M5 Twinkles: Variable Stars in a Globular Cluster

Globular clusters are ancient cosmic “cities” of stars, and Messier 5 is one of the brightest and most impressive among them. By photographing this cluster at different times and comparing the combined frames, I noticed that some stars literally twinkle, changing their brightness from dim to bright. These are variable stars — objects that reveal important clues about the nature and evolution of ancient stellar systems.

The variability in stellar brightness arises from internal pulsations of a star’s outer layers. In the case of RR Lyrae stars, which are commonly found in globular clusters, the star periodically expands and contracts, changing its radius and surface temperature. As a result, its luminosity varies in a regular and predictable way. Because of this, RR Lyrae stars are widely used by astronomers as “standard candles” to measure distances within our Galaxy.

Based on images of the M5 cluster taken at different times, I created an animation with a smooth transition that clearly shows how the brightness of these variable stars changes. The data for the first image were obtained almost 14 years ago, on March 31, 2012. The second image of the same cluster was taken a year later, on April 15, 2013.

Gear I used:

• Sky-Watcher 130 mm f/5 Newtonian reflector telescope
• Lumenera Lu070m monochrome camera
• Baader Planetarium RGB filter set
• Sky-Watcher EQ3 mount with a single-axis electric drive

In 2012, 920 frames were captured in the luminance channel with exposure times of 4.5–10 seconds. In 2013, 754 frames were taken with exposure times of 20–40 seconds. Different exposure times were necessary because a bright lamp was shining directly in front of the balcony, and thermal air currents from a nearby boiler ventilation sometimes rose from below. To avoid overexposure and image distortion caused by turbulence, shorter exposures were often required.

For color imaging, between 100 and 150 frames were captured for each RGB filter, with an exposure time of 12 seconds per frame. The image at the beginning of this post is a composite, combining all frames of M5 taken over two years.

Observing M5, I realized that even among millions of ancient stars, a few dozen live in a special rhythm, continuously changing their brightness. These small, flickering worlds provide a powerful tool for understanding distances and the long-term evolution of our Galaxy.