The open cluster team of M6 and M7, floating just north of the Scorpion's stinger, creates one of the prettiest binocular sights in the summer sky.  Even the smallest pocket binocular will show that the brightest stars in M6 form a rectangular pattern that inventive eyes can imagine as the outline of a butterfly.  M7 is even larger and brighter than M6, and bursts into an exceptionally striking assortment of stars covering an area larger than the Full Moon.  Several of its 80 stars show subtle hues of yellow and blue.

Although conventional wisdom says that M7 is far too large to enjoy through most telescopes because of their narrow fields of view, studying it through 6-inch (15-cm) or larger instruments reveals a buried treasure nestled near the cluster's western edge.  John Herschel is credited with discovering this little gem, NGC 6453, on June 8, 1837, while observing from the Cape of Good Hope in South Africa.  Herschel included it as h3708 in his 1864 Catalogue of Nebulae and Clusters of Stars, which subsequently led John Dreyer to include it in his New General Catalog as object number 6453.  Echoing Herschel's original observation, Dreyer described the cluster as "considerably large, irregularly round, pretty much brighter (in the) middle, round."

NGC 6453 shines at 10th magnitude and measures nearly 8' across.  Were it alone in the sky, it would be a fairly routine catch in 8-inch (20-cm) and larger apertures.  But NGC 6453 is not isolated by any means; instead, it is immersed in a heavily packed field of stars. Spotting it can be literally like finding a needle in a haystack! To catch a glimpse, start with a low-power, wide-field eyepiece in your telescope and aim toward M7.  From the jumble of bright stars in M7's center, look for HD 162391, a single 5th-magnitude point of pale yellowish light toward the cluster's northwestern edge.  Switch to about 100x and then look immediately to the star's southwest for a slender triangle of 8th- and 9th magnitude points.  By traveling away from HD 162391 along the triangle's long dimension, you'll bump right into NGC 6453.

At 112x, my 8-inch (20-cm) reflector displays an unresolved smudge of light buried in a stellar ocean.  Don't confuse the few faint points scattered across the globular for object resolution.  These are most likely foreground stars, since the cluster's brightest stellar citizens are no brighter than 14th magnitude.  That's because the cluster, which is 37,000 years from Earth, is positioned inward toward the galactic center.  Its location, only 3,260 light years from the core of the Milky Way, confounds our view because of many intervening clouds of cosmic dust.

The dust causes significant reddening and obscuration, complicating studies. To counteract the effects, astronomers use infrared (IR) techniques, as IR energy can penetrate dust more effectively than visible light. Doing this, astronomers have found that stars in NGC 6453 have low metallicity, indicating they formed early in the universe.

Studies also reveal that NGC 6453 may have originated in an ancient progenitor galaxy sometimes referred to as Kraken (Kruijssen et al. 2019 and 2020) or Koala (Forbes 2020). The Milky Way cannibalized this galaxy in a major accretion event that occurred between 9-11 billion years ago. 

The accretion of Kraken/Koala played a crucial role in shaping the structure and composition of the Milky Way, contributing to its growth through the addition of stars and clusters from the smaller galaxy. In addition to NGC 6453, 20 other globular clusters are thought to have originated in Kraken/Koala. These include NGC 5946, NGC 5986, NGC 6093, NGC 6139, NGC 6144, NGC 6256, NGC 6273, NGC 6287, NGC 6333, NGC 6401, NGC 6402, NGC 6517, NGC 6541, NGC 6544, NGC 6681, NGC 6712, NGC 6809, FSR 1735, and Ton 2. The study of the chemical compositions and orbital parameters of these clusters supports their common origin, as they share distinct similarities.

The identification of these clusters as remnants of the Koala galaxy helps astronomers understand the processes of galactic evolution and the role of dwarf galaxies in building larger galactic structures. The study of such ancient mergers allows researchers to piece together the Milky Way's past and offers insights into the broader mechanisms of galaxy formation and development.

Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's test.  Contact me through my website or post to this month's discussion forum.

Until next month, remember that half of the fun is the thrill of the chase.  Game on!