Terzan 5 is one of the most interesting globular clusters in the Milky Way, with four separate identified stellar populations within it of increasing metallicity with age. With stellar motions tracked by Hubble (and later JWST) since 2003, it may not be a globular cluster after all, but rather a bulge fossil fragment: the stripped, remnant core of an ancient cannibalized galaxy. (Credit: NASA/ESA/Hubble/F. Ferraro)

Famous globular cluster Terzan 5: an imposter after all

Globular clusters are some of the most ancient cosmic relics that still survive in our Milky Way today. Famed Terzan 5 isn’t one of them.

Quick take:

• Globular clusters are some of the nearest windows we have into the ancient Universe, as many of the 150+ globular clusters located within the Milky Way have stars inside that were formed more than 13 billion years ago: extremely early in cosmic history.
• However, a few of the globular clusters that we’ve studied may not actually be globular clusters at all, but rather may be stripped remnants of ancient galaxies that fell into the Milky Way billions of years ago: bulge fossil fragments, not globular clusters.
• With more than 20 years of data from Hubble and augmented by recent JWST studies, we now have evidence that Terzan 5, one of the most famed globular clusters in the entire galaxy, isn’t a globular cluster at all. Here’s how we learned what’s really going on.

Within the Milky Way, ancient relics still persist.

There are slightly over 150 globular clusters identified within 200,000 light-years of the galactic center, with an additional five or six to be found if we double the radius of this search. While most formed along with the Milky Way, a substantial fraction did not, and were brought in at later times when smaller galaxies, such as the Kraken and Gaia-Enceladus, were devoured and cannibalized. (Credit: Larry McNish/RASC Calgary)

Over 150 globular clusters exist within our galaxy’s disk, bulge, and halo.

This image shows Euclid’s high-resolution view of globular cluster NGC 6397: only 7800 light-years from Earth, here in our own Milky Way. This object, like many globular clusters within our Milky Way, is close enough that our best observatories can resolve the individual stars within it, even close to the central regions. If intermediate mass black holes are present within them, the closest globular clusters are the best candidates for finding them, with 2024 evidence providing strong evidence for one inside Omega Centauri. (Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi, CC BY-SA 3.0 IGO)

Most possess just a single stellar population formed billions of years ago.

The globular cluster Messier 69 is highly unusual for being both incredibly old, with indications that it formed at just 5% the Universe’s present age (around 13 billion years ago), but also having a very high metal content, at 22% the metallicity of our Sun. Its location may have something to do with its high metal content: it lies very close to the galactic center. The brighter stars are in the red giant phase, just now running out of their core fuel, while a few blue stars that can be picked out visually are the result of the mergers of initially lower-mass stars: blue stragglers. (Credit: Hubble Legacy Archive (NASA/ESA/STScI))

They typically possess precious few heavy elements inside.

The life cycles of stars can be understood in the context of the color/magnitude diagram shown here. As the population of stars age, they ‘turn off’ the diagram, allowing us to date the age of the cluster in question. The oldest globular star clusters, such as the very old cluster shown at right, have an age of over 13 billion years, but many globulars also exhibit a second, more youthful population of stars alongside the older one: evidence that they had more than one burst of star-formation within them. (Credit: Richard Powell (L), R.J. Hall (R))

One major star-forming episode expels any remaining gas.

We’re only capable of identifying individual stars within the nearest globular clusters, such as Messier 71, shown here as imaged by the Hubble Space Telescope, located only ~13,000 light-years away. However, by performing population analyses of the light emitted from the stars inside, we can determine what the age(s) of the multiple bursts of stars inside a globular were, and can tell whether the stars were all formed at once or over multiple “bursts” separated by billions of years. (Credit: ESA/Hubble and NASA)

Globular clusters then persist, dark matter-free, as collections of ancient stars.

The left-hand image shows the globular star cluster Omega Centauri. The middle panel zooms in on the central area, while the right panel shows the innermost ~0.3 light-years of the cluster itself. It’s in that right-most region that the seven key hypervelocity stars were found and measured. (Credit: ESA/Hubble & NASA, M. Häberle (MPIA))

Their populations often exceed 12–13 billion years in age.

