The oldest core of our galaxy was born somewhere else

Twelve clusters we thought were ours

The team re-dated 17 globular clusters in the inner Milky Way from Hubble imaging, bringing the number of clusters aged by the same method to 39. The ages and metal abundances of those 39 clusters do not fall on a single curve; they trace three separate sequences. One belongs to the Milky Way's own progenitor, one to the Gaia-Sausage-Enceladus merger 10 billion years ago. The third fits neither: 12 of the 15 clusters initially tied to the low-energy group gather there, and all of them sit within 6 kiloparsecs of the galactic centre.^[1]

The progenitor behind that third sequence was named Low-energy-Kraken-Heracles. Its stellar mass is roughly 500 million solar masses, the same scale as Gaia-Sausage-Enceladus, and it was accreted about 1.8 billion years earlier. The strength of that figure comes from its being relative: the paper measures the difference directly and anchors the absolute date to a reference value adopted for Gaia-Sausage-Enceladus. NASA's announcement places the merger about 11.8 billion years ago.^[1]

Where does "ours" begin?

Stop for a moment: some of the oldest, densest star cities in the galaxy you look up at were born somewhere else, before this galaxy had taken a spiral shape at all. That place has no name of its own, no remnant, and its stars are mixed in with ours. In this picture dwarf galaxies flow into larger ones, and part of what we now call ours was digested rather than borrowed. The scale of the intuition about origins shifts here: the question is less about how the galaxy looks today than about what was folded into it.^[1]

The deflationary reading is available too, and it deserves to be taken seriously. Cosmological simulations expect between one and four substantial mergers in the history of a Milky Way-like galaxy, so finding a third confirms that expectation. Cluster membership is assigned as a probability above 50 per cent, and 3 of the low-energy group's 15 clusters already fall outside this sequence. The result therefore turns a contested guess into a dated event; it does not reveal something about our origins that nobody suspected.^[1]

What will show whether the count grows?

The three-progenitor model is the best on offer, but it leads by a margin rather than a gulf. The two-progenitor option sits 5.3 in log evidence behind and is decisively ruled out; the four-progenitor option is only 2.2 behind. The authors also say they cannot exclude progenitors whose globular clusters were fully disrupted or that never had clusters. If the number of inner Milky Way clusters aged by the same method rises clearly above today's 39, by the end of 2027 that comparison will either strengthen the three-progenitor model — Milky Way, Gaia-Sausage-Enceladus and Low-energy-Kraken-Heracles — or promote the four-progenitor one. The signal to watch is whether newly aged globular clusters settle onto the intermediate age-metallicity sequence or open a fourth.^[1]

For one person the meaning of this is modest and real. The story of the galaxy's childhood now consists of a few dated events; before this it was a nameless pile of collisions. One of those events was the end of somebody else's home. Wonder can carry that much without having to wait for certainty.^[1]