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Messier Monday: The Wrong-Way Globular Cluster, M68

A surprising globular cluster found directly opposite to the galactic center!

“A friend who is far away is sometimes much nearer than one who is at hand. Is not the mountain far more awe-inspiring and more clearly visible to one passing through the valley than to those who inhabit the mountain?”
-Khalil Gibran

With a nearly full Moon not only out tonight, but located right in the Virgo Cluster, you’re going to have a terribly difficult time viewing one of those galaxies this fine Messier Monday. But the clusters of the night sky — both the open star clusters and the globular clusters — still make for spectacular viewing, and tonight’s object is one-of-a-kind!

Image credit: Greg Scheckler, from a 2008 Messier Marathon, via http://gregscheckler.wordpress.com/2008/03/11/messier-object-marathon-march-8-9-2008/.

While the 100+ globular clusters in our galaxy are roughly distributed spherically in a halo around our galactic center, we are located in the plane of the disk, about 27,000 light-years out. How unusual, then, would it be to find one of these objects in the exact opposite direction to the galactic center?

Yet 180° away, there it is: Messier 68. Here’s how to find it.

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

You can always navigate from the Big Dipper by following the “arc” of the handle to the bright orange giant, Arcturus, and then “speed on” to Spica, the bright blue star beneath. Tonight, you’ll also find Mars and the Moon nearby, but if you continue speeding on, at about 11:00 PM, you’ll encounter a few other prominent stars that will be your guide to Messier 68.

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

If you keep speeding on, you’ll run into the star β Corvi, which isn’t quite as bright as Spica but still is prominent, and can be found as the “lower-left” star of the almost-square in the constellation of Corvus. There will be two equally bright (but bluer) stars prominently above it: Algorab and Gienah, and if you trace from those stars “down” through β Corvi, you’ll come to a faint, but still naked-eye star: HIP 61621.

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

Train your telescope (or binoculars) on HIP 61621, and less than half a degree away, you’ll find the faint, extended but brilliant globular cluster: Messier 68. At least, it’s brilliant in larger telescopes; smaller ones are unable to resolve it into stars. Messier — the cluster’s discoverer — had the same problem:

Nebula without stars below Corvus & Hydra; it is very faint, very difficult to see with the refractors; near it is star of sixth magnitude.

Image credit: Joe Bohanon of http://www.joebohanon.com/wordpress/messier-marathon-part-3-galaxies-galaxies-galaxies/.

It is faint, and the brightest individual stars in it are as much fainter than HIP 61621 as that barely-visible star is from the brightest ones in the sky! It’s pretty cool that a globular cluster like this is actually found among the Messier objects; if it were in, say, the same direction as the galactic plane, it would have been invisible to Messier against the dense backdrop of stars!

As it is, however, its details can be brought out exquisitely with the right equipment.

Image credit: Geert Vandenbulcke of http://www.astronomie.be/Tranquility.Base/Messier%20pages/pagem68.htm.

The stars found in this cluster are — like the majority of globular clusters — very old, very metal-poor, and much dimmer (on average) than the typical stars found in our galaxy. These things all go hand-in-hand:

  • The stars are old because globular clusters tend to form when the Universe was much younger, and the ones that are still around tend to have been the most massive ones.
  • They’re very metal-poor because they formed out of gas that had only been enriched very slightly by prior generations of stars.
  • The stars inside are dim because the brightest stars — the O, B, A, and even most of the F-class stars — have all burned out by now!

And what we’re left with is a glittering jewel-box of ancient stellar relics from the young Universe.

Image credit: user Antilhue from Chile, via Astrosurf, at http://www.astrosurf.com/antilhue/m68.htm.

Up until the mid-20th century, there were claims that this cluster consisted of maybe 200-to-250 stars. Of course, that would be crazy for a globular cluster, which normally has at least tens-of-thousands!

At 33,000 light-years distant (so, about 60,000 light-years from the galactic center), it’s certainly one of the less impressive, less concentrated globulars, rating a concentration class of the rather loose X on a scale of I to XII. It’s also one of the most metal-poor clusters of all, containing just 0.6%-to-0.7% the amount of heavy elements that the Sun does! This is incredibly low, even for a globular cluster, and is suggestive that perhaps — as some have suggested — that it didn’t originate in the Milky Way, and was captured from a galaxy we absorbed long ago.

But a look at this cluster in the infrared teaches us something else interesting.

Image credit: Two Micron All-Sky Survey (2MASS).

There are maybe 250 giant stars inside, but thousands more. Based on the motion of the stars we see, the total mass of the cluster is probably around 200,000 times the mass of our Sun, telling us that there are probably hundreds of thousands of stars inside.

It’s also headed towards us at about 112 km/s, which really just tells us it’s moving towards the galactic center; this is totally expected behavior for a globular cluster in orbit on the outskirts of our galaxy.

Image credit: ESO, obtained with VLT KUEYEN on April 4, 1999.

But the core of this globular cluster — even though it’s a relatively low-concentration cluster — is still so dense! This image from the Very Large Telescope identified many thousands of stars in the central three light-years of this galaxy, which is very impressive.

But the best images of the core of this cluster come from the Hubble Space Telescope, which has observed it multiple times.

Image credit: NASA / ESA / Hubble, from WikiSky’s snapshot tool, via http://server1.sky-map.org/snapshot?img_size=&img_res=&ra=12.6577842&de=-26.74335&angle=0.0553424&projection=tan&rotation=-38.5669&survey=astrophoto&img_id=60446&width=4000&height=4000&img_borders=0&interpolation=bicubic2&jpeg_quality=0.95&max_preview_size=800&show_params=1&mouse_action=zoom.

In the innermost 30-or-so light-years, shown here (for a cluster that’s more than 100 light-years across), you can really see how many stars there are in this incredibly small region of space; it measures just a 20th-of-a-degree on a side!

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Still, I prefer this Hubble image of the same region, which showcases the relative brightness of what’s inside.

Image credit: ESA/Hubble & NASA, via http://www.spacetelescope.org/images/potw1231a/.

And finally, for those of you who are fans of slices-and-flythroughs of the spectacular Hubble images in full resolution, here’s the penultimate one in full, glorious resolution!

If you enjoyed today’s Messier Monday, why not take a look back at all the previous deep-sky wonders:

Next Monday, the Moon will be out of Virgo, and we’ll take a look at one of the highlights of the greatest galaxy cluster in the vicinity of our Universe! Until then, enjoy the night sky!

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