Globular clusters are packed with variable stars, some of which are easy to see and follow in amateur telescopes. We explore M5 and M13, two of the season's finest.

Take this one for a whirl
M5 is one of the oldest known globulars with an age 13 billion years. It's located 24,500 light-years from Earth. It contains at least 105 variables, most them pulsating RR Lyrae stars.
Robert J. Vanderbei

The globulars are coming! The globulars  are coming! Along with warmer nights, spring marks the return of globular clusters. Two of my favorites are M13 in Hercules and M5 in Serpens. These densely-packed stellar swarms make spellbinding sights on dark nights. For me their richness and sparkle synchronize with the abundant life that characterizes the season.

Most globulars are well along in age, with many of their members old enough to have evolved into giant stars with helium-burning cores wrapped in shells of fusing hydrogen. At this stage, their outer envelopes begin to expand and contract or pulsate, changing in brightness as they puff in and out. In time lapse animations (below), stars in the brighter globulars twinkle like swarms of fireflies on a summer evening as their assorted variables cycle from bright to faint. An amazing sight!

Giants in the house
This Hertzsprung–Russell diagram shows the location of several types of variable stars classed according to absolute magnitude (intrinsic brightness) and spectral type. Globulars are rich in Cepheids, especially RR Lyrae stars, but also have a share of long-period Mira-type stars and the related semi-regular variables.
Rursus / CC BY-SA 3.0

RR Lyrae variables, named for the bright prototype RR Lyrae, are thick as thieves in globulars. They're a fainter, low-mass version of the giant classical Cepheid variable stars that also inhabit these densely-packed clusters. Like Cepheids, they make excellent standard candles, helping us nail down the distances to the globulars with aplomb. Other geriatric pulsators you'll come across include Mira-type variables, RV Tauri stars (solar-mass stars but puffed up to supergiant status in old age), and semiregular variables, which are similar to the Miras but have light curves that vary from one cycle to the next.

Here's winking at you
Messier 5 is home to two of the brightest globular variable stars — v42 and v84. V42 is especially easy to see because it's bright and well away from the busy center. North is up. If you prefer a negative image with black stars on a white background, click here.
Robert J. Vanderbei / Annotations by author
Lively bunch!
You can easily see RR Lyrae and other Cepheids pulsing in this animated gif. V84 was caught "blinking," while v42 shows no variation due to a fluke of timing.
Robert J. Vanderbei

Most of the variables in globulars are faint, but several clusters, including M5 and M13, offer up a nice selection for telescopes as small as 6 inches. Two of the brightest wink from M5 — variable 42, a Cepheid, and variable 84, an RV Tauri star. At maximum, v42 shines at magnitude 10.5 and stands out as the brightest star in the entire cluster. Over 25.7 days, it fades to magnitude 12 and then returns to peak brightness as the star's outer shell expands and contracts between 10 and 20 percent. Take a look the next clear night and make a note of the star's magnitude using the chart below, then return to track its ups and downs.

Combo M5 variables
Use this combination wider-view map along with the photo to step from 5 Serpentis to M5's two bright variables. Click to enlarge.

The star v84 is tucked in closer to the cluster's center, so it takes just a bit more effort to see. Its light variations are somewhat more subdued, swinging from magnitude 10.8 to 12.3 and back again over 54 days. Star v50, located in a line of three faint stars north of the core region, belongs to the semi-regular clan and varies between 14.2 and about 15.0 with an indeterminate period. I include it because it was the third variable described in M5 after its brighter siblings. A 10-inch on a night of steady seeing should paw it into view at maximum.

On April 23.2 UT, v42 was near maximum at magnitude 11.1 and perfectly obvious in my 15-inch at 64×. Closer to the cluster's core and currently at minimum, V84 at ~12.3 magnitude took a bit more digging and magnification (142×) for a clear view. With v50 I had to crank it up to 400× and use averted vision to catch glimpses of this 15th-magnitude blip.

Flashy personality
This animated gif shows the "blinking" variables across the globular M13. Like blinking globulars? Click here for a fine animation of M3's variables in action.
Robert J. Vanderbei

Like M5, M13 is rich in variables. The brightest is v11 at magnitude 11.9, located a couple arcminutes southwest of the core. Despite its relatively easy visibility, this semi-regular variable has a limited amplitude of just 0.3 magnitude over 92.5 days. To its north is the fainter but more strongly varying v2. This Cepheid cycles from magnitude 12.6 to 13.5 and back again in just 5.1 days, making it more entertaining to watch.

