NASA’s Hubble opens the earliest window into the young Milky Way

The Milky Way, spanning over 100,000 light-years, is our galactic home.

Growing throughout cosmic history, it now possesses 400 billion stars, and 150+ globular clusters.

This growth required more than the steady accretion of infalling matter.

At critical moments throughout history, we swallowed other, smaller galaxies.

Some telltale signs persist from recent mergers: streams of stars encircling us.

But mergers from even longer ago also leave sustaining relics within galaxies.

These include globular clusters: carrying the metallicities and stellar ages of their original galaxy.

Globular clusters from any specific merger event should be spatially and energetically concentrated.

Stars of extragalactic origins should also have distinct properties from stars formed within the Milky Way.

Although cross-contamination of stars pose great difficulties, globular clusters retain their original stellar populations.

Therefore, examining Milky Way globular clusters can reveal their potential ancient, extragalactic origins.

Scientists using Hubble looked at 17 centrally located globular clusters.

They exquisitely determined their ages, linking 12 of them to the same formation event.

In addition to already-established mergers, a new, earlier event was uncovered.

11.8 billion years ago, a 500 million solar mass galaxy, Low-energy-Kraken-Heracles, was devoured.

It’s now the earliest Milky Way progenitor ever discovered.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.

This article is featured on Big Think.