Bok’s Discovery

Back in the 1940’s, Harvard Astronomer Bart Bok first observed small, dark clouds of interstellar dust silhouetted against bright emission nebulae. Bok and E.F. Reily published a pioneering paper in 1947 [ref 1] hypothesizing that these were compact regions of dust that were undergoing gravitational collapse. They compared these structures to stellar “insect cocoons” where stars would eventually emerge. After Bok’s death, analysis of infrared observations published in the 1990’s eventually confirmed that stars were indeed forming inside of these globules.

The globules that Bok discovered are still the subject of intense research. Some are known to be the coldest objects in the universe; although, some have imbedded warm regions where matter is being heated by gravitational compression. The “typical” globule is thought to contain about 10-15 solar masses within a region roughly 1-2 ly across and are thought to give birth to multiple star systems.

Thackeray’s Globules

The objects pictured here are located in IC 2944 (The Running Chicken Nebula) and are called “Thackeray’s Globules” after they were first identified in 1950 by A. D. Thackeray while working with the 1.9-m telescope at the Radcliffe Observatory in S. Africa using photographic plates [ref 2]. These globules are characterized by sharp and structurally complex edges that appear to glow with respect to the central regions. These objects at first appeared to be the same sort of globules that Bok discovered, but research has shown that they are quite different.

In 1997, Reipurth, Bally, Devine, & Heathcote published a paper [ref 3] analyzing data from observations made with the ESO 3.6m telescope at the European Southern Observatory in La Silla, Chile. Image data was taken in 1988 with a RCA 620 x 1024 pixel CCD sensor, operating at an image scale of 0.38 arcsec/px through a Ha filter with an 8 nm bandpass. In this paper, the authors used an estimated distance of 1,800 pc = 5,800 ly to compute the size and physical parameters of the globules. More precise measurements published in 2025 [ref 4] using Gaia DR3 analysis of the parallaxes of 11 massive stars near the globules put the distance at 2,331 +/-30 pc = 7,600 ly. <ED: Given that the uncertainty is provided in 10’s of pc, it seems odd that this value isn’t listed as “2,330 +/-30 pc”>. This was one of the first papers to try to unravel some of the mystery surrounding these objects. The authors were the first to label the major globules (shown below) and to make careful estimates of their masses. The image below is from Reipurht, et.al., 1997 [ref 3].

📷 image.png

It is interesting to compare the images published in this 1997 paper to what can be done today using modern CMOS cameras and better digital image processing methods using much smaller telescopes. I’ve oriented my image here to roughly align with this data to simplify comparison.

My image covers a field of 8.1’ x 5.9’. The size of the globules in this region ranges from roughly 1’ (for Th1) down to about 1” for the very smallest objects, which is close to the resolution of this image. Reipurth and his collaborators determined that these particular objects represent a different class of objects than what Bok originally discovered and hypothesized about. These objects are subject to intense UV radiation from the nearby stars HD 101205 (O7 III) and HD 101191 (O8 V) and so far, show no evidence of infrared emission or protostellar formation. The intense high energy ionizing radiation that bombards these objects is causing significant photoevaporation, which causes the exposed surfaces to appear to glow, turbulence within the clouds, and movement of the clouds as a whole. The authors speculate that these globules may be the remnants of an “elephant trunk” type pillar, similar to the well known “Pillars” in M16 that has been torn apart by the intense high energy radiation from nearby type ‘O’ stars. So, rather than being regions of stellar birth, these globules may simply be the remnants of larger decaying structures. The intense high energy radiation that bombards these objects is expected to cause them to evaporate far faster than the amount of time it would take for them to form into stars.

Movement of entire clouds is being caused by something called “the rocket effect”. In simple terms, that simply means that ejected matter is imparting a force that accelerates the entire cloud, which is loosely held together by gravitational forces. M. Fernandez-Lopez et al.(2025) [ref 4] concluded that the largest globule is actually composed of three loosely connected separate globules (Th1A, Th1B, and Th1C) that appear connected simply because they are separated along our line of sight. They were able to show that these globules are all moving independently due to different evaporation rates that depend on their orientation and proximity to the type ‘O’ stars.

