Mysterious Spectral Lines At The Heart Of The Milky Way Turn Out To Be Just An ‘Optical Illusion’

In the spring of 2017, a puzzling discovery announced that a group of stars found 25,000 light-years away in the galactic center of the Milky Way had an anomalous composition.

Known as red giants, these stars inhabit a central region of our galaxy known as the Nuclear Star Cluster (NSC) — a dense and massive collection of stars that sits right next to Sagittarius A, the supermassive black hole at the heart of the Milky Way.

Spotted less than three light-years from Sagittarius A, these red giant stars were found to contain unusually high amounts of scandium, vanadium, and yttrium.

The revelation came after astronomers took a look at the light coming from these stars and analyzed its spectrum. Since each chemical element emits and absorbs light at a specific wavelength, measuring a star’s spectral lines can help determine what it’s made out of.

In the case of these particular stars, the scientists detected strong spectral lines associated with the three elements, which suggested that the stars harbored “astonishing and dramatically high levels” of scandium, vanadium, and yttrium.

Stumped by the unexpected readings, researchers scrambled to find an explanation for this mystifying phenomenon.

At first, the enigmatic spectral lines were attributed to leftover material either from earlier stars devoured by the black hole or from epic collisions between neutron stars. But a team of astronomers led by Brian Thorsbro of Lund University in Sweden unraveled the mystery behind the unusual spectral lines, revealing them as nothing more than an “optical illusion,” reports Science Daily.

In a new study just published in The Astrophysical Journal, the astronomers argue that the false readings were prompted by the relatively low temperatures of these stars.

“These giant red stars have used up most of their hydrogen fuel and their temperatures are therefore only half of the sun’s,” explained Thorsbro.

As his team pointed out, the lower temperatures of the red giant stars influenced the behavior of electrons in the three elements, leading to an error in measurement of their spectral lines. This created the illusion that the stars contained more scandium, vanadium, and yttrium than they really do.

The scientists cracked the mystery after taking a closer look at these stars with the largest infrared telescope in the world, housed at W.M. Keck Observatory on Mauna Kea in Hawaii.

Their observations are part of an extensive project aimed at mapping the central regions of the Milky Way by “exploring the spectral lines in the light from different stars to find out which elements they contain,” notes Science Daily, citing Lund University.

“Our research collaboration is world-leading in terms of systematically mapping the elements contained in the huge central star cluster — the star cluster that surrounds the black hole,” said study co-author Nils Ryde, also from Lund University.

The astronomers studied the near-infrared spectrum of the stars in the NSC cluster and compared it with that of similar stars in our solar neighborhood. Their findings revealed strong spectral lined for scandium, vanadium, and yttrium in red giant stars from both samples.

According to the authors, this proves that the puzzling detection was “not unique to stars in the NSC,” but actually occurred due to “a property of the line formation process that currently escapes accurate theoretical modeling.”

This, in turn, suggests that the composition of the NSC stars shouldn’t be inferred as anomalous — at least “until we better understand how these lines are formed,” concludes the paper.