Hunting Barnard's Variable in the Globular Cluster M3

M3 was the first object in the Messier catalog discovered by Charles Messier himself and one of my favorite globular clusters. Here's what he saw:

“On May 3, 1764, when working on a catalog of the nebulae, I have discovered one between Boötes and one of the Hunting Dogs of Hevelius. That nebula which I have examined with a Gregorian telescope of 30 pouces [inches] focal length, which magnifies 104 times, doesn’t contain any star; the center is brilliant, and the light gets lost fading (outward); it is round, and could have 3 minutes of arc in diameter."

Twenty years later, William Herschel examined it in his 20-foot-long reflector and resolved the "nebula" into stars, describing it as “one of the globular clusters; very brilliant and beautiful. The compression of the stars begins to increase pretty suddenly from the outside at ¾ of the radius, and continues gradually up to its center."

M3 easily makes my top five list of globular clusters, with myriad stars heaped so thickly in its luminous core they seem to spill into tendrils in all directions. Despite its lush appearance in larger amateur telescopes, we see only a tiny fraction of its estimated 500,000 suns. M3 lies about 34,000 light-years from Earth in the Milky Way's halo far above the galactic plane. With a true diameter of around 180 light-years, the globular is so enormous that it would more than fill the space between the Earth and the Hyades Cluster.

Nearly as remarkable, M3 has 274 confirmed variable stars, far more than any other globular cluster. The majority are RR Lyrae stars, also called cluster variables — old, low-mass giants of spectral classes A (Vega-like) and F (Procyon-like) that date to the formation of the galaxy. After a stint as red giants, they now fuse helium, with hydrogen burning relegated to a shell outside the core.

RR Lyrae stars occupy the instability strip on the Hertzsprung-Russell diagram, along with several other variable types including the classical Cepheids and W Virginis stars, also known as the Type II Cepheids. All expand and contract in cyclic pulsations, unable to reach a stable equilibrium. During a pulsation, an atmospheric layer of the star contracts slightly, heats up, and traps the energy radiating from the core. The star dims. Rising pressure within the interior then pushes the layer back out. As it expands and cools, the trapped energy is set free, and the star brightens. The layer then falls back, and the cycle begins anew.

RR Lyrae stars pulsate with periods from a few hours to one day, while the W Virginis group has longer periods, between 10 and 20 days. Like the RR Lyrae stars they're ancient, low-mass suns that occupy the galactic halo and globular clusters. Classical Cepheids, the famed standard candles of cosmology, are young, massive supergiants within the galactic disk and don't concern us here.

You may already be familiar with the variables V42 and V84 in the globular cluster M5. Both are bright enough to follow in a 6-inch telescope under dark skies. I've watched their ups and downs for years. But I've always had my eye on V154 in M3, the first pulsating variable star ever discovered in a globular. A W Virginis star, its light varies from about 11.8 at maximum to 13.2 at minimum with a period of 15.3 days, making it an excellent challenge for an 8-inch scope.

Although the American astronomer Edward C. Pickering discovered the star in 1889, he didn't provide a map or light curve, so the object was never properly cataloged. Ten years later, another American astronomer, Edwin Emerson Barnard, rediscovered the variable and published a map and table of its light variations in 1906. For that reason it's sometimes called Barnard's Variable.

I've always lacked a proper map to pin down this elusive star, but I recently did some digging and found Barnard's original report published in the German astronomy journal Astronomische Nachrichten.

In it, he includes a hand-drawn map titled "Hart (sic) of M3 from micrometer measures." While the sketch proved dead-on accurate, it oversimplified the busy environment of the cluster's core region. Finding V154 required several tries during my first attempt in late March 2021 with my 15-inch Dob. But suddenly, there it was. And what a great thrill to finally see the star that caught the eye of one of astronomy's greatest visual observers more than a century ago.

The key to finding the variable is a tight curl of three 13th-magnitude stars I dubbed the Pointer. It lies in a relatively clear area southeast of the dense core region and 1.1′ east of V154. Once you locate this mini-asterism, you're practically there. Follow the arc of the curl to another larger, spoon-shaped pattern. V154 occupies one corner of a diamond that includes Barnard's #8, #19, and a third star I labeled X in the photo-diagram. For those who prefer black stars on a white background, I've prepared this inverted version.

By pure luck I happened to catch the variable near its 12th-magnitude maximum, so despite its location just south of the densest region of M3's core, it stood out clearly as one of the brightest stars in the area. Magnifications of 429× and 571× gave the best views (despite mushy stars), but once I knew exactly where to look, V154 was visible even at 142×.

Use the highest magnification you can muster and don't worry too much about blurry stars. You've got to pry those close-spaced suns apart — that's what counts. Overmuch magnification made all the difference in finding V154 as well as picking out the comparison stars necessary to track its light variations. At maximum, the star is clearly brighter than stars X and #19; at minimum, it's fainter than #8.

Four nights after my first observation, V154 had dimmed to around 13th magnitude, a dramatic drop in brightness that was immediately obvious. Then the clouds returned — for 10 nights. Yikes! When I observed V154 again on April 15th it was slightly fainter than magnitude 12 and fading once again. The star takes about 8 days to rise from minimum to maximum and about the same to decline to minimum. Based on my most recent observations I estimate that V154 will begin its rise toward maximum again on or about April 22nd .

One of the great pleasures of amateur astronomy is exercising our problem-solving skills to see something we've never seen before. You make a map, hatch a plan and then execute it with the equipment on hand and all the concentration you can bring to bear. Bad seeing, mosquitos, and clouds often get in the way. But when the moment happens, it's pure magic.