Astronomers peer into galactic bubbles where stars burst into life

In the constellation Carina, 20,000 light-years away from Earth, a dusty nebula bubbles like a boiling cauldron, and within the bubbles, new stars are born.

One of these bubbles surrounds the star cluster Westerlund 2 within the nebula. Astronomers have used the SOFIA telescope to capture a high-resolution image of the bubble – an expanding region of hot, ionised gas, or plasma.The image has revealed that new stars are forming within the bubble’s shell.

The study, by researchers at the University of Maryland, has disproven previous research suggesting that the star cluster was surrounded by two bubbles, not one, and has uncovered the source of the bubble’s expansion.

“When massive stars form, they blow off much stronger ejections of protons, electrons and atoms of heavy metal, compared to our Sun,” said Dr Maitraiyee Tiwari, a postdoctoral associate in the UMD Department of Astronomy.

“These ejections are called stellar winds, and extreme stellar winds are capable of blowing and shaping bubbles in the surrounding clouds of cold, dense gas. We observed just such a bubble centred around the brightest cluster of stars in this region of the galaxy, and we were able to measure its radius, mass and the speed at which it is expanding.”

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Previous images of the Westerlund 2 star cluster had been taken using waves in the radio and submillimetre (far-infrared and microwave) regions of the electromagnetic spectrum. The team captured the new, high-resolution image of the cluster by using light of all wavelengths, from low-energy radio waves all the way to high-energy X-rays. By combining all of this data, the team were able to see how the stellar winds were driving the bubble to expand.

The researchers also found that, roughly a million years ago, the bubble had burst. On one side, it had broken outwards, spewing out plasma. As a result, the expansion had slowed. However, around 200,000 to 300,000 years ago, a new star had evolved in the cluster, and its stellar winds gave the bubble new life.

“We saw that the expansion of the bubble surrounding Westerlund 2 was reaccelerated by winds from another very massive star, and that started the process of expansion and star formation all over again,” Tiwari said. “This suggests stars will continue to be born in this shell for a long time, but as this process goes on, the new stars will become less and less massive.”