A Glorious Globular Cluster — Part 2

The Infrared Astronomical Satellite was launched from Vandenberg Air Force Base, Calif., on Jan. 25, 1983. In the ten months that it made infrared readings, IRAS vastly enlarged our understanding of the universe.

The California Institute of Technology, Pasadena, credits IRAS with detecting about 75,000 galaxies undergoing intense star formation, cataloging more than 12,000 variable stars, showing the core of our Milky Way galaxy for the first time, and discovering around half a million infrared sources, which doubled the number known.

IRAS recorded an unusual infrared signature coming from the direction of the core of the globular cluster M22. Further studies proved that the source, first called IRAS 18333-2357, was indeed part of the cluster, and not a background or foreground star.

It was, according to a March 15, 1989 report in The Astrophysical Journal, “an extraordinary planetary nebula system in the galactic globular cluster M22.” A planetary nebula is called that because it looks a little like a planet. Actually, it’s gas and dust puffed off by a dying star about the size of our sun.

Spectrograph studies show the nebula in M22 is “extraordinarily oxygen-rich and neon-rich” compared with other stars in the group, says the report. It was written by F.C. Gillett, G.H. Jacoby and R.R. Joyce of Kit Peak National Observatory and J.G. Cohen, G. Neugebauer, B.T. Soifer, T. Nakajima and K. Matthews of Palomar Observatory.

The first planetary nebula inhabiting a globular cluster was found in M15 in the early 20th century. The one in M22 was renamed GJJC1, after the initials of Gillett, Jacoby, Joyce and Cohen. Since the mid-1980s, planetary nebulae were detected in two other globulars, announced in 1997.

More than 600 planetary nebulae are known, but only these four in globular clusters. About 140 globulars have been tallied in our galaxy.

Globular clusters are long-lived; they date from near the creation of the universe, 13.7 billion years ago. Planetary nebulae are short-lived, as their gas disperses relatively quickly. An April 2004 study in the Monthly Notices of the Royal Astronomical Society, London, estimates that GJJC1 is about 6,000 years old.

we’re lucky to find any brief phenomena in a bunch of ancient, ageless stars.

A British Internet site CLICK HERE says the “tiny planetary nebula … is just 10 X 7 arc seconds in size and is estimated to be around magnitude 13.8.” By comparison, a fist held at arm’s length is about 10 degrees high. One degree of the heavens is 60 arc-minutes and each arc-minute is 60 arc-seconds. So hold your fist at arm’s length. Estimate the height of one-tenth of the fist; divide one of those segments by 60; divide one of those unimaginably small parcels by 6, and you get an idea of the planetary nebula’s apparent size.

Now imagine you’re trying to photograph small details through a telescope, as I did the center of M22 last month. Everything south of Polaris is moving in the sky. My exposures were a minute and a half each, and the telescope had to track M22 with precision during the period. If it had wavered much during the exposure, the resulting photo would have been badly blurred.

[The center of M22 as I photographed it at dawn on March 28, 2009. This is a composite of six exposures totaling nine minutes. I’ve been asked for technical details, so: the telescope is a Meade LX200GPS 12″ and the camera is an SBIG ST-402ME. Guidance was through a 40-mm. guide scope using an Orion StarShoot guider]

With careful processing, individual stars show up in the middle of the M22 swarm, some large and bright, some dim, some even duller.

This Internet site, CLICK HERE, discusses “The Super Challenging Planetary Nebula GJJC1 In Globular Cluster M22.” The page, maintained by Doug Snyder of Plominas, Ariz., shows how to find the nebula.

Super challenging refers to the difficulty of observing the tiny nebula visually through a telescope, detecting it within the glare of tens of thousands of stars — not to photographing it. Snyder wrote, “Using magnifications over 400x, and an O III filter we just could not pick out this PN [planetary nebula] from the haze of unresolved stars in the core.” While I did not try to spot it visually, I enjoyed using his information to locate GJJC1 in my astrophoto.

[Location of the planetary nebula noted on my photo]

I had photographed this peculiar object and Snyder confirmed it when I sent him my photo.

[A closer version]

GJJC1 is a standout in infrared. But I wonder whether it looks much different in visible light than other faint stars in the cluster.
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What seemed to be another remarkable discovery involving M22 turns out to have been a mistake.

In 2001 NASA issued a news release saying Hubble Space Telescope researchers had discovered free-floating planets within the cluster, using a technique called microlensing. Light from a star beyond the cluster would flare briefly when a planet in M22 passed in front of it.

Kailash Sahu of the Space Telescope Science Institute (which operates Hubble), Baltimore, and colleagues “report six unusual microlensing events inside the globular cluster M22,” says the release. It adds that “to confirm these extraordinary, but tentative results, Sahu and colleagues next plan to monitor the center of the globular cluster continuously over a seven-day interval. They expect to detect 10 to 25 short-duration microlensing events, which will be well-sampled enough to yield direct measurements of the true masses of the small bodies.”

That was the last I heard of it until I recently emailed Ray Villard, the institute’s news chief, inquiring about the follow-up investigation.

He replied, “In fact we are in the process of preparing an update and correction to this release to the effect the M22 data are in error.

“I’m asking the investigator to send the details to you.”

Sahu, the principal investigator in the project, wrote:

“Dear Mr. Bauman,

“Unfortunately, further investigation by us showed these tentative detections to be due to superposition of cosmic rays in successive images. Such cosmic ray hits in successive images were thought to be unlikely, but our investigation showed that repeated cosmic rays had indeed mimicked microlensing signals. We published another paper (Sahu et al. 2002, Astrophysical Journal, vol 565, L21) which essentially retracts the earlier claims.

“Thanks for your interest.

“-Kailash”

Those who take photos of the cosmos using CCD cameras are all too well aware that cosmic rays frequently hit the chip array and cause a pinpoint of light to show up in a photo. That was the source of the “planets.”

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