Messier Monday: The Most Elusive Globular Cluster, M55

It took Messier himself more than a decade to observe it, but the reward is a glimpse into the ancient Universe!

Image credit: Daniel Verschatse, via http://www.astrosurf.com/antilhue/m55.htm.


“The only thing I have learnt over the years is that if you enjoy your work and put in the best efforts, it will show. If you follow this process, things work out. But if you go chasing a formula, success will elude you.” -Mahesh Babu

But if you go chasing objects in the skies, you’re far better off paying close attention to those who have found them before! That’s the whole point of astronomical catalogues, and of what we’re trying to highlight here on Messier Mondays. Intending to create an accurate catalogue — in both position and description — of the brightest deep-sky objects visible from the Northern Hemisphere, Charless Messier’s compilation of 110 astronomical curiosities has provided generations of astronomers, professional and amateur alike, with countless hours of viewing pleasure.

Image credit: © 2008 by Patrick Freeman, via http://www.astro-pat.com/poster-larger-031808.htm; today’s object highlighted by me.

But not all Messier objects are so easy to find, particularly the ones that are farthest below the celestial equator (from high northern latitudes), particularly the ones that are somewhat intrinsically faint and diffuse, and particularly the ones that don’t have any nearby, bright stars to guide you there. In the case of today’s object — Messier 55 — all three of these things are true. Yet, if you know when and where to look, a glimpse back in time to a relic of the Universe from when it was less than 10% of its present age will be your reward.

Here’s how to find it.

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

Shortly after sunset — and you’ll have to contend with a crescent Moon to find this object before it sets — the constellation of Sagittarius looms prominently in the far southern portion of the sky. As autumn begins, the famous star pattern of “the teapot” appears lower and further towards the west, setting earlier and earlier as we head towards winter. The easternmost of these eight prominent stars make up the teapot’s handle, and in particular, the two handle stars that jut out away from the kettle itself will guide you towards Messier 55.

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

These two stars — Nunki and the fainter τ Sagittarii — point in roughly a straight line towards our object, but as you can see clearly (above), there really aren’t any prominent stars in that region of sky to guide you. As it is, Messier 55 is a good 10° away from Nunki and τ Sagittarii.

However, there are three barely naked-eye stars (easily visible in binoculars or through the lowest powered telescope) that can help you along your way: a pair before you get to Messier 55 and another single one just a little beyond it.

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

The pair — an orange giant and a massive blue star — are still a good 3° away from Messier 55, while the single star is a blue giant that’s approaching us, just a single degree past today’s object. This object was first catalogued in 1752 by an observer (de Lacaille) in South Africa, with Messier looking for it in vain as early as 1764! Only in 1778 did he finally spot it, noting:

A nebula which is a whitish spot, of about 6' extension, its light is even and does not appear to contain any star. Its position has been determined from zeta Sagittarii, with the use of an intermediate star of 7th magnitude.

Through most telescopes, you’ll see something similar to this description.

Image credit: John C. Mirtle, via http://www.astrofoto.ca/john/m055.htm.

With better optics or with longer exposures, you’ll be able to make out something Messier wasn’t able to see: individual stars. What a large telescope with a long-exposure CCD camera can obtain is truly astounding, and sheds some remarkable light on this dense island of stars orbiting around the halo of our galaxy.

Image credit: Jim Mazur’s Astrophotography, via Skyledge at http://www.skyledge.net/Messier55.htm.

Because what you’re looking at isn’t just a faint, diffuse cluster of stars, these are stars that date back to some of the earliest times in the history of our galaxy! Our Sun contains lots of heavy elements: carbon, oxygen, silicon, sulphur, iron, and so on, and it’s the abundance of those heavy elements that allowed rocky planets to form around it. Stars that formed longer ago, and in regions that had fewer generations of stars live-and-die to enrich the interstellar medium, tend to be poorer in these heavy elements, and give us a glimpse of the stars that formed when the Universe was much younger.

Globular clusters tend to have older stars, but Messier 55 has just 1.1% of the heavy elements found in the Sun, one of the most metal-poor globulars known to exist!

Image credit: Peter Ward of Barden Ridge Observatory (Australia), via http://www.atscope.com.au/BRO/gallery44.html.

If you can head down to more southerly latitudes — those of you in Texas, Greece, or (ideally) Australia — are in for some truly spectacular sights, as without having the low-altitude horizon to contend with, this object truly glitters.

What you’re looking at is a collection of some 269,000 solar masses confined to a radius of just 48 light years, with a relatively diffuse core and a bright, extended density profile. On the Shapley-Sawyer Concentration Class scale for globular clusters, this is far towards the diffuse end, rating an XI on a scale of I to XII. But in a sense, this is better, because it makes the central core easier to study.

Image credit: Two Micron All Sky Survey (2MASS), via http://www.ipac.caltech.edu/2mass/gallery/messiercat.html.

So does looking in the infrared, which allows us to see through the galactic dust which blocks so much visible light. As it stands, that doesn’t have so dramatic an effect on this object, as it’s on the near side of the galaxy with respect to us (at just 17,000 light-years distant), and it’s far enough out of the galactic plane to have a relatively clear line-of-sight towards us.

That said, there are close to 100,000 stars inside, with many surprisingly long-lived giant stars, telling us that there may yet be something very special about the stars present in this globular.

Image credit: Hillary Mathis, REU Program / NOAO / AURA / NSF, via http://www.noao.edu/image_gallery/html/im0724.html.

What we’re learning is that, although the stars in here are old, they might not be quite the oldest stellar population we’ve encountered. Something may have happened to produce some excess helium before these stars first formed. Alternatively, evolutionary or environmental effects may have given this globular cluster some unusual properties, including the long-lived giant star population or the very small number of variable stars found inside.

Regardless, whether you’ve got a small amateur telescope…

Image credit: Jean-Luc Dighaye, via http://www.eurastro.de/missions/mali07/mali07.htm in Mali.

or a gigantic, world-class professional one…

Image credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit, via http://www.eso.org/public/images/eso1220a/.

the views that Messier 55 provides to us are not only teaching us about our galaxy’s ancient past, but also how stars might have evolved differently based on a slight difference in their super-ancient past!

To close today’s Messier Monday down, let’s have a look at this most spectacular view courtesy of the European Southern Observatory, in fantastic full-resolution!

Image credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit, via http://www.eso.org/public/images/eso1220a/.

The bluest stars you see are examples of recent mergers, while the brightest ones you see are giant stars, well on their way to becoming planetary nebulae! And with that, we come to the end of our 102nd Messier Monday, leaving just eight objects more. Have a look back at all our previous entries here:

And join us next week for one of the final eight objects left to explore in this remarkable catalogue, only here on Messier Monday!


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