Astronomers Trace the Family Tree of Stars Clustered in the Solar Neighborhood

When you look up tonight, you might notice that alongside brilliant stars are clusters that dot the sky. The star-pricked fuzz of the Pleiades, for example, contains members born together some 100 million years ago.

Astronomers are peeking back in time, showing that many stellar groups were themselves once organized into larger gatherings that have since split. The work, published in Nature and presented at the 244th meeting of the American Astronomical Society, paints a picture of the massive stellar nurseries that created many of the young stars within 3,000 light-years.  

“We're providing the origin stories for most nearby young star clusters surrounding the Sun,” says Cameren Swiggum (University of Vienna, Austria).

Cosmic Rewind

Swiggum and colleagues used the catalog compiled by the European Space Agency’s Gaia mission to track down the origins of young star clusters near the Sun. Gaia provides the three-dimensional position and velocity for the stars in these clusters — measurements that can be surprisingly hard to come by for nearby celestial objects. Even the Pleiades’ distance was long contested. But with Gaia’s information in hand, astronomers can now press the forward and/or rewind button on a cosmic video of the solar neighborhood.

“We can take the velocity vectors that we see for each of these star clusters, and we can just flip it backwards,” Swiggum explains. “We can trace these clusters backwards in time and find where they actually came from.”

What they found is that the clusters’ tracks converge. Selecting hundreds of clusters even younger than the Pleiades, Swiggum and his colleagues followed the motions of their stars over the past 30 million years. The researchers found that more than half of the clusters — 155 of 272 — came from one of three origin points: three huge stellar nurseries that birthed dozens of clusters each.

The results appear trustworthy, says Alice Quillen (University of Rochester), who has studied similar data but wasn’t involved in the current study. “The backwards integration is not going that far back (about ¼ [galactic] rotation) so perturbations from things like spiral arms are probably not going to affect the orbits that much,” she explains. “The fact that the birth places in the backwards integrations were confined to small regions seems really nice (and suggests their orbit integration is robust).”

Blast from the Past

Such large nurseries won’t just make low-mass stars like the Sun. They’ll also produce more massive stars, whose evolutions played out fast and furious. Indeed, not all of the clusters’ stars made it to the present-day: The team estimates that 200 of the massive stars born inside the clusters have since exploded in supernovae.

The energy pumped into the galaxy by these supernovae might be responsible for some of the dust-bounded shells that astronomers have observed nearby. For example, Swiggum says, supernovae in a cluster named CR 135 and its natal family might be responsible for clearing a huge dust-lined cavity — a 3,000-light-year-long galactic supershell known as GSH 238+00+09, first identified in the 1990s.

The chain of reasoning is impressive: From low-mass stars in young clusters, Swiggum’s team has inferred the existence — and explosions — of massive stars that we can no longer see. And by doing so, they can explain structures around the Sun today.

There are limits to this type of study, though. Using Gaia data, astronomers can peer back tens of millions of years to see where star clusters formed — but they cannot (yet) look back the billions of years necessary to see the Sun’s birthplace.

“We do not currently have the ability to trace the Sun’s orbit back to its birth-time 4.6 billion years due to the dynamic nature of the galactic environment over such long timescales,” Swiggum says. But he can make an educated guess: “Since the Sun most likely formed in a star cluster, we can speculate that it might have also formed in a cluster that was part of a greater family of clusters, similar to the three young families we see today.”

Explore some of the clusters in this study in this interactive graphic.