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Messier Monday: A Globular from the Galactic Center, M9

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Peeking out through the galaxy’s dust, this ancient relic has plenty to offer if you know where to look!

“When someone demands blind obedience, you’d be a fool not to peek.”
–Jim Fiebig

Imagine that you were a tiny, overdense region of space when the Universe was first beginning. If you were too small in terms of size, excess energy would stream out, forcing you back down to average. If you were too large in terms of size, you’d have to wait until the Universe was old enough for the speed of gravity to tell you to start collapsing. But if you look at the cosmic microwave background — or the leftover glow from the Big Bang — there’s a minimum size to the density fluctuations that survive.

Image credit: ESA and the Planck Collaboration, via http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_and_the_cosmic_microwave_background.

The “smallest large-magnitude fluctuations” that make it will collapse and grow into the first structures in the Universe: gravitationally bound clumps of dark matter and gas that weigh in at around a few hundred-thousand times the mass of our Sun. Perhaps coincidentally, the most numerous structures in the Universe are around that same mass: the globular clusters, which number in the hundreds or even thousands (or tens of thousands) for every galaxy in the Universe. Most of them wind up orbiting in a galaxy’s halo, and a full 29 of them are represented in the Messier catalogue, all but one of which is in our own galaxy.

Image credit: © 2008 by Patrick Freeman, via http://www.astro-pat.com/poster-larger-031808.htm.

Many of these globulars may be as old as or even older than our galaxy itself, and today’s object — Messier 9 — formed no later than 12 billion years ago. Located just 5,500 light-years from the galactic center itself, this clump of stars weighs in at nearly half a million Suns, and is visible with just a pair of binoculars or a small telescope if you know where to look. Even on a Moon-filled summer night like tonight, it can provide you with spectacular sights.

Here’s how to find it.

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

After the Sun goes down tonight, you’ll notice a bright, waxing Moon in the southern portion of the sky, along with a bright yellow dot very close by: that’s the planet Saturn. Further to the south and east, the bright orange giant, Antares, shines prominently. Moving further towards the east and a little bit north, you’ll find the bright blue star Sabik, the second brightest star in the constellation of Ophiuchus. (Interestingly enough, if you lived on the planet Uranus, Sabik would be your pole star!)

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

If you’re looking for Messier 9, Sabik is a great place to start. South of Sabik (closer to the horizon, if you’re looking at it after sunset), you’ll find two other prominent naked-eye stars that appear to make an “arc” facing away from Antares: ξ Ophiuchi and the brighter θ Ophiuchi.

It looks like this “arc” would be symmetric if only there were a prominent fourth star in between Sabik and ξ Ophiuchi, but there are no naked eye stars to be found. Instead, point your binoculars (or your low-power telescope) at the area where you want that fourth star to be!

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

Instead, in between the two bright Hipparcos stars (labeled top and bottom, above), you’ll find a faint, fuzzy ball that appears to fade out as you move away from the center. That’s Messier 9, one of the original discoveries of Charles Messier in 1764 and described by him as:

Nebula, without star, in the right leg of Ophiuchus; it is round & its light is faint.

Indeed, with the best equipment of 250 years ago, Messier was unable to see the individual stars making up an object like this.

Image credit: NASA / ESA / Digitized Sky Survey 2 (Acknowledgement: Davide De Martin).

But it isn’t because there aren’t stars in it; it’s because the stars in this object are 25,000 light-years away, about six times as far away as the most distant individual star visible to unaided human eyesight. We’re actually very lucky when it comes to this object, because if it were positioned only a quarter of a degree west/southwest from where it is right now, it would appear obscured by one of the Milky Way’s prominent dust lanes. As a result, it would have been rendered invisible to Messier (and to us, as well) in visible wavelengths of light!

Image credit: © Copyright 1970 — 2014 by Fred Espenak, via http://astropixels.com/globularclusters/M9-01.html.

At first glance, the cluster might look a little bit oval-shaped, but that isn’t because it is intrinsically oval-shaped, it isn’t at all. Instead, the dust from the plane of the Milky Way slightly darkens one side of this collection of stars, making it appear as though it’s larger on one side than the other.

If we look, instead of in visible light, in the infrared portion of the spectrum (which is transparent to dust), the symmetric nature of this cluster becomes more apparent.

Images credit: NOAO/AURA/NSF via http://noao.edu/image_gallery/html/im0573.html (L); Two Micron All Sky Survey (2MASS) via http://www.ipac.caltech.edu/2mass/gallery/messiercat.html (R).

The stars in this cluster are some of the oldest in our local neck of the Universe, too. Our Sun is relatively rich in elements heavier than hydrogen and helium, mostly because our galaxy burned through so many generations of stars prior to our own, and we formed in an area that was heavily enriched with the atoms recycled from those generations.

But the stars in Messier 9? They contain only 1.7% of the heavy elements found in our Sun, telling us that these stars are old. This is further confirmed by the fact that — with the exception of blue straggler stars, or stars formed recently from the merger of older stars — there are no O, B, A, or even bright F-class stars in Messier 9.

Image credit: Jim Misti of Misti Mountain Observatory, via http://www.mistisoftware.com/Astronomy/Clusters_m9.htm.

There are plenty of red giants in there, however, which is something the Sun will become for hundreds of millions of years after it’s through burning its core hydrogen. For a long time, it was known that this cluster is incredibly close to the galactic center, and was thought to have slightly over 100,000 stars in it. (Wikipedia’s still out of date, for those of your checking.)

But then, two years ago, Hubble imaged it.

Image credit: NASA & ESA, via http://spacetelescope.org/images/heic1205a/.

Yes, it is very close to the galactic center, but Hubble was able to image the inner core of this cluster, finding 250,000 individual stars in the inner region of the cluster alone! This is a class VIII cluster, a little on the less-dense end of the intermediate range, meaning that there is a noticeable central concentration here, but the outer regions are relatively rich in stars as well, and that the cluster extends for quite a distance — around 45 light-years — from the center.

A zoom into the cluster shows just how close to the galactic center it really is.

As a special treat, I’ve gone and taken the Hubble image and cut into it at full-resolution, to give you a scroll-through view of the innermost region at the highest resolution available. Hold your breath and take a look, and know that from the outskirts of the cluster, this is the brilliance of what your night sky would look like!

Image credit: NASA & ESA, via http://spacetelescope.org/images/heic1205a/.

And there’s simply no way to top that, so that will bring us to the end of another Messier Monday! Including today, we’ve taken on the following Messier objects:

Come back next week for another spectacular view of the deep-sky wonders of our Universe, only here and only on Messier Monday!

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