Our expanding Milky Way galaxy swallowed up Kraken

Editor’s Note: Don Lincoln is a senior scientist at the Fermi National Accelerator Laboratory. He is the author of several science books for general audiences, including the best-selling audio book “The Theory of Everything: The Quest to Explain All Reality.” He also produces a series of science education videos. Follow him on Facebook. The opinions expressed in this commentary are solely his. View more opinion articles on CNN.

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Of all of the sciences, astronomy may be the one that is most universally fascinating. Yes, biology affects us day to day, but it is perhaps a bit too familiar. Astronomy is more removed, more mysterious, more majestic. It is a rare person who can look at a clear midnight sky and not have their thoughts turn to the philosophical and spiritual – contemplating such things as eternity.

A large part of its grandiosity lies in the fact that the heavens seem to be immutable. Aside from a handful of wandering planets, the familiar moon changing its location throughout the month and, for the professional, the occasional comet and the slow change of the tilt of the earth’s axis over thousands of years, the sky seems fixed and unchanging. What we see when we look up at it is nearly identical to what was seen by the very first astronomers, many thousands of years ago.

Don Lincoln

But while it may appear to be a universal truth that the stars and our galaxy doesn’t change, that is not, in fact, the case. A recent paper by an international consortium of researchers from Harvard University, the University of Heidelberg, and Liverpool John Moore’s University, published in Monthly Notices of the Royal Astronomical Society, has upended that thinking. This group, led by Dr. Diederik Kruijssen from the Center for Astronomy at Heidelberg University, believes that they have untangled what they call the family tree of the Milky Way galaxy.

The Milky Way is our cosmic home. It is a barred spiral galaxy containing between 100 billion and 400 billion stars, and our sun is a resident of one of the minor arms.

Our galaxy has existed in one form or other for about 13.6 billion years. However, it has not always been the size it is, nor has it always had its spiral shape. Astronomers have known for a long time that, over the eons, the Milky Way has gobbled other small dwarf galaxies like the familiar Magellanic Clouds.

Our galaxy is a cosmic cannibal.

Kruijssen’s group studied globular clusters in the Milky Way – clusters of as many as a million stars. Each globular cluster is bound tightly together by gravity and they are found in all galaxies. The researchers created computer simulations, called E-MOSAICS, which were able to simulate the path of globular clusters as small galaxy after small galaxy was absorbed by the Milky Way.

This program, which relies on artificial intelligence algorithms, was able to reconstruct the family tree of the Milky Way, including the merging of a previously unknown dwarf galaxy they called “Kraken.” As Kruijssen noted in an interview, prior to this work, the most significant galactic merging of which researchers were aware was an early collision between the Milky Way and a dwarf galaxy called Gaia-Enceladus, which occurred about nine billion years ago. The Kraken/Milky Way merger occurred about two billion years prior, when the Milky Way was about a quarter of its current size. The researchers claim that this earlier merging was the most impactful one in our galaxy’s history.

In total, their research shed light on five galactic merging events: Sagittarius, Sequoia, Kraken, the progenitor of the Helmi streams, and Gaia-Enceladus-Sausage. Each of these collisions between the Milky Way and smaller galaxies were crucial to the history of our galaxy.

While astronomers have long known that today’s galaxies have been assembled from earlier and smaller ones, the new research has helped us better understand our galactic history. It also reminds us that the heavens only seem unchanging on human time scales. The dinosaurs peered at a different sky.

And the process isn’t over. The giant spiral galaxy Andromeda is on a current collision course with our own Milky Way. Astronomers have long thought that the Andromeda galaxy is between three and five times more massive than our own Milky Way, although more recent research suggests that it may in fact be about the same size as the Milky Way.

No matter their sizes, in about four billion years, the two of them will likely collide in a spectacular show of cosmic fireworks. Astronomers even already have a new name picked out for the combined galaxy – Milkomeda.

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    So, what does this realization of the ever-changing nature of the heavens mean for humanity? Day-to-day, perhaps not much. But it reminds us of a classic truth, which is that change is inevitable. Perhaps the most poetic reminder is found in the lines of Percy Shelley’s Ozymandias, a sonnet describing the fall of kings – an apt metaphor for those who expect the heavens to never change.

    The research also reminds us of the truly awesome capabilities of modern science. Astronomers are able to reconstruct events that occurred over 11 billion years in the past – before our sun and earth even came into existence. It is an extraordinary accomplishment for a hairless ape who evolved only some hundreds of thousands of years ago. And I am sure that future astronomers will learn even more about our constantly changing cosmos. Permanence is an illusion, and the only eternal truth is change.