eight scenarios for the interactions of the five stars. They then ran simulations to see which was most likely to be correct. “We can sort of play the billiard ball game to see what is the most likely shot,” Portegies Zwart says. The winning scenario had VFTS 590 initially bound to Mel 39 in a triple-star formation. When this triple star encountered the Mel 34 binary 50,000 years ago, the five stars spiraled around each other for several hundred years before they all got an ejection boost.
Astronomer Hugues Sana from KU Leuven in Belgium says that the researchers make a convincing case that the proposed five-star interaction—as opposed to a simpler three- or four-star interaction—provides the best fit to the data. He says that if such stellar ejections are common, they could affect galaxy evolution. Massive runaways like Mel 34 will eventually explode as supernovae, injecting energy far from their birth clusters, thus affecting the overall structure of their host galaxy. “I have always been amazed by how the life and death of massive stars have played a fundamental role in shaping our Universe over cosmic times,” Sana says.
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.
References
- S. Portegies Zwart et al., “Origin of the most recently ejected OB runaway star from the R136 cluster,” Phys. Rev. Lett. 135, 021201 (2025).
- M. Stoop et al., “Two waves of massive stars running away from the young cluster R136,” Nature 634, 809 (2024).
- A. M. T. Pollock et al., “Melnick 39 is a very massive intermediate-period colliding-wind binary,” arXiv:2503.17150.