NASA’s Hubble space telescope reveals a galactic ‘string of pearls’

When galaxies collide, their stars are not actually destroyed. These rough-and-tumble dynamics actually trigger the formation of new generations of stars, and potentially even planets to accompany them.

[Related: Behold six galactic collisions, masterfully captured by Hubble.]

Astronomers using the Hubble Space Telescope have taken a closer look at 12 of these interacting galaxies. These galaxies all have long tails of gas, dust, and multitudes of stars. Hubble can detect ultraviolet light and has uncovered 425 clusters of newborn stars located along these galactic tails that resemble a string of lights or pearls. Each of these clusters is packed with as many as one million newborn blue stars. The findings were described in a study published in the Monthly Notices of the Royal Astronomical Society in September 2023. A new image of the string of pearls galaxy (Galaxy AM 1054-325) was released by NASA on February 8. 

Galactic collisions and high pressure

When galaxies interact with each other, gravitational tidal forces will pull out long streams of gas and dust from the material that make up each galaxy. The Antennae and Mice galaxies have long, narrow, finger-like tendrils and are common examples of what these galactic tails look like.

“As galaxies merge, clouds of gas collide and collapse, creating a high-pressure environment where stars could form,” study co-author and Penn State University astronomer Jane Charlton said in a statement. “The interiors of these mergers have been well studied, but less was known about possible star formation in the debris that results from these mergers, such as in the tidal tails.”

Tails of young stars

In their study, a team of scientists used new observations and archival data to estimate the ages and masses of tidal tail star clusters. At only 10 million years old, these clusters are very young and appear to be forming at the same 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,” study co-author and Randolph-Macon College astronomer Michael Rodruck said in a statement. “With tidal tails, you will build up new generations of stars that otherwise might not have existed.”

The tails appear to be taking the spiral arm of a galaxy and stretching it further out into space. The exterior arm is pulled in a gravitational tug-of-war between the two interacting galaxies. 

Galaxies as a time capsule

Before these galactic mergers even happened, the galaxies were rich in dusty clouds of molecular hydrogen. These clouds may simply have been unable to move, but the clouds eventually jostled around and began to bump into each other. This activity then compressed the hydrogen to a point where it created a huge storm of star birth. 

[Related: How do you make cosmic sausage?]

Scientists are still not sure what the fate of these strung-out clusters of stars will be. They could stay gravitationally intact and eventually change into globular star clusters. It’s possible that they may also disperse to form a halo of stars around their host galaxy, or even be cast off and become wandering intergalactic stars.

Nearby galaxies observed by Hubble like these can be used as a proxy for what happened in our universe millions of years ago and are a way to peer into the distant past.

“We think that star clusters in tidal tails may have been more common in the early universe,” said Charlton, “When the universe was smaller and galaxy collisions were more frequent.”

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