How to find it
Even with the best of viewing conditions, the M5 globular star cluster is barely detectable to the naked eye as a faint star. In binoculars, it still appears as a rather faint, fuzzy star. However, the deep-sky observer Stephen James O’Meara asserts that M5 is the finest globular cluster north of the celestial equator for small telescopes – even better than the celebrated M13 Great Hercules cluster.
M5 is located in the constellation Serpens Caput (the Serpent’s Head). In mid-May, M5 is due south and highest in the sky around midnight (1 a.m. daylight savings time). Because the stars (and star clusters) return to the same place in the sky some 2 hours earlier with each passing month, M5 is highest up in the south at about 10 p.m (11 p.m. daylight saving) in mid-June, and 8 p.m. (9 p.m. daylight saving) in mid-July.
Using a fist at an armlength for a guide, M5 resides a good 2 fist-widths to the southeast of yellow-orange Arcturus, summertime’s brightest star. M5 is also three fist-widths to the east of blue-white Spica, the brightest star in the constellation Virgo. Plus, M5 is about one fist-width to the north (above) Zubeneschamali. These stars give you at least a rough idea of M5 whereabouts in the heavens.
Practiced skygazers star-hop to M5 by way of two faint yet visible Virgo stars: 109 Virginis and 110 Virginis. They draw an imaginary line from 109 Virginis through 110 Virginis, and go twice the distance to land on the star 5 Serpentis. M5 is only 1/3 degree to the northwest (upper right) of this star. The distance from 109 Virginis to M5 spans about 8 degrees of sky. For reference, the width of 4 fingers at an arm length away approximates 8 degrees.
M5 is a globular star cluster. Unlike M5, the Pleiades and the Hyades clusters are open star clusters. Open star clusters are born within the galactic disk, and live out their lives within the galactic disk. In contrast, globular clusters reside within the galactic halo – a sphere-shape region of the Milky Way that extends above and below the galactic disk. If we liken the disk to a hamburger, then the bun would be the galactic halo.
Sky chart showing M5
Globular star clusters contain tens to hundreds of thousands of stars tightly bound by gravity, whereas open star clusters are only loosely held together by gravity, with rarely as many as a thousand stars. The relatively young stars of open clusters disperse after hundreds of millions of years. The stars in globular clusters still remain intact after many billions of years.
As you gaze at M5, you’re looking at an object that’s around 13 billion years old, nearly three times older than our solar system, and almost as ancient as the universe itself. Considering that M5 lies some 25,000 light-years distant, we can only imagine what this stellar city would look like if it were at the Pleiades’ distance of 430 light-years!