Mergers between the Milky Way and long-ago dwarf galaxies have enriched our skies with dozens of iconic globular clusters. Many are visible in small telescopes.

Captured globulars
Each of these bright globular clusters once belonged to separate dwarf galaxies that merged with the Milky Way between about 5 and 11 billion years ago. They're now permanent residents. Clockwise from upper left they originally belonged to the Sagittarius Dwarf Elliptical Galaxy, Gaia-Enceladus, Sequoia, Koala, the Helmi streams, and the High-energy group, respectively.
Top row: ESO / Mike Brown / Hunter Wilson; Bottom row: Manfred Höcherl / Hunter Wilson / NASA / ESA

The Milky Way is a magnificent barred spiral galaxy more than 100,000 light-years in diameter and home to several hundred billion suns. But 13.6 billion years ago — within spitting distance of the Big Bang — it coalesced from clouds of gas into a lumpy, irregular mass bearing little resemblance to the place we know today. As its gravitational prowess grew it drew in more gas and spun faster and faster. The lumps smoothed out, the galaxy flattened, and its signature spiral arms took shape.

Milky Way anatomy
An artist's impression of our Milky Way galaxy, a roughly 13-billon-year-old barred spiral galaxy home to several hundred billion stars. The face-on view (left) shows the spiral arms, where the majority of stars are located, mixed with gas and dust. The disk measures about 100,000 light-years across, with the Sun located about halfway between the center and edge. The edge-on view (right) reveals the galaxy's flattened disk, central bulge and halo, home to the globular clusters.
NASA/JPL-Caltech (right), ESA and ATG medialab

Gravity molded the abundant gas into the first generation of stars which in turn exploded as supernovae and enriched the cosmic medium with more complex elements that built later generations of stars including globular clusters, the galaxy's most ancient inhabitants. Tightly bound, nearly spherical agglomerations of stars, these cosmic disco balls are thought to have formed even before the Milky Way flattened into a disk, which is the reason they reside in the halo. Many of the currently known galaxy's ~160 globular clusters coalesced from massive molecular clouds 12–13 billion years ago and have persisted to this day. Having "seen it all," we might regard them as the galaxy's historians.

In the early universe collisions and mergers were more common because galaxies were packed closer together. The Milky Way grew up during this rough-and-tumble time, expanding its borders and diversifying its holdings by ingesting smaller galaxies. Big galaxies eat little galaxies even to this day, gobbling up stars and pickpocketing star clusters. In a familiar example, M81 and M82 in Ursa Major joust like two knights of old. But instead of lances they use gravity to inflict bursts of star formation and disk-warping tidal forces on each other. The battle will finally conclude when the two merge into a single entity several billion years hence.

In this animated computer simulation, a galaxy like the Milky Way (stars in blue) and a smaller galaxy (stars in red) collide and merge into a single larger galaxy. This has happened many times over the course of the Milky Way's lifetime.
Video: H.H. Koppelman, A. Villalobos and A. Helmi / Kapteyn Astronomical Institute / Univ. of Groningen

Merger Timelines

Diederik Kruijssen (Heidelberg University, Germany, and Harvard) and colleagues, as well as Duncan Forbes (Swinburne University, Australia), have studied the ages, dynamics, and chemical compositions of the Milky Way's globular clusters to unravel the history of our galaxy's cannibalistic ways. Their research reveals that between about 11 billion and 8 billion years ago at least five former dwarf galaxies merged with our own, "donating" approximately a billion stars and up to 76 globular clusters.

Below is a play-by-play of what went down long before any of us looked up. Because the timelines and naming schemes in the research cited above vary somewhat, for consistency's sake I've primarily referred to Forbes's data. Merger dates are approximate.

