These star clusters are on a collision course

NASA’s Hubble Space Telescope recently observed two clusters full of massive stars that appear to be in the early stages of merging.

The clusters are located some 170,000 light-years away in the Large Magellanic Cloud, a small satellite galaxy to our own Milky Way.

What at first was thought to be only one cluster in the core of the massive star-forming region 30 Doradus (also known as the Tarantula Nebula) has been found to be a composite of two clusters that differ in age by about one million years.

The entire 30 Doradus complex has been an active star-forming region for 25 million years, and it is currently unknown how much longer this region can continue creating new stars. Smaller systems that merge into larger ones could help explain the origin of some of the largest known star clusters.

Lead scientist Elena Sabbi of the Space Telescope Science Institute in Baltimore, Md., and her team first observing the sector while searching for runaway stars, which she describes as fast-moving stars that were ejected from their stellar nurseries where they first formed. 

“Stars are supposed to form in clusters, but there are many young stars outside 30 Doradus that could not have formed where they are; they may have been ejected at very high velocity from 30 Doradus itself,” Sabbi explained.

The astronomer also noticed something unusual about the cluster when looking at the distribution of the low-mass stars detected by Hubble. 

“It is not spherical, as was expected, but has features somewhat similar to the shape of two merging galaxies where their shapes are elongated by the tidal pull of gravity,” said Sabbi. “Hubble’s circumstantial evidence for the impending merger comes from seeing an elongated structure in one of the clusters, and from measuring a different age between the two clusters.”

According to some models, the giant gas clouds out of which star clusters form may fragment into smaller pieces. Once these small pieces precipitate stars, they might then interact and merge to become a bigger system. This interaction is what Sabbi and her team think believe are observing in 30 Doradus.

In addition, there are an unusually large number of high-velocity stars around 30 Doradus. Astronomers believe these stars, often called “runaway stars” were expelled from the core of 30 Doradus as the result of dynamical interactions. 

These interactions are quite common during a process called core collapse, in which more-massive stars sink to the center of a cluster by dynamical interactions with lower-mass stars. When many massive stars have reached the core, the core becomes unstable and these massive stars start ejecting each other from the cluster.

The big cluster R136 in the center of the 30 Doradus region is too young to have already experienced a core collapse. However, since the core collapse is much faster in smaller systems, the large number of runaway stars that has been found in the 30 Doradus region may be better explained if a small cluster has indeed merged into R136.

Follow-up studies are slated to analyze the area in more detail and on a larger scale to see if any more clusters might be interacting with the ones observed. In particular, the infrared sensitivity of NASA’s planned James Webb Space Telescope (JWST) will allow astronomers to peer deep into the regions of the Tarantula Nebula that are obscured in visible-light photographs. In these areas cooler and dimmer stars are hidden from view inside cocoons of dust. Hopwfully, Webb will better reveal the underlying population of stars in the nebula.

The 30 Doradus Nebula is particularly interesting to astronomers because it is a prime example of how star-forming regions in the young universe may have looked. This discovery could go a long way in helping scientists understand the details of cluster formation and how stars formed in the early universe.