Comparison with HST Data

In 2003, Reipurth, B.,Raga, A., Heathcote S. published another paper to look in more detail at these objects using HST data. Below is a side-by-side comparison of my image with the Hubble data. One thing that jumps out is the usefulness of looking at combined color images to better reveal structure. Most researchers use monochrome or near monochrome image data, which is good for radiometric studies but it lacks the subtleties that can be revealed when colors are mixed in. Even with the very mediocre seeing that I experienced, the level of detail that I was able to achieve compares favorably with the low resolution Hubble image. Remember that the high resolution cameras in Hubble can probably provide somewhere between 5x - 10x the image resolution that the wide field, low resolution camera provides. So, be careful with any notion that my little 0.6 m scope can compete with Hubble in terms of raw resolution! That is simply not even close to being true. Below is a direct comparison of my image to the low resolution HST image with similar scaling taken from Reipurth, et.al., 2003 [ref 5].

📷 image.pngMy Own Brush with Bart Bok

I can’t resist the opportunity to take a brief detour to say a bit more about Bart Bok and my memory of him while I was a student. Professor Bok was certainly an interesting guy. He worked at and eventually became the director of the Harvard Observatory. He remained there for almost thirty years from 1929 until 1957 where he eventually held the Robert Wheeler Wilson Chair in Astronomy as a full professor. He eventually left Harvard to join Steward Observatory at the University of Arizona as the director for many years. I was very lucky to get to attend one of his lectures at an observatory public night while I was an undergraduate studying physics and astronomy. Professor Bok very much looked the part of a learned and distinguished professor of astronomy and he was an excellent speaker. So, the room was always packed whenever he gave a talk. It’s been so long that I don’t recall the exact subject of his talk but I remember that as an aspiring astronomy student, I was very inspired by him.

Gathering the Data and Processing Notes

I’ve had it in the back of my mind that I wanted to have another go at imaging this region for quite some time. The last time I imaged it was with my former 20” scope and I was never super happy with the details that I was able to to achieve—even though I liked the overall look of the image. Then, a little while ago, Wolfgang Proper (@Wolfgang Promper ) posted his absolutely world class image of the Running Chicken and that reminded me that time was running out before it sunk below the horizon for yet another year. So, as the moon started to wax, I switched over to narrow band with the idea that I’d just take enough data to get an acceptable image. I didn’t have another NB project on deck so I let it run for longer than I originally expected, which resulted in a very clean data set. The only problem is that this isn’t the time of year for good seeing conditions in Chile. So, this image is not composed of extraordinarily sharp data. The best that I saw in the Ha channel was only 1.2” with a median of 1.7”, which is pretty poor for this scope in this location. As you can see, the worst is above 2”, which is downright terrible for this location. The distribution given below shows what I got for the Ha data. Regardless, I tossed it all into WBPP and let it sort it out based on the PSF signal weighting.

📷 Screenshot 2026-05-03 at 1.04.05 PM.pngI snagged enough data to put together an image but I didn’t grab any RGB data for the stars. I happened to mention to Adam Block (@Adam Block) that I needed to grab a bit of RGB data and he reminded me about the PlanckStarColorMapping script by Rainer Raupach (@Rainer Raupach) . DOH! I had seen it when Rainer first posted it and I had been meaning to try it so, this was the perfect opportunity. Thanks for the reminder Adam! Rainer applied a very nice bit of black body physics to allow using narrowband data to simulate RGB star colors and it worked beautifully! It not only saved me a lot of time, but it produced a pretty nice looking result. The one thing that I found is that I had to apply pretty high saturation to get the colors to show up well after screening the stars back onto the no-stars image.

I have a few more things in mind that I may still do with this data set so if it works out, there may be more to come. As always, I’m happy to entertain feedback and questions so feel free to fire away. I love learning more about the story of little objects like these and I hope that you do too! I’d love to revisit these objects with some better seeing just to see how much more detail could be revealed.

John

=========== REFERENCES =============

(1) Bok, B. J., & Reilly, E. F. 1947,
“Small Dark Nebulae,”
The Astrophysical Journal, 105, 255.

(2) Thackeray, A. D. 1950,
“Dark Nebulae in IC 2944,”
Monthly Notes of the Astronomical Society of South Africa, 9, 37.

(3) Reipurth, B., Bally, J., Devine, D., & Heathcote, S. 1997,
“Thackeray’s Globules in IC 2944,”
Astronomy & Astrophysics, 323, 577–582.

(4) Fernández-López, M., Zapata, L. A., Reipurth, B., et al. 2025,
“Thackeray’s Globules in IC 2944: The Rocket Effect Revisited by ALMA,”
Monthly Notices of the Royal Astronomical Society.

(5) Reipurth, B.,Raga, A., Heathcote S., 2003,
“Fragmentation of Globules in HII Regions: Hubble Space Telescope Images of Thackery’s Globules,”
The Astronomical Journal, 126:1925–1932, 2003 October.