Astronomers discover strange new type of star hidden in the centre of our galaxy

An artist's illustration depicts an an old smoker star, or an aging red giant star releasing a thick cloud of smoke and dust. (Philip Lucas/University of Hertfordshire)

A decade-long survey of the night sky has revealed a mysterious new type of star astronomers are referring to as an "old smoker."

These previously hidden stellar objects are aging, giant stars located near the heart of the Milky Way galaxy. The stars are inactive for decades and fade until they're almost invisible before belching out clouds of smoke and dust, and astronomers think they could play a role in distributing elements across the universe.

Four studies detailing the observations published January 25 in the Monthly Notices of the Royal Astronomical Society.

Astronomers observed the old smoker stars for the first time during the survey that involved monitoring nearly a billion stars in infrared light, which is invisible to the human eye.

The observations were carried out with the Visible and Infrared Survey Telescope, situated at a vantage point high in the Chilean Andes at the Cerro Paranal Observatory.

The search for newborn stars

The team's initial goal was searching for newborn stars, which are hard to detect in visible light because they are obscured by dust and gas in the Milky Way. But infrared light can pierce through the galaxy's high concentrations of dust to pick out otherwise hidden or faint objects.

While two-thirds of the stars were easy to classify, the remainder were more difficult, so the team used the European Southern Observatory's Very Large Telescope to study individual stars, said Philip Lucas, professor of astrophysics at the University of Hertfordshire. Lucas was the lead author of one study and a coauthor on the other three.

As astronomers monitored hundreds of millions of stars, they tracked 222 that experienced noticeable shifts in brightness. The team determined that 32 of them were newborn stars that increase in brightness by at least 40 times, and some as much as 300 times. A large percentage of the eruptions are ongoing, so astronomers can continue monitoring how the stars evolve over time.

"Our main aim was to find rarely-seen newborn stars, also called protostars, while they are undergoing a great outburst that can last for months, years, or even decades," said Dr. Zhen Guo, Fondecyt Postdoc Fellow at the University of Valparaiso in Chile, in a statement. Guo was the lead author of two studies, and coauthor on the other two.

"These outbursts happen in the slowly spinning disc of matter that is forming a new solar system. They help the newborn star in the middle to grow, but make it harder for planets to form. We don't yet understand why the discs become unstable like this," Guo said.

An unexpected stellar discovery

During their observations of stars near the galactic centre, the team identified 21 red stars that experienced unusual changes in luminosity that puzzled astronomers.

"We weren't sure if these stars were protostars starting an eruption, or recovering from a dip in brightness caused by a disc or shell of dust in front of the star — or if they were older giant stars throwing off matter in the late stages of their life," Lucas said.

The team focused on seven of the stars and compared the new data they collected with data from previous surveys to determine that the stellar objects were a new type of red giant stars.

Red giants form when stars have exhausted their supply of hydrogen for nuclear fusion and begin to die. In about 5 or 6 billion years, our sun will become a red giant, puffing up and expanding as it releases layers of material and likely evaporating the solar system's inner planets, although Earth's fate remains unclear, according to NASA.

But the stars spotted during the survey are different.

"These elderly stars sit quietly for years or decades and then puff out clouds of smoke in a totally unexpected way," said Dante Minniti, a professor in the department of physics at Andrés Bello University in Chile and coauthor on three of the studies, in a statement. "They look very dim and red for several years, to the point that sometimes we can't see them at all."

The stars were largely found in the innermost nuclear disc of the Milky Way, where stars are more concentrated in heavy elements. Understanding how the old smokers release elements into space could change the way astronomers think about the way such elements are distributed across the universe.

Astronomers are still trying to understand the process behind the stars' release of dense smoke, and what occurs after.

"Matter ejected from old stars plays a key role in the life cycle of the elements, helping to form the next generation of stars and planets," Lucas said. "This was thought to occur mainly in a well-studied type of star called a Mira variable. However, the discovery of a new type of star that throws off matter could have wider significance for the spread of heavy elements in the Nuclear Disc and metal-rich regions of other galaxies."