Messier Monday: The First Extragalactic Globular, M54

Ethan Siegel
Sep 22, 2014 · 10 min read

Beyond our own galaxy, this ancient relic holds a key to our cannibalistic past.

Image credit: NASA / Hubble / Wikisky.

“A hungry man can’t see right or wrong. He just sees food.” -Pearl S. Buck

But unlike our bodies, where hunger can be sated by consuming something, a galaxy simply grows hungrier and hungrier the more it eats. When it comes to gravitation, the greater your accumulation of mass, the greater your gravitational force gets. Over the span of billions of years, this explains why the Universe is networked with huge galactic clusters connected by tenuous filaments and separated by vast cosmic voids. And where we are in our little corner of the cosmos, looking up at the night sky from most northern latitudes of Earth, there are 110 deep-sky objects that stand out in a small, amateur telescope more brilliantly than all the rest.

Image credit: Rich Richins’ Messier Marathon, via http://starizona.com/acb/hyperstar/images/MessierMarathonRichins.jpg.

Of all the objects in the Messier catalogue, forty of them are galaxies beyond our own, while the rest are star clusters, nebulae or stellar remnants belonging to our own Milky Way. All of them, that is, except for today’s object: a bright globular cluster that’s beyond our own galaxy!

That object is Messier 54, and here’s where you can look to find it.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

Shortly after sunset, once the sky darkens, you can look to the south and find the constellation of Sagittarius, which includes the famous asterism of “the teapot,” shown above. With the handle towards the east and the spout towards the west, it’s one of the most recognizably human of star patterns in the sky.

And if you’re looking for the most distant of all Messier globulars, just look towards the star connecting the bottom of the handle to the base: Ascella.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

Many deep-sky objects can be found in and around this region of space, as the galactic plane and center run through this constellation. However, Messier 54 is all the more interesting because, despite being located in this part of the sky, it doesn’t have anything to do with our galaxy at all, but rather belongs to one of the dwarf galaxies in our local group, one that’s destined to be eaten by the Milky Way in just a few hundred million years.

In the meantime, we can simply look through the plane of our Milky Way, and see this object clear on the other side.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

Just 0.5° south and 1.5° west of Ascella, with the Hipparcos stars shown above flanking it, lies the most distant of all Messier globulars: Messier 54. Unsurprisingly, Messier himself was unable to resolve it into stars, seeing a:

“Very faint nebula, discovered in Sagittarius; its center is brilliant & it contains no star…”

Unlike all the other Messier globulars, even with a relatively good-sized telescope, you’re unlikely to see individual stars when you find it.

Image credit: Ole Nielsen of http://www.ngc7000.org/ccd/messier.html.

What you’re likely to see is a fuzzy but bright central core, gradually fading out as you move towards the outskirts. But just what is it that we’re looking at here?

Most globular clusters are collections of anywhere from a few tens-of-thousands of stars all the way up to a few million stars. They tend to be very dense and clustered around galaxies, with our Milky Way containing about 150–200 total and the largest galaxies — like Messier 87 — containing around 10,000!

Image credit: © 2005–2009 by Rainer Sparenberg, in collaboration with S.Binnewies and V.Robering, via http://www.airglow.de/html/starclusters/m54.html.

While the other globular clusters in the Messier catalogue are members of our Milky Way, it was only in the 1990s that we discovered that this one, M54, is not a part of our own galaxy! While the spiral of our Milky Way might be some 50,000 light-years in radius, with our Sun’s location maybe halfway from the edge to the center, globular clusters are located in a roughly spherical distribution all around the galaxy’s halo.

But unlike the others bound to our galaxy, M54 is some 87,000 light-years distant, and belongs not to the Milky Way, but to the Sagittarius Dwarf Elliptical Galaxy, a small satellite of the Milky Way in the slow-and-gradual process of being gobbled up by our own.

Image credit: Chris W. Purcell, James S. Bullock, Erik J. Tollerud, Miguel Rocha, and Sukanya Chakrabarti, via http://hipacc.ucsc.edu/html/MilkyWayImpact.html.

It’s traditionally very difficult to see through the plane of the Milky Way, as the intragalactic gas and dust conspires to block most of the visible light. But the path to the core of the Sagittarius Dwarf Elliptical Galaxy is fortuitously clear, and allows us to image its core.

Perhaps surprisingly, that’s precisely where Messier 54 resides.

Image credit: REU Program / NOAO / AURA / NSF.

Although it was originally thought that this was the dense, stripped core of the once-proud galaxy, we now believe that Messier 54 instead is and always was a globular cluster: the first globular cluster ever discovered outside our own galaxy! It’s simply the case that globulars tend to migrate towards the center of galaxies thanks to dynamical friction, meaning that the interstellar medium within the galaxy interacted with the moving globular as a “braking force.” Over billions of years, that resulted in this globular cluster winding up at the core of the dwarf galaxy.

And since then, the galaxy has begun to be ripped apart due to its gravitational interactions with the Milky Way, but the dense globular has remained intact!

Image credit: Jim Misti of Misti Mountain Observatory.

As far as globulars go, this one’s quite interesting in its own right. It’s one of the most dense types out there, coming in at class III on the I-to-XII concentration scale; it’s very large at 153 light-years in radius; it’s extremely bright, intrinsically, with 850,000 times the luminosity of our own Sun; and finally, it contains multiple stellar populations: the main population that’s approximately 13 billion years old, but four more that (probably) belonged to its parent dwarf elliptical galaxies, that formed 6 billion, 4 billion, 2.3 billion and (most recently) only a few hundred million years ago!

And finally, the very core of the galaxy shows evidence that there’s an intermediate mass black hole of approximately 9,400 solar masses at its core!

Image credit: NASA / Ata Sarajedini / Hubble Legacy Archive / STScI.

This was the first intermediate-mass black hole ever discovered: more massive than could possibly be formed from a single star but far less massive than the million-plus-solar-mass behemoths found at the centers of most galaxies.

What’s amazing is that, over the long term, this galaxy will be completely captured by the Milky Way, but it will likely remain in a very long-period orbit for many billions of years, remaining intact even as its parent galaxy is cannibalized by our own. In the meantime, star formation is probably done in this guy, so the stars we’re seeing inside it now are the largest numbers of stars we’re ever likely to observe. At the highest resolution possible, have a look — thanks to Hubble — for yourself!

Image credit: ESA/Hubble & NASA, via http://www.spacetelescope.org/images/potw1145a/.

That’s more than a million stars shining so brightly at such a distance you might mistake it for a single star through a telescope. But if you look closer — with great magnification — you’ll uncover the unbelievable story of the first globular cluster ever found beyond our own galaxy!

And with that, we wrap up our tenth-to-last Messier object of the whole catalogue! Have a look back at all our previous Messier Mondays:

And join us next week for another deep-sky wonder and story of the Universe, only here and only on Messier Monday!


Starts With A Bang!

The Universe is out there, waiting for you to discover it.

    Ethan Siegel

    Written by

    The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.

    Starts With A Bang!

    The Universe is out there, waiting for you to discover it.