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Messier Monday: The brightest Messier globular, M22

With a planetary nebula, over 80,000 stars and a distance of only 10,000 light-years, it’s one of the most rewarding globulars of all!

Image credit: R. Columbari, via http://asterisk.apod.com/viewtopic.php?t=31833.

“I love the sea’s sounds and the way it reflects the sky. The colours that shimmer across its surface are unbelievable.” –John Dyer

When you look out at the deep-sky objects in the sky, whether nebulae, star clusters or distant galaxies, one of the things that first-time skywatchers find most surprising is how faint these wondrous sights look through an eyepiece. In particular, the nebulae and galaxies appear more diffuse and faint than people expect, particularly given the long-exposure astrophotos that they’re used to. Perhaps this is something you’ve experienced yourself, looking through a telescope, at one of the many wonders of the Messier catalogue.

Image credit: © Ole Nielsen 1999–2007, via http://www.ngc7000.org/ccd/messier.html.

Although they may be intrinsically quite bright, their great distances combined with the very large, continuous surface area that their brightness is spread over makes them appear less spectacular than photos would lead you to suspect. But the closest objects — the star clusters within our galaxy — can appear far more spectacular to a casual observer.

While globular clusters tend to be significantly farther away than the open star clusters, the brightest ones provide an absolute feast for the avid astronomer, and Messier 22 — the brightest Messier globular of them all — is perhaps the most classic example. Here’s how to find it.

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

The onset of northern hemisphere winter brings with it dark skies quite early in the night; at high northern latitudes you’ll achieve total darkness by 8 PM local time. Despite the fact that Sagittarius is very much a summer constellation, it’s quite visible even as late as October, with the famed teapot-like collection of stars prominent just above the southern horizon in the early part of the night. As this region of space is very close to the galactic center, there are many deep-sky objects to keep an eye out for, but to find Messier 22, focus on the “top” of the teapot.

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

The four stars of the handle, shown at the bottom-left of the image above, will guide you to a number of deep sky wonders, as will the three stars at the top-left, all members of Sagittarius. But it’s the very apex of the teapot — the star Kaus Borealis — that will guide you to Messier 22.

Simply draw an imaginary line connecting Kaus Borealis to the westernmost of those three stars “behind” the teapot, ξ Sagittarii, which is actually an easily-split double star. If you navigate from Kaus Borealis and go just shy of 3° towards ξ Sagittarii, it’ll be unmistakable.

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

Appearing almost midway between a bluish-white star and a red one of nearly equal brightnesses, Messier 22 stands out as a brilliant, round object with a bright core that fades away as you move farther out. According to Messier himself, it’s a:

Nebula, below the ecliptic, between the head and the bow of Sagittarius, near a star of 7th magnitude, 25 Sagittarii, according to Flamsteed, this nebula is round, it doesn’t contain any star, & one can see it very well in an ordinary telescope…

And so can you.

Image credit: © 2006 — 2012 by Siegfried Kohlert, via http://www.astroimages.de/en/gallery/M22.html.

Of all the known globular clusters, this one has the honor of being the very first discovered, by Abraham Ihle all the way back in 1665! While pretty much all globular clusters are giant collections of tens-to-hundreds-of-thousands of stars, most of which formed more than ten billion years ago, there are a few properties of Messier 22 that make it a unique treat for observers here on Earth.

Image credit: Messier 22 and Jupiter, by John C. Mirtle of http://www.astrofoto.ca/john/m022.htm.

One is that Messier 22 is extremely close to the ecliptic, meaning that it’s quite frequent that planets come very close to it. Above is Jupiter’s near-conjunction with this globular in 1996, and in fact that’s how Ihle discovered it all the way back those 3½ centuries ago: observing Saturn during a chance conjunction with this cluster!

A second is that while globular clusters are distributed in roughly a halo centered on the galaxy’s core, this one is extremely close to us on a galactic scale. While there are 29 globular clusters in the Messier catalogue (and about 150-to-200 in the galaxy total), only one of them — Messier 4 — is closer to us that today’s object!

Image credit: James Cormier of flickr, via https://www.flickr.com/photos/12598495@N08/7777609032/.

In fact, Messier 4 was the first globular resolvable into stars (by Messier), but at a distance of only 10,600 light-years (with an uncertainty of about 1,000 light years), Messier 22 is just 47% farther away than its close cousin, and its stars are clearly visible through most small telescopes today.

A third is that, despite being located in the direction of the galactic center, it’s also located along a relatively dust-free line-of-sight. (Yes, there’s some, but not so much.) While it’s more spectacular from more southerly locations where it rises higher above the horizon, the cluster itself appears little different in the visible from the infrared. Sure, different stars are prominent, which you’d expect since infrared and visible measure different temperatures, but very few new stars appear entirely, which is what would happen if dust were significant. This teaches us that the effect of extinction due to dust is small, and our path to M22 is relatively clear.

