Yellowballs offer insights into star formation

Irregular green-yellow band with many small dots circled.
This false-color infrared image shows a swath of the Milky Way, our home galaxy, used in an analysis of what astronomers call yellowballs (circled in this image). They appear to be places where young stars are heating the leftover gas and dust from which they were born. Image via Charles Kerton, Iowa State University/ NASA/ Spitzer Space Telescope/ PSI.

Stars are born from clouds of gas and dust in space. What astronomers call yellowballs are thought to be clusters of still-forming young stars, heating the gas and dust of their surroundings. Citizen scientists in the Zooniverse-based Milky Way Project first noticed yellowballs in 2011 and 2012. These regions don’t appear yellow to the eye; they’re observable so far only in the infrared part of the electromagnetic spectrum. Astronomers said on April 12, 2021 that yellowballs are helping them understand the diverse cosmic environments that form stars and star clusters, very early in their development, when the stars are only about 100,000 years into their lifespans of billions of years. Grace Wolf-Chase at the Planetary Science Institute in Tucson, Arizona, led the study. She explained:

This is the point at which their presence is first revealed, but they remain embedded in their dusty birth cocoons. This allows us to link the properties of stars with their birth environments, as if a human were giving birth to a hundred or so infants at once.

The peer-reviewed Astrophysical Journal published the new yellowballs study on April 12. A preprint is available at

Grace Wolf-Chase, a senior scientist at the Planetary Science Institute, is the lead author on the study. Image via Planetary Science Institute.

This new research shows that forming star clusters – sometimes called protoclusters – containing stars in a range of different masses go through a yellowball stage. The scientists’ statement explained that some of these protoclusters form massive stars greater than 10 solar masses that will sculpt their environments into what are called bubbles, which form because strong stellar winds and harsh ultraviolet radiation push out the surrounding dust and ga. A typical yellowball has a diameter of about a light-year. Meanwhile, over the course of a million years, the bubbles associated with star formation can expand to tens of light-years across.

These cosmic bubbles were a focus of the original Zooniverse Milky Way Project. Yellowballs, when they were first spotted, were thought to become bubbles over time. Wolf-Chase told EarthSky:

We developed the Milky Way Project to have citizen scientists identify and measure the sizes of bubbles. We wanted to catalog these features across the Milky Way because massive stars are so important to the evolution of stars and chemical elements in general. Massive stars build heavy elements through nuclear fusion in their cores over the course of a few million years and then explode as supernovae, enriching their surroundings with these heavy elements. These heavy elements can be incorporated in future generations of stars.

Yellowballs (left) are compact objects in the evolution of a star, with a diameter of about a light-year. Bubbles (right) can grow to tens of light-years as they expand outward. Seen here in infrared images from NASA’s Spitzer Space Telescope, the false-color application of red-blue-green allows for the identification of Yellowballs, which are otherwise not prominent. Image via PSI/ NASA/JPL-Caltech.

Bubbles, as Wolf-Chase noted, develop when young massive stars – more than 10 times our sun’s mass – are present. This, in turn, destroys the material closest to the stars, leaving behind a rim of infrared light that looks a lot like a bubble.

The scientists’ statement explained:

During the course of searching for ‘bubbles’ in the Milky Way Project, citizen scientists used the project’s discussion board to tag small, roundish objects that appear ‘yellow’ in the representative color infrared images.

Wolf-Chase explained:

Scientists initially thought these might be very young versions of the bubbles and we included identifying yellowballs as a principal goal in a version of the Milky Way Project that was launched in 2016. This resulted in the identification of 6,176 yellowballs over more than 1/3 of the Milky Way. Their distinctive ‘yellow’ appearance relates to wavelengths that trace complex organic molecules and dust as they are warmed by very young stars embedded in their birth clouds.

Our paper analyzes a subset of 516 yellowballs and shows only about 20% of yellowballs will form the bubbles associated with massive stars, while about 80% of these objects pinpoint the location of regions forming less massive stars.

This work shows the great value of citizen science in opening a new window into our understanding of star formation.

Artist's concept of a telescope in space.
Mid-infrared images from the Spitzer Space Telescope in the red, blue, and green allowed citizen scientists and scientists to see yellowballs, which had been missed in other surveys.

Bottom line: The unexpected discovery of yellowballs – clusters of young stars surrounded by dust and gas – by citizen scientists is shaking up astronomy, and giving scientists a new window into the birth and evolution of young stars.

Source: The Milky Way Project: Probing Star Formation with First Results on Yellowballs from DR2

Via Planetary Science Institute

April 15, 2021

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