‘String of pearls’ star clusters form when galaxies collide

• When galaxies collide, they cook up new generations of stars. These galactic collisions cause a gravitational tug-of-war, and gas and dust are drawn out into large streamers.
• The knots of gas gravitationally collapse to create about 1 million newborn stars per cluster in the long tidal tails.
• These “string of pearls” features are probably more common in the early universe when galaxies collided more frequently.

NASA published this original article on February 8, 2024. Edits by EarthSky.

String of pearls are born when galaxies collide

Contrary to what you might think, galaxy collisions do not destroy stars. In fact, the rough-and-tumble dynamics trigger new generations of stars, and presumably accompanying planets.

Now NASA’s Hubble Space Telescope has homed in on 12 interacting galaxies that have long, tadpole-like tidal tails of gas, dust and a plethora of stars. Hubble has uncovered 425 clusters of newborn stars along these tails, looking like strings of holiday lights. Each cluster contains as many as 1 million blue, newborn stars.

We’ve known about clusters in tidal tails for decades. When galaxies interact, gravitational tidal forces pull out long streamers of gas and dust. Two popular examples are the Antennae and Mice galaxies with their long, narrow, finger-like projections.

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A new study of tidal tails

A team of astronomers published a new study on September 29, 2023, using a combination of new observations and archival data to get ages and masses of tidal tail star clusters. They found that these clusters are very young: only 10 million years old. And they’re forming at the same rate along tails stretching for thousands of light-years.

Lead author Michael Rodruck of Randolph-Macon College in Ashland, Virginia, said:

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

The tails look like they are taking a galaxy’s spiral arm and stretching it out into space. The exterior part of the arm gets pulled like taffy from the gravitational tug-of-war between a pair of interacting galaxies.

Before the mergers, the galaxies were rich in dusty clouds of molecular hydrogen that simply may have remained inert. But the clouds got jostled and bumped into each other during the encounters. This compressed the hydrogen to the point where it precipitated a firestorm of star birth.

What will happen next?

The fate of these strung-out star clusters is uncertain. They may stay gravitationally intact and evolve into globular star clusters, like those that orbit outside the plane of our Milky Way galaxy. Or they may disperse to form a halo of stars around their host galaxy, or get cast off to become wandering intergalactic stars.

This string-of-pearls star formation may have been more common in the early universe when galaxies collided with each other more frequently. These nearby galaxies that Hubble observed are a proxy for what happened long ago. Therefore, they are laboratories for looking into the distant past.

Bottom line: This beautiful galaxy is one of many that Hubble has observed showing a “string of pearls.” When galaxies collide, new star formation creates these clusters in the tidal tails.

Source: Star clusters in tidal debris

Via NASA