Images credit: Two-micron all-sky survey (2MASS) (L); N.A.Sharp, REU program/AURA/NOAO/NSF (R).

In some ways, the cluster is fairly typical of globulars in our galaxy:

  • It contains about 3.2% the heavy elements in our Sun,
  • It’s dated at about 12 billion years old,
  • It contains 32 known variable stars,
  • It’s of average concentration — Class VII — on a scale of I to XII,
  • Its mass is around 300,000 Suns, and
  • It spans about 100 light-years in diameter at its estimated distance.

But its very close proximity to us gives us the opportunity to discover things that may be common in globular clusters, but that are extremely difficult to detect.

Images credit: IRAS (Gillett et.al., 1989) (L); George Jacoby, KPNO (center and R), all via http://messier.seds.org/more/m022_pn.html.

Near the center of the globular cluster is the object identified as IRAS 18333–2357 (or GJJC1, labelled above), which is one of only four planetary nebulae ever found in a globular cluster. Formed from a dying Sun-like star that blows off its outer layers after burning through its fuel in its giant phase, this nebula exhibits a strong signature of doubly-ionized oxygen (O[III]), and yet is very rare for planetary nebulae in that it seems to be totally devoid of hydrogen. The age of the nebula is estimated, based on its size and the luminosity of its main star, to be only 6,000 years old.

Considering there are at least 83,000 stars in Messier 22 (at last count), there was only a chance of a few percent that we’d be serendipitous enough to find a planetary nebula in there at this point in time. But there’s another recent discovery that’s even more world-changing.

Images credit: ground-based telescope (A), Hubble legacy archive (B and C), Very Large Array / NRAO (R); all via Anna Rosen at Astrobites.com.

Just two years ago, astronomers working at the Very Large Array were searching for signals of a massive black hole at the center of this cluster. Other globulars have been found with them, and they’re theorized to be at the center of virtually all galaxies, so why not here as well? Gravitational and frictional effects should have brought all of these black holes together at the cluster center, certainly after 12 billion years!

But what they found were stellar mass black holes instead: two of them, with one about 0.8 light-years and the other 1.4 light-years from the cluster’s center. These are only visible because they’re actively feeding off of companion stars, and comparisons of those locations with X-ray observations from Chandra gives us the black hole masses: between 10-and-20 solar masses. This early data is suggestive of the fact that — given how rare feeding black holes are — there might be as many as 100 black holes in this globular cluster, and hence, possibly most globular clusters too!

Image credit: NASA, Kailash Sahu, Stefano Casertano, Mario Livio, Ron Gilliland (STScI), Nino Panagia (ESA/STScI), Michael Albrow and Mike Potter (STScI) (main Hubble image); Nigel A.Sharp, REU program/AURA/NOAO/NSF (NOAO inset).

But perhaps most spectacularly, this globular is close enough to us that we can use the Hubble Space Telescope to not only monitor those 83,000 individual stars inside, but they can look for increases in brightness among those stars. This would be expected if there were intervening, rogue planets passing along the line-of-sight between us and the stars, with gas giant-sized planets capable of causing the hypothetical, transient brightness Hubble would see. What did they find?

From February 22 to June 15, 1999, Hubble’s Wide Field and Planetary Camera 2 looked through this central region and monitored 83,000 stars. During that time the orbiting observatory recorded six unexpectedly brief microlensing events. In each case a background star jumped in brightness for less than 20 hours before dropping back to normal. These transitory spikes in brightness mean that the object passing in front of the star must have been much smaller than a normal star. Hubble also detected one clear microlensing event. In that observation a star appeared about 10 times brighter over an 18-day span before returning to normal. Astronomers traced the leap in brightness to a dwarf star in the cluster floating in front of the background star.

In other words, rogue planets, or planets without a parent star to call their home, may be extremely common in globular clusters. (Alternately, these stars may have massive exoplanets around them, some of which just happen to be fortuitously aligned along our line-of-sight.)

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

At full-resolution, the Hubble image of this cluster is spectacular.

Image credit: ESA / Hubble and NASA, via the Wikisky snapshot tool and Wikimedia Commons user Friendlystar.

All of which is to say, this is the final globular cluster left in the Messier catalogue, and the perfect one to leave you with as Messier Monday approaches its end. With just seven objects left, why don’t you take a look back at all our previous Messier Mondays here:

And come back next week for one of the most glorious sights in the deep-sky, a perfect treat for your October nights!

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