Galaxy collisions: Hubble spots a 'string of pearls' star cluster 

NASA’s Hubble Space Telescope has unveiled a celestial phenomenon that illustrates the awe-inspiring aftermath of galaxy collisions. Through its sharp and sensitive vision, Hubble has traced a “string of pearls” cluster of newborn stars along the tidal tails formed from the interactions of 12 galaxies. 

This discovery sheds light on the dynamic processes that birth new generations of stars during such cosmic encounters.

When galaxies collide

Galaxy collisions are vast, slow-moving events occurring over millions to billions of years, where two or more galaxies gravitationally interact with each other. Despite the dramatic name, these collisions are not as chaotic as one might expect due to the vast distances between stars within galaxies. 

It’s rare for stars within colliding galaxies to actually collide. Instead, the gravitational forces significantly alter the galaxies’ shapes and structures.

While it may seem that galaxy collisions would destroy stars, the reality is that these encounters trigger explosive star formation. The collisions catalyze the birth of new stars and, presumably, their accompanying planets. 

“String of pearls” star clusters 

The gravitational tug-of-war between interacting galaxies draws out gas and dust into extended streamers, setting the stage for the creation of star clusters. These clusters, each containing up to one million blue, newborn stars, are strung along the tails like luminous beads, forming a spectacular “string of pearls.”

The Hubble Space Telescope, which is particularly sensitive to ultraviolet light, has been instrumental in uncovering 425 of these star clusters. The interactions stretch the galaxies’ material into long, tadpole-like tails that look like strings of holiday lights.

A firestorm of star birth 

Such phenomena, while known for decades through examples like the Antennae and Mice galaxies, have never been observed with such clarity and detail. 

Before their encounters, the galaxies were rich in dusty clouds of molecular hydrogen, which, without the violent interactions, might have remained dormant. However, the collisions jostle these clouds, compressing the hydrogen and igniting a firestorm of star birth.

Very young star clusters 

A team of astronomers, led by Michael Rodruck of Randolph-Macon College in Ashland, Virginia, analyzed a combination of new observations and archival data to determine the ages and masses of these tidal tail star clusters. 

Remarkably, the clusters are very young, only about 10 million years old, and appear to form at a consistent rate along tails that stretch for thousands of light-years. 

“It’s a surprise to see lots of the young objects in the tails. It tells us a lot about cluster formation efficiency,” said Rodruck. “With tidal tails, you will build up new generations of stars that otherwise might not have existed.”

Processes that shaped the universe 

The future of these star clusters, however, remains uncertain. They could evolve into globular star clusters, similar to those orbiting the Milky Way, disperse to form a stellar halo around their host galaxy, or even become wandering stars cast off into the intergalactic medium.

This observation not only provides insight into the mechanisms of star formation but also serves as a window into the past. In the early universe, when galaxy collisions were more common, this “string-of-pearls” star formation may have played a crucial role in the cosmic landscape. The galaxies observed by Hubble act as proxies for these ancient events, offering a unique opportunity to study the processes that shaped the universe.

More about galaxy collisions 

When galaxies collide, their gas and dust can be compressed, triggering bursts of star formation that illuminate the merging galaxies. This process can lead to the creation of stellar nurseries, where new stars are born at an accelerated rate compared to the more steady pace observed in solitary galaxies. 

Over time, the gravitational pull between the galaxies weaves them into a new, often larger and irregularly shaped galaxy. The Milky Way itself is on a collision course with the Andromeda galaxy, an event predicted to occur in about 4 billion years.

These cosmic encounters are not only visually stunning but also scientifically invaluable, offering insights into galaxy evolution, dark matter distribution, and the cosmic web that structures the universe. The resulting galaxies can be elliptical, spiral, or irregular in shape, depending on the mass and composition of the colliding galaxies and the dynamics of their interaction.

Through the study of galaxy collisions, astronomers can better understand the life cycle of galaxies, from formation to eventual merger, shedding light on the future of our own galaxy within the ever-expanding universe.

Image Credit: NASA, ESA, STScI, Jayanne English (University of Manitoba)

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