We often think that in order to see pretty much anything in the sky other than the Moon or some bright stars we have to use a telescope. Believe it or not, there are some objects located thousands of light-years away that even our bare eyes can see them (under dark skies, of course), and one such object is overhead at nightfall this time of year – the Great Cluster of Hercules. The cluster, also known as M13 as it was the 13th entry on 18th-century comet hunter Charles Messier’s list of non-comets, is a grand example of a globular cluster, a dense, gravitationally bound group of stars that can have over a million members. In this case, M13 has about 300,000-500,000 stars contained in a sphere about 150 light-years wide, which equates to about one star every four to six cubic light-years. By comparison, looking out 10 light-years in all directions from our Sun (a volume of about 4,000 cubic light-years), we only find about a dozen stars!
How globular clusters formed is still not well understood. One hypothesis is that many of the stars of a cluster formed at a prolific rate from a large cloud of gas, and the similar ages and chemical compositions seen in many of the stars of a cluster provides supporting evidence for this. However, some clusters show two or more populations of stars, suggesting that star formation bursts happened at several different periods. Some large clusters, such as Omega Centauri, even resemble small spheroidal dwarf galaxies. We know that the Milky Way has interacted gravitationally with some of the dwarf galaxies orbiting it, so it is possible some globular clusters are the remains of dwarf galaxies after they have been stripped of stars thanks to close interactions with our galaxy. Globular clusters are also extremely old objects with M13 estimated to be about 11.5 billion years old – almost three times the age of our Sun. Our Milky Way has around 160 such clusters arranged in a roughly spherical distribution around it, and globular clusters are seen around many other galaxies. The giant elliptical galaxy M87, home to the supermassive black hole that we first glimpsed in 2017, has an estimated 13,000 globulars swarming around it. In fact, some of the “stars” that you see surrounding very photogenic galaxies such as the Andromeda Galaxy (M31), M87, and the Sombrero Galaxy (M104) are actually globular clusters.
In the early 20th century, astronomer Harlow Shapley (who coincidentally was the PhD advisor of Dyer Observatory’s founding astronomer Dr. Carl Seyfert) worked to create a three-dimensional map of the locations of the clusters. He observed a type of variable star in the clusters known as RR Lyrae stars. Like their bigger cousins, the Cepheid variables, RR Lyrae stars show a variation of pulsation that is directly related to their true luminosity. By observing how long it takes them to repeatedly change in brightness, Shapley could determine their true luminosities, which in turn could provide distances based on how bright the stars appeared in our sky. Armed with distances and their coordinates on the sky, he was able to create a three-dimensional map of their distribution. Interestingly, they seemed to be distributed spherically in space, and it appeared our solar system was offset from the center. Shapley correctly surmised the clusters were centered on the Milky Way, which gave us one of our first estimates of the distance to the center of the galaxy and the size of the galaxy itself. As it turns out, Shapley was unable to accurately take into account the dimming effects of dust within the galaxy that made the clusters appear dimmer and thus farther away, so his size estimate was too large by a factor of 2-3. Still, the idea was correct!
At a distance of approximately 27,000 light-years, M13 is the farthest object associated with our galaxy that one can typically see with the unaided eye. A pair of binoculars easily brings it into view, and it resembles a dusting of glitter in medium to large telescopes. M13 also has an interesting feature that becomes apparent in long-exposure photographs – the “propeller.” At first, this feature may go unnoticed, but once you see it you cannot unsee it. The propeller isn’t due to a structure blocking the light of stars behind. If you zoom in on high-resolution images, you will notice that it just so happens there aren’t really bright stars in the areas of the propeller, making it appear that there are dark lanes present. Still, it is pretty impressive coincidence that the three lanes are so uniformly spaced.
M13 is one of just a few globular clusters that doesn’t require a telescope to be able to see. Under dark spring skies well away from city light pollution, the cluster appears to the naked eye as a faint fuzzy star lying between two bright stars in the “keystone” of Hercules. M13 is also a relatively easy target to find thanks to its placement in Hercules. The torso of Hercules is marked by four naked-eye stars that form a trapezoid often referred to as “the keystone of Hercules,” and the stars Rutilicus and Eta Herculis form the western side of the keystone. Connect the two with a line and move a third of the way from Eta Herculis to Rutlicus and you will land you on M13. Still not sure? M13 also lies almost halfway between the very bright stars Arcturus and Vega. If globular clusters pique your interest, another that can be picked up by binoculars but seen in better detail telescopically is M4, one of the oldest globulars known (~12.2 billion years of age) and the closest globular cluster to our solar system (a mere 6,000 light-years away). Just find the orange-red star Antares (the brightest star of the constellation Scorpius) and look slightly more than a degree to its west.
