Astronomers discover new type of star hidden at the center of our galaxy

A decade-long survey of the night sky has revealed a mysterious new type of star that astronomers call an “ancient smoky.”

These previously hidden stellar objects are aging giant stars located near the heart of the Milky Way. Stars lie dormant for decades and fade until they are almost invisible before spewing clouds of smoke and dust, and astronomers believe they could play a role in distributing elements throughout the universe.

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

Astronomers observed the ancient smoldering stars for the first time during research 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 Research Telescope, located at an observation point high in the Chilean Andes, at the Cerro Paranal Observatory.

The search for newborn stars

The team's initial goal was to look for newborn stars, which are difficult to detect in visible light because they are obscured by the dust and gas of the Milky Way. But infrared light can pass through the galaxy's high concentrations of dust to detect objects that would otherwise be hidden or faint.

Although two-thirds of the stars were easy to classify, the rest were more difficult, so the team used the European Southern Observatory's giant 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 co-author of the other three.

As astronomers monitored hundreds of millions of stars, they tracked 222 that experienced noticeable changes in brightness. The team determined that 32 of them were newborn stars whose brightness increased at least 40 times, and some up to 300 times.

“Our main goal was to find rarely seen newborn stars, also called protostars, while they are undergoing a huge explosion that can last for months, years or even decades,” said Dr. Zhen Guo, Fondecyt Postdoctoral Fellow at the University of Valparaíso, in Chile, in a statement. Guo was the lead author of two studies and co-author of the other two.

“These explosions happen in the slowly rotating disk of matter that is forming a new solar system. They help the newborn star in the middle grow, but hinder the formation of planets. We still don't understand why disks become unstable in this way,” said Guo.

An unexpected stellar discovery

During their observations of stars near the galactic center, the team identified 21 red stars that experienced subtle changes in luminosity that confused astronomers.

“We were unsure whether these stars were protostars initiating an eruption or recovering from a drop in brightness caused by a disk or layer of dust in front of the star, or whether they were older giant stars expelling matter in the final stages of life,” Lucas said.

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

Red giants form when stars exhaust 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, swelling and expanding as it sheds layers of material and likely evaporating the solar system's inner planets, although Earth's fate remains uncertain, according to to NASA.

But the stars detected during the search are different.

“These elderly stars remain quiet for years or decades and then expel clouds of smoke in a completely unexpected way,” said Dante Minniti, professor in the physics department at Andrés Bello University in Chile and co-author of three of the studies, in an affirmation. “They look very dark and red for several years, to the point that sometimes we can’t even see them.”

The stars were largely found in the Milky Way's innermost nuclear disk, where stars are most concentrated in heavy elements. Understanding how ancient smokers release elements into space could change the way astronomers think about the distribution of these elements throughout the Universe.

Astronomers are still trying to understand the process behind stars releasing dense smoke and what happens next.

“Matter ejected from ancient stars plays a key role in the life cycle of 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 the Mira variable. However, the discovery of a new type of star that releases matter could have broader significance for the propagation of heavy elements in the Nuclear Disk and metal-rich regions of other galaxies.”