Hunting Barnard’s Variable in the Globular Cluster M3

Hubble photo of M3
While it might look daunting, discovering V154 (circled) in the globular cluster M3 is not as laborious as you suppose. V154 was the first variable star ever found in a globular. When brightest, at magnitude 11.8, it stands aside from the blizzard of suns. South is up.
ESA / Hubble and NASA G. Piotto et al.

M3 was the first object in the Messier catalog found by Charles Messier himself and considered one of my favourite globular clusters. Here’s what he noticed:

“On May 3, 1764, when engaged on a catalog of the nebulae, I’ve found one between Boötes and considered one of the Hunting Dogs of Hevelius. That nebula which I’ve examined with a Gregorian telescope of 30 pouces [inches] focal size, which magnifies 104 occasions, doesn’t include any star; the middle is good, and the mild will get misplaced fading (outward); it’s spherical, and will have 3 minutes of arc in diameter.”

Charles Messier portrait
Charles Messier, seen right here at age 40, found 13 comets and compiled a catalog of 110 deep-sky objects.
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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 wide field locator map
M3, positioned in Canes Venatici, is a simple star-hop from Arcturus or Beta (β) Comae Berenices. The globular shines at magnitude 6.3, vibrant sufficient to see in binoculars.
Stellarium

M3 simply makes my high 5 listing of globular clusters, with myriad stars heaped so thickly in its luminous core they appear to spill into tendrils in all instructions. Despite its lush look in bigger newbie telescopes, we see solely 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 real diameter of round 180 light-years, the globular is so huge that it could greater than fill the space between the Earth and the Hyades Cluster.

Nearly as exceptional, M3 has 274 confirmed variable stars, excess of another globular cluster. The majority are RR Lyrae stars, additionally referred to as cluster variables — outdated, low-mass giants of spectral lessons A (Vega-like) and F (Procyon-like) that date to the formation of the galaxy.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 outdoors the core.

M3 time lapse movie
This time lapse film of M3 was taken over a single evening. Most of the variable stars twinkling in the picture are short-period RR Lyrae stars that may shortly double their brightness whereas additionally turning into noticeably bluer.
J. Hartman and (Harvard CfA) and Ok. Stanek (Ohio State University)

RR Lyrae stars occupy the instability strip on the Hertzsprung-Russell diagram, together with a number of different variable sorts together with the classical Cepheids and W Virginis stars, also referred to as the Type II Cepheids. All broaden and contract in cyclic pulsations, unable to achieve a secure equilibrium. During a pulsation, an atmospheric layer of the star contracts barely, heats up, and traps the power radiating from the core. The star dims. Rising stress inside the inside then pushes the layer again out. As it expands and cools, the trapped power is about free, and the star brightens. The layer then falls again, and the cycle begins anew.

RR Lyrae stars pulsate with durations from a couple of hours to at some point, whereas the W Virginis group has longer durations, between 10 and 20 days. Like the RR Lyrae stars they’re historic, low-mass suns that occupy the galactic halo and globular clusters. Classical Cepheids, the famed customary candles of cosmology, are younger, huge supergiants inside the galactic disk and do not concern us right here.

HR diagram and Instability Strip
The pulsating RR Lyrae, W Virginis, and classical or Type I Cepheid variable stars all lie alongside the instability journey on the Hertzsprung-Russell diagram. The H-R diagram plots stellar luminosity towards spectral classification.
Rursus / CC BY-SA 3.0

You might already be conversant in the variables V42 and V84 in the globular cluster M5. Both are vibrant sufficient to observe in a 6-inch telescope beneath darkish skies. I’ve watched their ups and downs for years. But I’ve at all times had my eye on V154 in M3, the first pulsating variable star ever found in a globular. A W Virginis star, its mild varies from about 11.8 at most to 13.2 at minimal with a interval of 15.3 days, making it a wonderful problem for an 8-inch scope.