Globular clusters are round, symmetric collections of stars filled primarily with older stars: formed in bursts many billions of years ago. This object, although it looks like a globular cluster visually, is some ~200,000 light-years away and consists exclusively of young stars; it is instead an open star cluster, but one that has not yet begun to dissociate. (Credit: ESA/Hubble & NASA)

However, astrophysical appearances can be deceiving.

Liller 1, long thought to be a globular cluster, is located just 30,000 light-years away near the center of our Milky Way: within the galactic bulge. It possesses an ancient population of stars that is dated to 12 billion years old, but also another, metal-rich population formed just 1–2 billion years ago. It may not be a globular cluster, but could instead be a bulge fossil fragment. (Credit: ESA/Hubble & NASA, F. Ferraro)

Some globular-like objects could be remnants of ancient galaxies.

This ground-based image of globular cluster Terzan 5 was conducted in the near-infrared, allowing astronomers to peer through the intervening galactic dust and to identify the stars inside. The brighter stars are evolved, cool giant stars, shining brightly in infrared light, while the more common, smaller-looking stars are the relics from several different populations of stars that formed within it throughout cosmic history. (Credit: ESO/F. Ferraro)

Liller 1 and Terzan 5 are the first-ever candidates for bulge fossil fragments.

The identification of multiple populations of stars formed with very different ages and metallicities, identified here in this 2021 paper about “globular cluster” Liller 1, suggests that perhaps this is not a globular cluster, but rather a fossil fragment of an ancient galaxy now located in the Milky Way’s galactic bulge. Terzan 5 is the second such candidate thus far identified, but many more may be hiding in plain sight. (Credit: F.R. Ferraro et al., Nature Astronomy, 2021)

They possess multiple populations of stars with disparate ages and metallicities.

Discovered in 1968, this object, known as Terzan 5, has long been called a globular cluster. First imaged by Hubble in 2003, it was found to have multiple stellar populations inside in 2009, and long-term proper motion studies of stars have enabled astronomers to precisely determine which stars are cluster members and which ones are not. It now appears to host four populations of stars inside: formed 12.5, 4.7, 3.8, and 2.5 billion years ago. (Credit: NASA, ESA, CSA, STScI, Giorgia Zullo (University of Bologna), Francesco Ferraro (University of Bologna); Processing: Alyssa Pagan (STScI))

With Hubble and JWST, we’ve tracked Terzan 5’s stars since 2003.

This diagram shows the total stellar population in the vicinity of Terzan 5 (black, left) and the stars that were determined to actually be cluster members based on 20+ years of peculiar velocity data allowing us to track stellar motions (red, right). The stars identified as being within the cluster could then subsequently be analyzed for properties like color and age. (Credit: G. Zullo et al., Astronomy & Astrophysics, 2026)

Stellar motions of its outermost stars suggest dark matter’s presence.

This image of Terzan 5 was created with a mix of Hubble (F606W, F814W) and JWST NIRCam (F200W) filters. Although the stars are largely red in color, indicating an old age, there are several bright giants and a few blue stars: a mix of blue stragglers, younger-population stars, and foreground stars not associated with the cluster at all. (Credit: Meli thev/Wikimedia Commons)

Four stellar populations exist inside, formed:

• 12.5,
• 4.7,
• 3.8,
• and 2.5 billion years ago.

These curves show the stars identified as being cluster members of Terzan 5 (right), with three stellar populations of 12.5 billion years (red), 4.7 billion years (blue), 3.8 billion years (green), and a fourth population of 2.5 billion years (yellow, right) shown in the other two panels. This image is a composite of Figures 9 and 11 from the linked paper, published in 2026. (Credit: G. Zullo et al., Astronomy & Astrophysics, 2026)

More recent populations possess increasingly more heavy elements.

This color-coded map shows the heavy element abundances of stars in the Milky Way, with red indicating the most metal-rich stars and dark blue indicating the most metal-poor stars. The candidates for bulge fossil fragments appear within the red zone near the center of the Milky Way, suggesting but not proving an interplay between our galaxy’s gas and a replenishment of star-forming material in the bulge fossil fragment over time. (Credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO)

Though appearing like globular clusters, they’re probably bulge fossil fragments instead.

