Our home, the Milky Way galaxy, has a long and fascinating history. It wasn’t always the grand, spiral structure we see today. Instead, it was built up over billions of years through the merging and combining of smaller galaxies.

Now, astronomers have made an exciting discovery – they’ve identified what could be the very first building blocks that went into creating our Milky Way. These are the remnants of ancient, smaller galaxies that joined together to form the beginnings of our galactic home between 12 and 13 billion years ago.

The astronomers have given these ancient structures the names Shakti and Shiva, after important figures in Hindu mythology. They found these clues by combining data from the European Space Agency’s Galactica satellite and observations from the Sloan Digital Sky Survey.

Imagine our Milky Way galaxy as a big city. Shakti and Shiva would be like the first small settlements that eventually grew into the sprawling metropolis we see today.

When galaxies collide and merge, a lot of complicated things happen. The clouds of gas in each galaxy get scrambled together, sparking the birth of new stars. And the stars from the different galaxies also get mixed up. So how can astronomers tell which stars came from which original galaxy?

It comes down to some basic physics. Stars in the same galaxy tend to share similar properties, like their energy and the way they’re moving. By looking for groups of stars with these matching characteristics, astronomers can identify remnants of the ancient mergers that built up the Milky Way.

Another clue is the chemical makeup of the stars. Newer stars have more “heavy” elements like iron and silicon, while the oldest stars are “metal-poor.” By finding metal-poor stars, astronomers can trace the earliest building blocks of our galaxy. This kind of galactic archaeology has only become possible in the last few years, thanks to incredible new data from telescopes and satellites like Galactica.

This spacecraft has mapped the positions and motions of nearly 1.5 billion stars in the Milky Way, giving astronomers an unprecedented view of our galaxy’s history. The Galactica data has already revealed other ancient structures within the Milky Way, like the Galactica Enceladus/Sausage stream – a remnant of a major merger that happened 8 to 11 billion years ago.

And now, Shakti and Shiva seem to be two even older additions to our galactic family tree. Astronomers think Shakti and Shiva could be among the very first contributors to the central “heart” of the Milky Way, helping it grow into the grand spiral galaxy we see today. Future observations will help confirm their status as the earliest known building blocks of our cosmic home.


Max Planck Institute for Astronomy | Khyati Malhan, Hans-Walter Rix, Shiva and Shakti: Presumed Proto-Galactic Fragments in the Inner Milky Way. The Astrophysical Journal 964 104. DOI 10.3847/1538-4357/ad1885

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