Chemical fingerprints reveal hidden spiral arms in the Milky Way

An international team has revealed parts of the Milky Way that ordinary star counts have missed. The researchers read elemental clues in stars to outline two inner spiral arms and a faint bridge between them.

The experts worked with a carefully selected set of roughly 5,000 stars, focusing on patterns in their chemistry rather than just positions. The result reframes how we chart the galaxy’s most crowded regions.

Spiral arms concealed in the Milky Way

The work was led by Carlos Viscasillas Vázquez, an astrophysicist at Vilnius University. His research centers on the chemical and dynamical evolution of the Milky Way.

The team’s analysis appears in a peer-reviewed study that maps chemical patterns across the inner disk.

The experts searched for regions where some elements are slightly more or less abundant than their surroundings, using data from the Gaia-ESO Survey on the Very Large Telescope.

This public survey provides high-quality stellar measurements across the Milky Way.

Chemical clues of subtle changes

Chemical patterns help researchers compare parts of the Milky Way that formed under very different conditions. The patterns offer a steady record of how star-forming regions produced and recycled elements over long periods of time.

These clues also highlight places where earlier models missed subtle changes. They reveal contrasts between nearby zones that look similar in brightness but differ in the way stars enriched the surrounding gas.

Astronomers often track spiral arms with young stars and clusters. Earlier maps based on Gaia data traced nearby arm segments with upper main sequence stars and Cepheids.

This study leans on spectroscopy, the identification of elements by analyzing starlight. Element ratios change with time because different supernovae enrich space on different schedules.

A key tracer is metallicity, the abundance of elements heavier than helium. A second tracer is the magnesium to iron ratio in a star, which captures the balance of fast core collapse supernovae and slower Type Ia events.

The Milky Way’s inner spiral arms

Patterns in iron and magnesium mark the Scutum and Sagittarius arms near the Galactic center. The team also identified a spur, a short arm-like bridge, that links the two structures.

“By applying a technique that allows us to see even small differences in chemical abundances, the arms clearly emerged,” said Dr. Laura Magrini, an astronomer at the Arcetri Observatory in Italy.

“This result highlights the close connection between the dynamics and chemistry of stellar populations. Spiral arms enhance star formation and leave chemical signatures behind,” said Vázquez.

Why chemical patterns matter

Chemical trends help show how star formation changed across different parts of the Milky Way, even when dust blocks the view. 

The chemical fingerprints allow astronomers to compare regions that formed stars quickly with those that evolved at a slower pace.

These patterns also help connect local structures to larger spiral features. They reveal how nearby clusters, older field stars, and dense inner arm regions share linked histories that would otherwise be hidden.

How the method was tested

To check interpretations, the team compared their maps with theoretical models of chemical evolution that include multiple spiral patterns. 

The observed element variations match the idea that arms pass through the disk and briefly boost star formation.

Previous research using Gaia’s measurements pinpointed gentle chemical shifts in stars close to the Sun. Stars inside the spiral arms carry signs that the gas around them changed more quickly, which helps explain how those arms grew.

Growing chemical insights

Chemical patterns offer a way to study how star-forming regions change across time and space.

They help researchers notice slow shifts in how elements spread through the Milky Way and how different stellar groups formed under different conditions.

These maps also show how vertical structure varies above and below the disk. They make it possible to trace slight changes in metallicity with height, which helps explain how older and younger populations ended up arranged the way they are.

Studying patterns in star chemistry

Historically, radio masers and gas maps sketched the galaxy’s scaffolding over thousands of light years. 

Chemistry adds a different lens, one that works even when stars are packed together and dust is thick.

The new study demonstrates that stellar chemistry can map spiral arms, highlight minor branches, and track changes through the disk’s height. These chemical patterns turn stars into durable records of past star formation.

Future surveys with more detailed spectra and improved distances will make the method even clearer – tying together how stars move, form, and are arranged across the Milky Way.

The study is published in the journal Astronomy & Astrophysics.

Image Credit: ESA/Gaia/DPAC, Stefan Payne-Wardenaar

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