E. E. Barnard portrait
Edward Emerson Barnard
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Although the American astronomer Edward C. Pickering discovered the star in 1889, he did not present a map or mild curve, so the object was by no means correctly cataloged. Ten years later, one other American astronomer, Edwin Emerson Barnard, rediscovered the variable and revealed a map and desk of its mild variations in 1906. For that motive it is typically referred to as Barnard’s Variable.

I’ve at all times lacked a correct map to pin down this elusive star, however I just lately did some digging and located Barnard’s original report revealed in the German astronomy journal Astronomische Nachrichten.

Barnard's Variable original map
In his map revealed in the January 1906 version of the Astronomische Nachrichten, Barnard labels three stars close to the core of M3. The star marked “7” is the variable V154. Objects #19 and #8 are comparability stars he used to estimate V154’s mild variations. In the article Barnard notes that #19 additionally confirmed small mild variation of 0.3–0.4 magnitude. Compare this determine to the picture of the identical area under. South is up.
Copyright John Wiley and Sons

In it, he features a hand-drawn map titled “Hart (sic) of M3 from micrometer measures.” While the sketch proved dead-on correct, it oversimplified the busy setting of the cluster’s core area. Finding V154 required a number of tries throughout my first try in late March 2021 with my 15-inch Dob. But abruptly, there it was. And what an ideal thrill to lastly see the star that caught the eye of considered one of astronomy’s best visible observers greater than a century in the past.

V154 variable in M3 locator map
The circled star is the variable V154. Barnard’s #8 and #19, together with “X” and the helpful telescopic asterisms, the Pointer and Spoon, are additionally proven. The three vibrant stars under the Spoon (north of cluster middle) seem on Barnard’s map as properly. When V154 shines at most, you may’t miss it! South is up.
Robert J. Vanderbei / CC BY-SA 3.0

The key to discovering the variable is a decent curl of three Thirteenth-magnitude stars I dubbed the Pointer. It lies in a comparatively clear space southeast of the dense core area and 1.1′ east of V154. Once you find this mini-asterism, you are virtually there. Follow the arc of the curl to a different bigger, spoon-shaped sample. V154 occupies one nook of a diamond that features Barnard’s #8, #19, and a 3rd star I labeled X in the photo-diagram. For those that want black stars on a white background, I’ve ready this inverted version.

By pure luck I occurred to catch the variable close to its Twelfth-magnitude most, so regardless of its location simply south of the densest area of M3’s core, it stood out clearly as considered one of the brightest stars in the space. Magnifications of 429× and 571× gave the finest views (regardless of mushy stars), however as soon as I knew precisely the place to look, V154 was seen even at 142×.

Use the highest magnification you may muster and don’t be concerned an excessive amount of about blurry stars. You’ve obtained to pry these close-spaced suns aside — that is what counts. Overmuch magnification made all the distinction in discovering V154 in addition to choosing out the comparability stars mandatory to trace its mild variations. At most, the star is clearly brighter than stars X and #19; at minimal, it is fainter than #8.

M3 globular wide view
The Pointer is circled in this wider view that reveals the complete cluster. Whether or not you determine to seek for V154, be sure to place M3 in your listing of spring must-sees. Even a 6-inch begins to interrupt down its nebular look into granules of starlight.
Jim Misti

Four nights after my first commentary, V154 had dimmed to round Thirteenth magnitude, a dramatic drop in brightness that was instantly apparent. Then the clouds returned — for 10 nights. Yikes! When I noticed V154 once more on April fifteenth it was barely fainter than magnitude 12 and fading as soon as once more. The star takes about 8 days to rise from minimal to most and about the identical to say no to minimal. Based on my most up-to-date observations I estimate that V154 will start its rise towards most once more on or about April twenty second .

One of the nice pleasures of newbie astronomy is exercising our problem-solving abilities to see one thing we have by no means seen earlier than. You make a map, hatch a plan after which execute it with the tools available and all the focus you may deliver to bear. Bad seeing, mosquitos, and clouds typically get in the way. But when the second occurs, it is pure magic.


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