  • 11 billion years ago — Koala bears sleep 18–22 hours a day so it seemed appropriate to dub the low-energy dwarf galaxy that merged with the Milky Way Koala. The one-time dwarf possessed about a billion stars and numerous globular clusters, 21 of which are now bona fide members of our own galaxy. (Koala is the same object Kruijssen and collaborators call Kraken.)
  • 10 billion years ago — Sequoia not only relinquishes an unknown number of suns but also nine globular clusters including Omega Centauri, its likely nucleus.
  • 9–11 billion years ago — Gaia-Enceladus, also known as the Gaia Sausage, collides with the Milky Way and contributes 28 globular clusters. Its nucleus now lives a second life as the massive globular NGC 1851 in Columba.
  • 8–9 billion years ago — The Sagittarius Dwarf Elliptical Galaxy adds nine clusters including its former nucleus, M54.
  • 5–8 billion years ago — A small galaxy torn into rivulets of stars named the Helmi streams parts with nine globulars.
  • The final bunch called the "High energy" group and totaling 11 globulars don't have a common origin. They likely arrived singly or perhaps two at a time from the merger of small, irregular galaxies.

Except where noted in the table below, all globular clusters listed are probable members of the dwarf galaxies described in Forbes's article "Reverse Engineering the Milky Way." Messier objects are bolded. Kruijssen's group's "Kraken" article and this earlier Sky & Telescope post provide additional details.

KoalaNGC 5946, NGC 5986, M80, NGC 6139, NGC 6144, M10, NGC 6256, M19, NGC 6287, M9, NGC 6401, M14, NGC 6453, NGC 6517, NGC 6541, NGC 6544, M70, NGC 6712, M55, FSR 1735, Ton 2
SequoiaNGC 3201, Omega Centauri, NGC 5466, NGC 6101, NGC 6535, NGC 7006, IC 4499, FSR 1758, Palomar 13
Gaia-EnceladusNGC 288, NGC 362, NGC 1261, NGC 1851, NGC 1904, NGC 2298, NGC 2808, NGC 4147, NGC 4833, NGC 5286, NGC 5897, M13, NGC 6229, NGC 6235, NGC 6284, M92, M56, M75, M2, M30, NGC 7492, IC 1257, Djorg 1, Terzan 10, ESO-SCO6, Palomar 1, Palomar 15. Tentative membership: Palomar 2
SagittariusNGC 2419, NGC 5824, M54, Arp 2, Palomar 12, Terzan 7, Terzan 8, Whiting 1, AM 4
Helmi streamsM68, M53, NGC 5053, M3, NGC 5634, M5, M72, Palomar 5, Ruprecht 106
High-energy groupNGC 5694, NGC 6426, NGC 6584, NGC 6934, AM 1, Eridanus, Pyxis, Palomar 13, Palomar 4, Palomar 14, Crater

Hello, Stranger

Omega Centauri as galactic nucleus
The iconic globular cluster Omega Centauri globular cluster, home to 10 million stars, is likely the former nucleus of the Sequoia dwarf galaxy.

Frankly, I was blown away when I saw how many bright, familiar globular clusters were not from here. Across a lifetime of observing I had assumed that most had formed in place. It would appear that nearly half originated in unremembered galaxies! The Great Hercules Cluster M13 a vagabond? M5 snatched from a drowsy Koala?

The European Space Agency's Gaia mission has been instrumental in helping astronomers reconstruct the Milky Way's history. Launched in 2013 and expected to operate until 2025 the satellite has determined precise locations, velocities, and chemical compositions of more than two billion stars and star clusters. Every nugget of knowledge gleaned extends the trail of breadcrumbs further and further back in time.

Gaia meets Milky Way
This visualization depicts the collision between the Milky Way galaxy (left) and the Gaia-Enceladus galaxy (also called the Gaia Sausage) some 10 billion years ago. M13 and M56 were among the clusters our galaxy inherited from the merger. Stars ripped from the dwarf followed long, sausage-shaped orbits, hence the name.
V. Belokurov (Cambridge, U.K.; and CCA, New York, U.S.) based on the image by ESO/Juan Carlos Muñoz

Earlier this month I spent two precious nights as a time-traveling galactic archaeologist. My goal was to visit 20 different globular clusters representing all five-plus mergers. I began in Coma Berenices with the Helmi stream trio of M53, NGC 5053, and M3 and finished at M54, the former nucleus of the Sagittarius Dwarf Elliptical Galaxy. At each stop I tried to appreciate how each donor galaxy enriched the halo of our own with these shimmering chandeliers of stars. M54 struck me as particularly impressive with its intensely bright, compact core surrounded by a symmetrical haze of minute, faint stars. It took little imagination to picture it dominating the center of a smaller galaxy 8 billion years ago until the heavy hand of the Milky Way plucked it like a ripe plum.

