The still rather mysterious object that resides in the heart of our Milky Way could easily remain silent for decades, even approaching invisibility during that period, and then suddenly spew out enormous clouds of gas.

Scientists discovered this previously unknown type of star during a long-term study that lasted 10 years, during which they observed almost a billion stars in infrared light. They initially used the Visible and Infrared Survey Telescope (VISTA for short). But at a later stage they also appealed to the Very Large Telescope, explains researcher Philip Lucas. “Almost two-thirds of the stars were easy to classify. The rest was a bit trickier, so we used ESOs Very Large Telescope to obtain an individual spectrum for many of these stars. A spectrum reveals how much light a star emits at different wavelengths, giving a much clearer picture of what exactly we are looking at.”

Red stars
Such follow-up observations proved necessary after the researchers detected 21 red stars in the inner part of the Milky Way, whose brightness changed enormously during the 10 years in which they observed them. “We weren’t sure whether they were explosive protostars or whether they were recovering from a decrease in brightness caused by dust passing in front of the star. A third option was that they were older giant stars that ejected matter towards the end of their lives and puffed away gas like old smokers.”

Old smokers
Follow-up observations of seven of these seemingly mysterious stars revealed that they belong to a previously unknown class of stars that researchers informally refer to as “old smokers.” “These older stars lie dormant for years to decades and then suddenly puff out gas clouds in a completely unexpected way,” says researcher Dante Minniti. “They are then very faint and red for several years in a row; sometimes so bad that we can no longer see them at all.”

Heavy elements
With the discovery of these ‘old smokers’, our picture of the star population in the Milky Way has become slightly more complete. But not only that; The research can also provide more insight into how (heavy) elements are spread through space. For example, the researchers point out that the ‘old smokers’ are located in the innermost regions of our Milky Way, where stars are richer in heavy elements than elsewhere in our galaxy. And some of those heavy elements are apparently puffed away by the ‘old smokers’. New stars and planets can emerge from these heavy elements at a later date. “The discovery of a new type of star that ejects matter could inform our understanding of the distribution of heavy elements,” Lucas said.

In addition to the ‘old smokers’, the researchers have also made other interesting discoveries in their long-term study. They also encountered dozens of previously unobserved newborn stars in our Milky Way – also called protostars. Most of these stars are obscured by dust. But because infrared light – unlike optical light – travels effortlessly through dust, the researchers were able to see these stars for the first time. In total, they discovered 32 of these protostars still surrounded by a gas disk whose brightness (in infrared) appeared to increase at least 40 times and sometimes even 300 times. “This outburst (increase in brightness, ed.) takes place in the slowly rotating disk of matter from which a new solar system is formed,” explains researcher Zhen Guo. “These outbursts help the newborn star at the heart of the disk to grow, but make it more difficult for planets to form. We don’t yet understand why the drives become so unstable.”

With the discovery of newborn and very old stars that cannot even be classified into any existing class, the researchers once again demonstrate that there is still plenty to discover in our Milky Way. At the same time, the discoveries also almost automatically raise new questions. Researchers cannot yet properly explain why the disk around protostars is disturbed. It is also unclear what exactly causes old smokers to puff away large clouds of matter.

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