The Space Between - M72 & M73

Messier 72 (M72) and Messier (M73) are very different objects. M72 is a globular cluster, which makes it very interesting. M73, it turns out, is an asterism comprising four stars, which makes it very boring. But the field of view around them is replete with faint dust, perhaps what scientists call an integrated flux nebula (IFN). IFN is always fascinating to me. More on that below. The most interesting thing about M72 is the controversy it ginned up. More on that below, too.

Messier 72

At 50,000 light years from Earth, M72 is one of the most distant known Milky Way globular clusters. M72 is pretty diffuse as globular clusters go. It’s the sixth least dense Milky Way globular cluster known.

Globular clusters are ancient collections of stars that are gravitationally bound together in a spheroidal shape. Some are almost as old as the universe itself. They can contain anywhere from tens of thousands of stars to millions of stars. The stars they house tend to be very old as well. Star formation inside the cluster has largely ceased long ago. The formation of globular clusters is not well understood. Current research leans toward the idea that they formed from very dense molecular clouds in the early universe. Some larger globular clusters may once have been dwarf galaxies whose larger star populations were stripped away from the core by larger galaxies.

M72 Facts

Distance: 50,000 light years

Number of stars: 100,000 to 200,000

Diameter: 106 light years

Age: 9.5 billion years

Messier 73

For many years, scientists thought that M73 might be a very small open star cluster containing the four stars Messier catalogued and perhaps a few more. Astronomers argued back and forth on this subject for several years beginning in the early 2000s. In March 2000, L.P. Bassino, et al, published a paper claiming that spectral data showed that the stars in M73 were the remnants of an old open star cluster.

But two months later, G. Carraro published a paper that stated unequivocally that M73 was an asterism rather than an open cluster. The nail in the open-cluster coffin came in 2002 when M. Odenkirchen and C. Soubiran published a paper showing that the stars were at very different distances and were moving in different directions. More confirmation of M73’s asterismness came with the 2018 data release from the Gaia satellite.

Integrated Flux Nebula (IFN)?

When Messier first observed M73, he remarked that it was a cluster “of three or four small stars, which resembles a nebula at first glance, containing very little nebulosity.” The claim of some small amount of nebulosity could not be confirmed for centuries. But with modern telescope gear, we can clearly find faint dustiness blanketing the entire region.

It may be that this dustiness is integrated flux nebula (IFN), but I cannot confirm that with the tools I have at hand. Yet the region containing M72 and M73 is high above the galactic plane where one would normally find IFN, so it’s quite possible that it appears in this image. And perhaps Charles Messier’s eyes were good enough to pick up on that somehow. Integrated flux nebulas (IFNs) are the result of extremely diffuse gas and dust at or beyond the perimeter of the galaxy reflecting the light of all the stars in the Milky Way. IFNs are very faint and are extremely hard to capture with a telescope, often requiring dozens or even hundreds of hours of exposure time to unravel. 

Notwithstanding all that, one thing has never been in doubt: I’ve run my highly sophisticated analysis, and it’s clear that Dez made that catch. The ref isn’t close and he’s not even looking. He’s busy downing a big gulp of vodka from his “water” bottle.

Never one to pass up posting a link to a favorite song, I feel like this one is actually appropriate given the back and forth involving the objects in this image. There's also the large gap between M72 and M73 in the image that seems to play into this theme: