Great Balls of Fire! — The Oldest Things You’ll Ever See

With so much to see with my smart scopes and so little time between the interference of the Moon, clouds and this year's damn Canadian fires, I often find myself working on refining observing plans. But in June this planning becomes quite easy because of all the globular clusters (GC’s) lining up to come “on stage.” With their intrinsically intense surface brightnesses they image quickly too.

Ophiuchus and Sagittarius are filled with GC’s so they are prime hunting grounds for our smart scopes. If I just start with the ones in the Messier Catalog, I have my observing queue ready to go in June — for Ophiuchus: M9, 10, 12, 14, 19, 62 &107; and for Sagittarius: M22, 28, 55, 69, 70 & 75. Before you read on, make sure they’re on your observing check list, unless, of course you are in the Southern Hemisphere.

Finished with the list? Yes. Okay we can go on. Oh, and don’t forget about the magnificent M13. Here’s a Hubble image of it. (The one in the header is one of early eVscope shots.) Note the faint blue stars. We’ll come back to them later. We can’t see them with our scopes, but they’re there and they’re sort of like zombies—which I’ll explain later.

Globulars are relatively dense, spherical clusters of stars. They typically contain 100,000 members packed into gravitational pockets of some 130 light years across. Their average densities are ten times higher than open clusters. In their central regions the density of stars rises to nearly 100 times greater than their average density.

Their compact nature is their key to their continued survival for it means the mutual gravity of all the stars is high enough to keep them from being pulled apart by the gravitational tides of our galaxy. The very appearance of GC’s as nice round star clusters means they’re also what’s called, dynamically relaxed — their stars have orbited about their centers of gravity for many orbits.

Twenty seven globular clusters make up 25% of the Messier Catalog alone. And there are many, many more. In fact two of the best are in the southern skies which would have been below Charles Messier’s horizon.

Observing GC’s is quite straightforward since their surface brightnesses are high and they have lots of articulated detail. Filters are as unproductive on them as they would be on galaxies, because they’re primarily balls of reddish stars — “Late-Type” stars to use the astronomical jargon. In addition, although GC’s may not be show stoppers like HII regions or M51, they provide much to contemplate about the deep universe and deep time.

Speaking of time, I thought I’d mention here that when I observe with one of my smart telescopes, I feel like I’m part of an astronomical Time Team. I’m a huge fan of the old BBC archaeology television series by this title and which ran for 21 seasons and delivered 286 episodes! I even received a Time Team t-shirt for my birthday earlier this year.

The Time Team, for those of you who haven’t seen it, was based upon the leadership of a hand full of landscape archaeologists, with the added support of historians and other artifact or biological specialists. They used state-of-the-art geophysical ground penetrating imaging and aerial survey to scout out where to dig. They gave themselves just three days to complete their work. And, what especially humored me is that they often dug up average people’s back yards based upon a rumor that Druids, Vikings or Romans had been hanging about once. In the end, they wound up testing their hypotheses and, if they had good luck, they collected shards of pottery, bones, coins or even ancient jewelry.

In my mind when I’m observing with a smart scope I sometimes fantasize I’m part of some sort of Cosmic Time Team. Here’s why. Our observations also require high-tech mapping and orientation when we use the plate solving and tracking capability of the scopes. And just as the archaeological Time Team use JCB backhoe loaders to speed their task, so too do we use image stacking to “excavate” our signals out of our “dirty” light-pollution noise. Our images are put together quickly — a lot faster than 3 days, we hope. And finally, I think we can think of our images as “shards” of ancient cosmic history.

The Royal Astronomical Society of Canada’s “Observer’s Handbook, 2023,” lists 42 GC targets for the amateur if you want to go beyond my initial list. And, if you’re really intrepid and able to get to the southern hemisphere too, there are some 119 GC’s known with magnitude brighter than 12. All of these are, in theory within the capabilities of our scopes. I’m not that intrepid, however, I just love M13 because it was my “first” cluster I imaged, and M22 is a favorite because of its commanding size because it’s so close.

As much as I enjoy these two, I have to admit very jealous of those of you who live at more southern latitudes. For you are able to collect the photons from the two most magnificent GCs and also one of the most curious. Omega Centuari is spectacular and is over twice the diameter of M13! (You can see a nice set of comparison photos by Lorenzo Comoli here: http://www.astrosurf.com/comolli/d159.htm). And 47 Tucanae is yet another impressive globular to image if you are in the south.

Finally, the southern globular, NGC 6397, in the constellation Ara is relatively close to us at ~8,000 ly but is extremely compact. NGC 6397 has been in the news recently because the Hubble Space Telescope has been used to uncover evidence that it contains a central swarm of some 20 black holes!

Before I get too distracted with black holes and the like, let me review the general things we should be thinking about when we image GC's:

• GC’s as a “system” form a halo that orbits the center of our Milky Way Galaxy. Harlow Shapley, the famous director of the Harvard College Observatory determined this mapping in the 1920s, thereby situating the Solar System in the distant suburbs of the galaxy.

• GC’s contain mostly very old “late type” stars that didn’t have the opportunity to acquire “metals” from earlier generations of stars. (Note that astronomers are very primitive when it comes to chemistry. Any element heavier than helium we call a “metal”.)

• GC’s are some of oldest structures in the universe — possibly older than the galaxies about which they swarm. Most globulars are 12-13 billion years old. Unless you can detect infrared galaxies like the JWST or sense the Cosmic Microwave Background, these star clusters are the oldest things you’ll ever see.

• The stars in a GC have been in a rapid dance with their siblings for so long that there have been many collisions. As a result, there are some younger, re-cycled stars called “blue stragglers” in some GCs. Sounds like a real class of blue zombies to me. Team them up with a fair number of black holes known to be in GCs and we have the cast for a cosmic episode of “The Walking Dead.”

But most of all QC’s are quick and easy delights on a summers night and your closest connection to the really ancient universe. For me, 6-8 minutes on a GC is about as good as I’m going to get and that’s fine. What I plan to do is to pour myself a cold Blue Zombie cocktail, listen to Nils Fram's "Says" and enjoy the show.

Credits:

> First M13 is J.Sweitzer. Time Team Logo from the BBC.

> Hubble M13: NASA, ESA and the Hubble Heritage Team (STScI/AURA) Acknowlegement: A. Cool (San Francisco State Univ.) and J. Anderson (STScI)

> Artist’s interpretation of black holes in NGC 6397: Credit:ESA/Hubble, N. Bartmann