This annotated photo of M13 locates 14 different variables. Most are semi-regular Mira-type stars that vary by only 0.1 to 0.2 magnitude. To find the more strongly varying v11 and v2, use the three asterisms southwest of the cluster's core as guides. Magnitudes are given for one of the figures. North is up. Click here for a negative image.
Robert J. Vanderbei / Annotations by author

Both are located several arcminutes southwest of the cluster's center. Use the maps and asterisms to point you to them. The featured variables located well beyond the core will be much easier to spot and all should be visible in telescopes 8 inches and larger using a magnification of around 150×. For variables in the crowded central region, increase the power to 250× or higher. I typically start with 286×, but often switch to 357× to better "break apart" the little starry clumps some of these variables hide in. High power usually means mushy images, but I never care so long as it helps me arrive at my destination.

On April 23.2 UT in moderately good seeing, I estimated v11 at magnitude 12.2 and v2 at ~12.8. I'm eager for the next clear night to see if the latter is on its way up or down.

Asterism hopping to v11
In this wider-field view of M13 I've marked the location of V11 using the same asterisms shown in the earlier map. As a further guide, two 10.8 magnitude stars lies just outside the frame. North is up.
Jim Misti

I circled an additional dozen of reasonably bright semi-regular variables around the periphery of the cluster. They range from magnitude 11.9 to 12.4 and show only slight variations from 0.1 to 0.4 magnitude.

Most of us love the cumulative effect of so many stars heaped together in glittering piles in globular clusters, but getting to know some of these huffing-and-puffing giants individually adds character and distinction to what are otherwise anonymous points of light in a crowd.



Variable stars in the globular clusters M5 and M13
Star Magnitude range or amplitude Location Period Type
v42 10.5 — 12 M5 25.7 days Cepheid
v84 10.8 — 12.3 M5 54 days RV Tauri
v50 ~14.2 — 15.0 M5 ------ Semiregular (SR)
v11 0.3 magnitude M13 92.5 days SR
v2 12.6 — 13.5 M13 5.1 days Cepheid
v15 12.1 mag., 0.1 mag. range M13 ------ SR
v17 12.0, 0.4 mag. M13 43 days SR
v18 12.3, 0.1 mag. M13 41.2 days Slow, irregular
v19 12.1, 0.2 mag. M13 ------ SR
v20 12.1, 0.2 mag. M13 ------ SR
v24 12.0, 0.2 mag. M13 45 days SR
v33 12.1, 0.2 mag. M13 33 days SR
v39 12.0, 0.3 mag. M13 56 days SR
v40 12.1, 0.1 mag. M13 33 days SR
v42 11.9, 0.2 mag. M13 40 days SR
v43 12.4, 0.1 mag. M13 ------ Slow, irregular
v59 12.3, ? mag. M13 ------ SR?


Image of Rod


April 24, 2019 at 12:29 pm

"The globulars are coming! The globulars are coming!", yes indeed they are coming. On 22-April-19 near 2230 EDT, I enjoyed some good views of M13 in Hercules using my 90-mm refractor and M53 in Coma Berenices. On 26-Mar-19, I enjoyed excellent views of M3 in Canes Venatici using my 10-inch Newtonian. I will soon (weather permitting) plan some observations of M13 using my 10-inch. M3 was a great view, and I think M13 should be spectacular. The 90-mm refractor was quite nice so folks with smaller telescopes can also enjoy M13 in Hercules, however, larger telescopes will resolve many more halo stars in a spherical shape around this globular cluster. M3 was just like that when I viewed using my 10-inch.

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Image of Bob King

Bob King

April 25, 2019 at 9:53 am

You might just be able to see those brighter semiregular variables in your 90mm. Clear skies!

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Image of Rod


May 3, 2019 at 1:11 pm

FYI. I did enjoy some great views of M13 in Hercules and also M92 globular clusters shortly after mid-night on 29-April. I viewed at 34x to 154x using my 10-inch Newtonian. I did not look for variable stars - too many in the field of view so I just enjoyed looking 🙂 My first time viewing M13 using the 10-inch telescope. It was great!

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