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Starts With A Bang

Messier Monday: A Titan in a Teapot, M69

An ancient relic from the young Universe is covered in surprising riches near the galactic center.

Image credit: Paul Chasse (astronewb11) of flickr, via https://www.flickr.com/photos/astronewb2011/7247070648/.

“Ancients knew that you need guidance, patronage and protection as you move from one place or state to another, whenever you cross a bridge.” -Richard Rohr

When you look up at the night sky, you find that it’s littered with stars. If you look a little deeper — beyond, perhaps, what you can see with your naked eye — you find that it’s filled with clusters, nebulae and galaxies, some of which are only thousands of years old while others are nearly as old as the Universe itself. This Messier Monday, let’s have a look at one of the most ancient oddities we’ve ever discovered.

Image credit: Sky & Telescope’s Messier Card, via https://www.shopatsky.com/product/Messier-Card/.

You see, the Universe started off composed almost entirely of hydrogen and helium, and so the first stars that formed were practically metal-free, having very little iron, for example. So you’d expect that if we find a star cluster that’s ancient — that formed when the Universe was less than a billion years old — its stars would have very little iron, too. That’s the case, in fact, for most of them, but every once in a while, there’s an exception. Today, let’s have a look at one of the most puzzling objects in our galaxy and its cosmic story: Messier 69. Here’s how to find it.

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

After sunset tonight, the sky will darken with the Moon hovering low on the horizon in the southwest. Due south, however, you’ll find the constellation of Sagittarius, heralded by a collection of stars that looks like a teapot, one of the most recognizable asterisms of the summer-and-early-autumn skies. If you’re at high northern latitudes, these stars will appear low on the horizon and will therefore be washed out by the atmosphere, appearing dimmer than they would otherwise. Nevertheless, look towards the lowest star on the teapot: the one at the foot of the spout.

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

That’s Kaus Australis, a luminous blue giant that’s the brightest star in the entire constellation, and your starting point to find Messier 69. If you follow the imaginary line across the teapot’s bottom (towards Ascella), you’ll come to two stars that are brighter than all the others in between Kaus Australis and Ascella: HIP 91014 and HIP 90763. Look just north of the latter one — the one closer to Kaus Australis — and a fuzzy-looking star that won’t quite focus will appear.

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

That’s Messier 69, one of many original discoveries by Charles Messier himself! His discovery notes attest to how faint it appears from northern latitudes when it’s low on the horizon:

Nebula without star, in Sagittarius, below his left arm & near the arc; near it is a star of 9th magnitude; its light is very faint, one can only see it in good weather, & the least light employed to illuminate the micrometer wires makes it disappear.

Nevertheless, if you can find it, its sights are tremendously rewarding!

Image credit: © 2005–2009 by Rainer Sparenberg; photo by R.Sparenberg, S.Binnewies, V.Robering; editing by Stefan Binnewies; via http://www.airglow.de/html/starclusters/m69.html.

When you look at a large collection of stars like this — in this case, hundreds of thousands of them — you can measure their colors and brightnesses to determine just how old and evolved the stars inside are. When a star cluster first forms, it contains all the different classes of stars, from the brightest and bluest down to the dimmest and reddest: O, B, A, F, G, K, and M, in descending order. As the stars age, they start to evolve and die: first the Os, then the Bs, and so on. As the blue stars disappear, the cluster turns white, and will eventually turn yellow, orange and even red as time goes on.

Image credit: Paul and Liz Downing, of Messier 69, via http://www.paulandliz.org/Star_Clusters/Globulars.htm.

Messier 69 is so old that all the O, B, A, and even F-class stars have run through their entire life cycles. Even the brightest and bluest G-stars have died; the most massive main-sequence stars left in Messier 69 are G2-stars, the same class as our humdrum Sun. That places the age of this cluster at around 13.1 billion years, meaning that the stars in here formed when the Universe was only 700 million years old!

Image credit: Jim Mazur’s Astrophotography, via http://www.skyledge.net/Messier69.htm.

This isn’t that strange; globular clusters are often among the oldest objects known in the Universe, and even the Messier catalogue contains some that are older than this one. But when we look at the elements present inside, that’s where Messier 69 starts to look funny.

You see, our Sun formed relatively late in the history of the Universe, meaning that many generations of stars have had the chance to live-and-die prior to the Sun’s creation, enriching the environment in which we formed with riches of the heavier elements in the periodic table. The globular clusters that formed early on didn’t have this, with some containing just 1% of the heavy elements found in the Sun.

Image credit: REU program / NOAO / AURA / NSF, via http://www.noao.edu/image_gallery/html/im0630.html.

So why is it, then, when we look at this object — formed so long ago — that it contains 22% of the amount of iron our Sun has, or more than ten times as much as the other globulars of a comparable age?

The key, as any real estate agent (cosmic or terrestrial) will tell you, is location! This globular cluster isn’t located in the galactic halo or in the outskirts of the Milky Way like most of them, but rather very close to the galactic core: only 6,200 light-years away, compared to 25,000 for us. Therefore, it’s associated with the galactic bulge, a location that evolves much more quickly in terms of heavy elements and the number of generations of stars that pass by than any other in our galaxy.

Image credit: Larry McNish of RASC Calgary Centre, via http://calgary.rasc.ca/globulars.htm.

In fact, the globulars that we do find in the galactic bulge — and there are a handful of them — show that they’re all more metal-rich for their ages than any other globulars within our galaxy. So that’s the resolution to the mystery: this object is as old as it looks, but the place where it formed in the Universe (the center of our galaxy) aged much more quickly in terms of forming heavy elements than everywhere else. That’s why, if you only used the heavy element abundance, this cluster would fool you into thinking it formed later than it really did!

Image credit: Hubble Legacy Archive (NASA / ESA / STScI), via HST / Wikimedia Commons user Fabian RRRR, with original data from http://hla.stsci.edu/hlaview.html.

This cluster — although quite old — is relatively typical for globular clusters in most other ways. It contains a smattering of red giants, which is what happens to evolved stars when they run out of hydrogen in their core, it’s of about average concentration towards the core (class V on a scale of I to XII), and about the only other “oddity” in here is how few variable stars it has: only about a dozen are known. Considering the advanced age and the 30,000 light-year distance to this ancient relic, that might not be too surprising!

The best view I can give you for appreciating just how rich a globular cluster is in its central region — even in a modestly concentrated object like this — comes courtesy of the Hubble Space Telescope, and is certainly worth taking your time with.

Image credit: ESA / Hubble & NASA, cropping by me, via http://www.spacetelescope.org/images/potw1240a/.

And with that spectacular view of this unusually rich, ancient globular, we’ll come to the end of today’s Messier Monday! Including today’s object, we’ve covered 98 out of the 110 deep-sky wonders in the Messier catalogue. Have a look back at all our previous Messier Mondays:

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

And join us next week as we begin counting down the final 12 Messier objects, each one with its own, unique story, only here on Messier Monday!


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