Let's Go There!

Messier globulars
Many of the bright Messier globular clusters once belonged to dwarf galaxies that merged with the Milky Way. Here their names are color-coded by origin. Yellow = Gaia-Enceladus; blue = Koala; orange = Helmi streams; and purple = Sagittarius Dwarf.

My 20 representatives showed great diversity in structure from tiny and faint NGC 6144 (Koala) near Antares to the splashy spangles of M10 (Koala) in Ophiuchus to the faint glitter bomb NGC 5897 (Gaia-Enceladus) in Libra. Summer nights are ideal for visiting as many of these galactic emigrants as your scope can tackle since many are well-placed at nightfall. A 6-inch scope will show at least 30 from dark skies. The map above will help you find some of the brighter Messier objects. For the others, I recommend using Sky & Telescope's Pocket Star Atlas or a free planetarium program such as Stellarium or Cartes du Ciel. The online interactive Stellarium on the Web is also excellent.

This animation depicts the collision between our Milky Way galaxy and the Andromeda galaxy. Pulled together by their mutual gravity they will collide in about 4.5 billion years and merge into a single galaxy some 6 billion years from now. The third galaxy in the video is M33 (Triangulum Galaxy) which may also merge with the Andromeda-Milky Way pair.
Credits: Visualization Credit: NASA; ESA; and F. Summers, STScI; Simulation Credit: NASA; ESA; G. Besla, Columbia University; and R. van der Marel, STScI

Although things have been pretty quiet in the galactic neighborhood for the past 5 billion years, the Milky Way's merging days are hardly over. Our biggest satellite galaxy — the Large Magellanic Cloud — is on a collision course and will share its riches in approximately 2.5 billion years. Whatever treasures the Milky Way has gathered over its lifetime will all become shared property in 4.5 billion years. That's when our galaxy and Andromeda are expected to merge. Impersonal as the cosmos may be there is a lesson in all of this. You can't hold onto anything forever. Letting go lets you grow.

Read more on how to observe galactic globular clusters in the June issue of Sky & Telescope magazine.


Image of Andrew James

Andrew James

June 28, 2023 at 10:54 pm

Usage of B.P. here? What's billion years compared to having starting date of the 1st January 1950? I think you'll find B.P. is far more related to carbon dating than these astronomical aeons. Moreover, 'ago' is simpler for any novice reading this article!

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Bob King

June 29, 2023 at 12:20 am

Hi Andrew,
You really make a good point. It's funny because I originally had another description but chose B.P. for brevity. But I don't really like it, so I went with your suggestion. Thank you!

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June 30, 2023 at 6:58 pm

Bob, It would be nice to know with these things, besides the remnants of the Galaxies in GC's, is the remnant Black Holes that might have been there centers. Do any of these GC's have especially massive BH's at there centers? If not where do you think there BH's went?

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Bob King

June 30, 2023 at 10:59 pm

Hi Skynrt,
While many globulars may have black holes there are only a few confirmed examples. The ones I'm aware of are M15, NGC 6397 and Palomar 5. Of the three, only Palomar 5 — which arrived via the Helmi streams — is on the list of the 76 clusters cobbled by the Milky Way from past dwarf galaxies. It's possible others do but we simply haven't discovered their black holes yet. So it's probably too soon to know whether any of the clusters lost their black holes. In fact, Palomar 5 has more than 100 in its center!

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Anthony Barreiro

July 1, 2023 at 10:31 pm

Thank you Bob. This is wonderful. I regularly observe M3, M5, M13, M92, M10, and M2 with 10x42 binoculars through urban light pollution from my back yard. It is wild to think that they're from other galaxies!

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Bob King

July 4, 2023 at 1:08 am

Hi Anthony,
Thank you! I know — my reaction was the same. Like having someone around all these years you thought you knew only to find out they've been keeping their identity secret!

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