The elliptical galaxy adjacent to Andromeda, Messier 32 (M32), has properties that are very similar to what we expect a stripped galactic core that was gas-rich and that gave rise to stellar streams around a galaxy would look like. Although this hypothesis is not yet proven, recent (2020s-era) Hubble data about Andromeda and its satellites supports this picture quite strongly. (Credit: NASA, ESA, Benjamin F. Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern); Processing: Joseph DePasquale (STScI))

Other globular-like objects are known to host multiple populations of stars inside.

Within the field of galaxy cluster MACS J1423, a few stretched-out arcs can be seen: examples of lensed background galaxies. One such arc corresponds to the Firefly Sparkle galaxy, with ten component star clusters inside of it. Nearby, two other lensed galaxies, only 6500 and 42,000 light-years away, respectively, indicate young proto-galaxies in the process of assembling into a larger, more modern galaxy. The Firefly Sparkle galaxy may represent an early version of what Terzan 5 and/or Liller 1 are thought to be today. (Credit: NASA, ESA, CSA, STScI, Chris Willott (NRC-Canada), Lamiya Mowla (Wellesley College), Kartheik Iyer (Columbia))

The Firefly Sparkle galaxy,

This NIRCam view of a selection of the gravitationally lensed region surrounding galaxy cluster SMACS 0723 contains multiple lensed galaxies, including the thrice-appearing Sparkler galaxy, highlighted here. The “sparkles” have been identified as star-forming knots of gas appearing atop already-existing globular clusters: evidence for a second, more metal-rich population of stars forming alongside the ancient, metal-poor ones. Below the left-center of the second image of the Sparkler galaxy, a foreground star within the Milky way shows the characteristic diffraction spike pattern for JWST. (Credit: NASA, ESA, CSA, STScI; Annotation: E. Siegel)

the “Sparkler” lensed by SMACS 0723,

Here in the heart of Omega Centauri, one of the largest, richest globular clusters visible from Earth’s location within the Milky Way, lots of stars of various colors have been imaged. Owing to the dense nature of this environment, gravitational interactions between stars and stellar systems are common, often resulting in ejections, gravitational captures, and sometimes, low-mass stars (or even failed stars) winding up in tight orbits with millisecond pulsars. Multiple populations of stars exist within Omega Centauri, one of the most massive globular clusters known. (Credit: NASA, ESA, and the Hubble SM4 ERO Team)

ultra-massive Omega Centauri,

Globular cluster Messier 54 may turn out to be a bulge fossil fragment of a different galaxy: the Sagittarius Dwarf Elliptical Galaxy. Located around 80,000 light-years away, it’s very large for a globular cluster, more than 300 light-years across, and contains at least three and perhaps up to five stellar populations, formed 13, 6, 4, 2.3, and possibly a new one less than 1 billion years ago. (Credit: ESA/Hubble & NASA)

and extragalactic Messier 54,

Messier 3, a globular cluster located 33,900 light-years away, as seen through a 24″ telescope. The stars within this globular cluster are approximately 11.4 billion years old, and it can be seen with the naked eye under ideal viewing conditions. With just one stellar population inside with a low metallicity, this is a more “typical” globular cluster. (Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona)

may all ultimately be globular cluster imposters, too.

While globular clusters are always round and typically consist of very old stars, open star clusters are newly formed and have a variety of concentrations and shapes. Two open star clusters are shown here: one older and more spherical (at left), NGC 265, and one younger and more irregular (at right), NGC 290. Like NGC 411, both of these open clusters are present in the Small Magellanic Cloud. (Credit: ESA and NASA; Acknowledgment: E. Olszewski (University of Arizona))

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

*Starts With A Bang is written by Ethan Siegel, Ph.D., author of (affiliate links following) Beyond The Galaxy, Treknology, [The Littlest Girl Goes Inside An Atom](https://amzn.to/44FQQOl), and [Infinite Cosmos](https://amzn.to/3zEUHjX). His latest, The Grand Cosmic Story, is out now!*