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Joe Bauman
The globular cluster M-5, photographed before dawn on June 15, 2018, in Tooele County, Utah.

Editor's note: A version of this was previously published on the author's website.

An unforgettable experience at the eyepiece, almost akin to seeing Saturn, is one's first look at a globular cluster. Hanging in the black of space is a spherical mass of stars whose center is so tightly packed that individual orbs cannot be picked out, while around the ball are stellar streamers and loops, the whole conglomeration glowing like gems. They are a galaxy's brilliant diamond brooches.

As far as we know, star clusters are of two types, open and globular (the preferred pronunciation starts with "glob"), with the difference reflecting not only appearance but origin.

NASA, Hubble Space Telescope, ESA
A Hubble Space Telescope view of the center of M-5 shows it's a glorious object with "100,000 stars or more, bound by gravity and packed into a region around 165 light-years in diameter. It lies some 25,000 light-years away," says NASA.

Open clusters, also called galactic clusters, are collections of stars that condensed together from the same vast cloud of gas and dust in a typical stellar nursery. Residing within a galaxy in star-forming regions, an open cluster can number several thousand young stars that formed about the same time. Eventually the stars consume nearly all of the cocoon of primordial material, but younger open clusters retain nebulous filaments of the original cloud. The stars may be scattered haphazardly. The best-known open cluster is the Pleiades, called the Seven Sisters (after the number of stars easily visible to the naked eye) although it has more than 3,000 dimmer stars.

Globular clusters orbit around the plane of the galaxy, rather than residing in the disc. Imagine a DVD: open clusters are part of the disk while globulars go up over the hole (representing the nucleus of the galaxy) and around it. Globular clusters contain some of the oldest stars in the universe. Instead of the nonchalant scatterings in open clusters, globulars' stars are concentrated in fuzzy spheres.

Our galaxy is circled by around 150 globular clusters, Georgia State University notes. Each contains "hundreds of thousands, and sometimes millions of stars. … It is thought that globular clusters formed very early in the vast halo surrounding the nascent galaxy before it flattened to form the spiral disc. Star formation would have stopped in these clusters maybe 13 billion years ago, so only old stars are expected to be found there."

Without star evolution and supernova explosions to seed heavier material into space, globular clusters probably don't have habitable planets.

According to William E. Harris, Department of Physics and Astronomy at McMaster University, Hamilton, Canada, "Globular clusters are likely to be the oldest directly visible star entities, and virtually all large galaxies appear to contain them." The number varies from galaxy to galaxy.

Certain small galaxies are deficient in globular clusters, apparently because nearby behemoths stripped them away. A decade ago, Hubble Space Telescope studies identified over 11,000 globulars in the Virgo galaxy group. Astronomers examined 100 galaxies, among the 2,000 galaxies in the group, and discovered that globular clusters are more common in areas where denser molecular clouds promote more star-formation. Speaking of M-87, the most massive galaxy in the bunch, a NASA news release says, "Many of M87's star clusters may have been snatched from smaller galaxies that ventured too close to it."

NASA, ESA and the Hubble Heritage Team (STScI/AURA); Acknowledgment: P. Cote (Herzberg Institute of Astrophysics) and E. Baltz (Stanford University)
NASA refers to the enormous elliptical galaxy M-87 as dominant among the Virgo Cluster's 2,000 galaxies, noting that M-87 houses "several trillion stars" and is approximately 54 million light-years away. A jet of material flares from its central black hole. It is believed to have stripped many of its 15,000 globular clusters from smaller galaxies. Globular clusters of stars show up as bright points in this view.

The statement quotes Eric Peng of Peking University, Beijing, China — the lead author of the study — saying, "We found few or no globular clusters in galaxies within 130,000 light-years from M87, suggesting the giant galaxy stripped the smaller ones of their star clusters." The other galaxies' loss was M-87's gain.

M-87 has an estimated 15,000 globular clusters; the supermassive galaxy has a superabundance of ancient star collections.

Until now, estimates of the distance to globular clusters in our galaxy were based on color and luminosity, comparing these to properties of stars nearer the solar system. The calculation could not be precise because of unknowns such as the amount of dust between Earth and the globulars. NASA says this caused uncertainties of 10 to 20 percent.

On April 4, the agency announced it had made the first direct measurement of the distance to a globular cluster, using geometric measurements of the minute difference in angles between one of the closest globulars and Hubble from different points of the space telescope's orbit. The result: NGC 6397, the globular cluster, is 7,800 light-years away, plus or minus 3 percent.

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Knowing the distance, luminosity, color and other factors concerning the cluster's stars, scientists were able to pinpoint its age at 13.4 billion years, not much younger than the universe itself. A 2013 report pegs the age of the universe at 13.8 billion years.

Globular clusters, either formed in place or stolen from other galaxies, are remnants of the early universe. NASA's April 4 release explains, "These spherical, densely packed swarms of hundreds of thousands of stars are the first homesteaders of the Milky Way."