Newly discovered open star cluster hints at other massive groupings hidden in the Milky Way

May 18, 2021

Dust clouds hide open star cluster Valparaiso 1. By removing the dust and isolating it from the background, a spectacular cluster was discovered. (IAC/Gabriel Pérez Díaz)

Scientists have found a sprawling cluster of stars in the Milky Way's Sagittarius Arm, a discovery that suggests many similar groupings may also be hiding within the densely populated inner regions of our galaxy.

The researchers used measurements from the European Space Agency's Gaia satellite to identify stars gravitationally bound together, revealing a cluster that belongs to an age group that is underrepresented in star cluster counts. The team reported the findings on April 24 in Monthly Notices of the Royal Astronomical Society.

"It shows us that traditional methods for cluster detection may fail quite spectacularly," said Ignacio Negueruela, a professor of astronomy at the Universidad de Alicante, in Spain, and first author of the study. "The cluster should be easily detectable with a small (backyard) telescope but has escaped detection until now."

Stars are born in clouds of dust and gas, forming clusters that can include dozens to hundreds of thousands or millions of stars. These include enormous globular clusters that can offer a window into the Milky Way's early history, and scientists recently reported that there may also be several as yet unidentified globular clusters on the galaxy's outskirts. The stars in open clusters are fewer in number, which means their mutual gravitational pull doesn't bind them together as tightly. These much younger clusters are less symmetrical in shape than globular clusters and can shed light on how stars form and evolve.

"Open clusters are fundamental tools to study stellar physics," Negueruela said.

The more massive a cluster is, he said, the more useful it becomes. A larger stellar population means more opportunities to find similarly sized stars — which scientists can compare to investigate how the stars are affected by other variables — and to catch stars in rare and short-lived evolutionary phases. 

Scientists have spied massive open clusters less than 30 million years of age in the Milky Way, as well as ancient clusters billions of years old. However, there's a dearth of objects in between these extremes, probably because they are so difficult to detect. Very young clusters have extremely bright stars called supergiants that outshine everything around them, making them easy to spot. 

"But older clusters have less bright stars and get lost in the confusion of these crowded environments," Negueruela said.

He and his colleagues first suspected the presence of a young open cluster in the Sagittarius Arm — which lies next to the smaller spiral arm housing our solar system — in 2011. But they didn't pay much attention to it until the Gaia satellite provided more detailed measurements several years later. The satellite's instruments can measure the minute motions and distance of faraway stars with unprecedented accuracy. 

The cluster, which the researchers named Valparaiso 1, showed up as a group of stars about the same distance from Earth that were moving together in exactly the same way, Negueruela said. 

When he and his team took a closer look at the cluster they'd found, he said, "We realized it was really huge."

Most clusters tend to have high concentrations of stars in their centers that stand out from the background, but Valparaiso 1 is unusually spread out. Additionally, there are so many other stars in front of and behind Valparaiso 1 that it wasn't previously clear which stars in the crowded patch of sky belonged to this particular cluster. 

"This combination of low stellar density and huge number of unrelated stars in the same field is what has kept it hidden," Negueruela said. "And it is precisely the high number of members [that] makes it stand out in the Gaia data."

He and his colleagues estimate that the cluster is roughly 7,500 light-years from Earth and 75 million years old. Two stars within the cluster are particularly intriguing, he said. One is a Cepheid variable, a kind of star that astronomers use to determine distances to other galaxies. 

"This Cepheid had been heavily studied in the past, but no one realized it was connected to a cluster," Negueruela said. 

The other object hadn't been previously identified and belongs to a group called asymptotic giant branch stars, which represent the final stage of a star before it dies.

"Finding these interesting objects in open clusters allows us to assign them masses and an evolutionary context," Negueruela said.

In addition to suggesting that many similar clusters may be awaiting discovery in the Milky Way, he said, the findings show how star clusters may be formed with varying amounts of compactness. 

Further measurements from Gaia will give the researchers an even more accurate sense of how far Valparaiso 1 lies from Earth, as well as the cluster's age and size, Negueruela said. 

The study, "A massive open cluster hiding in full sight," published April 24 in Monthly Notices of the Royal Astronomical Society, was authored by I. Negueruela and A. Marco, Universidad de Alicante; A-N Chené, Universidad de Valparaíso and Gemini Observatory; H. M. Tabernero, Universidad de Alicante and Universidade do Porto; R. Dorda, Universidad de Alicante, Instituto de Astrofísica de Canarias and Universidad de La Laguna; and J. Borissova and R. Kurtev, Universidad de Valparaíso and Instituto Milenio de Astrofísica.

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