The Milky Way’s ‘yellowballs’ are clusters of baby stars

Astronomers have cracked a curious cosmic case: What are “yellowballs”? These mysterious space objects were first thought to be signs of young, supermassive stars. Scientists now have confirmed that they do mark stellar nurseries. But these birthplaces for stars can host many types of stars with a wide range of masses.

Researchers shared their discovery April 13 in The Astrophysical Journal.

The stars in the clusters are relatively young, only about 100,000 years old. “I think of these as stars in utero,” says Grace Wolf-Chase. She’s an astronomer at the Planetary Science Institute and lives in Naperville, Ill. For comparison, massive stars forming in the Orion nebula are already 3 million years old. Our sun, at 4.6 billion years old, is considered middle-aged.

Volunteers with the Milky Way Project were the first to spot the unknown objects. The splotches showed up in pictures of the galaxy taken by the Spitzer Space Telescope. That telescope, which worked until last year, saw the cosmos in infrared light. And Spitzer’s images were like a sort of stellar ultrasound. They let astronomers “probe what’s going on in these cold environments before the stars are actually born,” explains Wolf-Chase.

yellowball cluster of baby stars
Astronomers first thought “yellowballs” (circled left) were precursors to gas bubbles blown around massive, young stars (right). A new study instead suggests yellowballs are actually clusters of smaller stars.JPL-Caltech/NASA

Citizen scientists had been scouring the images for signs of baby stars and their birthplaces. The babies were expected to be at least 10 times the mass of our sun. And they blow giant bubbles of gas that is electrically charged, or ionized. A year or two into the project, some users noted small yellow blobs in the false-color images. They began tagging the objects #yellowballs. Between 2010 and 2015, the volunteers found 928 such yellowballs.

Wolf-Chase’s team first thought the balls signaled early-stage gas bubbles. But the researchers wanted more data to get a better look. The first yellowballs to be tagged were a lucky discovery. The researchers knew they probably hadn’t caught enough of them to definitively ID these objects. In 2016, the team asked volunteers with the Milky Way Project to find more. By the next year, that group had spotted more than 6,000 additional yellowballs.

Wolf-Chase and colleagues studied about 500 of those balls more closely. They compared the balls to catalogs of star clusters and other known structures to figure out what they were. “Now we have a good answer: They’re infant star clusters,” Wolf-Chase says. The clusters blow ionized gas bubbles of their own. Their bubbles are similar to the ones blown by single young, big stars.

Wolf-Chase hopes the work will help researchers spot yellowballs with newer telescopes. One, the James Webb Space Telescope, is due to launch in October. Such images could reveal more about the balls’ physical traits.

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