Scientists Found Stars That May Be the Remains of a Galaxy the Milky Way Absorbed Billions of Years Ago
The Milky Way galaxy you see as a glowing band across the night sky did not simply appear fully formed. Like a city that grew over centuries by absorbing surrounding towns, our galaxy has spent billions of years pulling in smaller galaxies and making them part of itself. Scientists believe this process, called galactic merging, has happened at least a dozen times throughout the Milky Way's 12-billion-year history. Now, a new study suggests astronomers may have found the ancient remains of one of those absorbed galaxies — and they have given it the nickname Loki, after the trickster god of Norse mythology.
The evidence comes from 20 unusual stars discovered orbiting surprisingly close to the galactic disk — the flat, rotating region of the Milky Way where most stars, including our sun, are found. What made these stars stand out is their chemistry. Stars in the galactic disk are typically younger and rich in heavy elements like iron and calcium, which astronomers group together under the term metals. This might seem confusing, since we usually think of metals as things like copper or aluminum, but in astronomy, any element heavier than hydrogen and helium counts as a metal. The 20 newly studied stars contained very few of these elements, which means they formed extremely early in the universe's history, when very few heavy elements existed yet. Finding old, metal-poor stars this close to the disk was unexpected and exciting.
To investigate further, the research team observed these stars using a powerful spectrograph — a scientific instrument that splits starlight into a spectrum, similar to how a prism splits white light into a rainbow — at the Canada-France-Hawaii Telescope. By analyzing that light, they could determine the chemical makeup of each star. They also used data from the European Space Agency's Gaia space telescope, which precisely tracks the positions and movements of stars across the galaxy. The combination of chemistry and motion data gave the researchers a clearer picture of where these stars might have originally come from.
One of the most puzzling clues was the direction the stars were traveling. Some moved in the same direction as the Milky Way's rotation — known as prograde orbits — while others moved in the opposite direction, called retrograde orbits. These two groups had almost identical chemical profiles, suggesting they once belonged to the same galaxy. But how could stars from one place end up moving in completely opposite directions? Computer simulations of galaxy formation provided the answer. If the merger happened very early, when the young Milky Way had not yet settled into a stable spinning disk, an infalling galaxy would have had the freedom to scatter its stars in many different directions, like tossing a handful of pebbles onto a spinning turntable. The simulations estimated this event occurred roughly 10 billion years ago, and that Loki had a total mass of about 1.4 billion times the mass of our sun.
Scientists outside the study have described the findings as promising, though more evidence is needed before Loki can be confirmed as a real distinct galaxy rather than some other grouping of stars within the Milky Way. Studying each star through a high-resolution spectrograph takes about four hours of telescope time, which is why only 20 stars have been analyzed so far. Future telescopes and larger surveys will allow astronomers to study hundreds of similar stars, potentially revealing more hidden remnants of the Milky Way's ancient, chaotic past. Every ancient star identified could be like finding one more bone of a creature that lived billions of years before humans ever existed — helping us piece together the full skeleton of our galaxy's history.
Source: Live Science