NGC 6544

Lore & Background

NGC 6544 was first observed by William Herschel in 1784 using his 18.4-inch reflector; he described it as fairly bright and large, circular, and resolvable into stars. The New General Catalogue provides a similar description. Its location is easily recoverable as it lies just 50 arcminutes southeast of the famous Lagoon Nebula, on the edge of a rich star field.

The cluster has an estimated distance of 2900 parsecs from the Sun, corresponding to 6100 parsecs from the center of the Milky Way and only 100 parsecs from the galactic plane, making it one of the closest globular clusters to the galactic plane known. Its main body has an angular size of just under 4 arcminutes, corresponding to 3.2 parsecs, making it one of the smaller globular clusters known. Its orbit is prograde but otherwise unusual, and it has a high proper motion; it is unclear whether it formed with the Milky Way or came from elsewhere. In 2021, a low-luminosity globular cluster named VVV CL160 was discovered with a similar location, metallicity, and trajectory, and may be associated with NGC 6544.

In 1999 a millisecond pulsar in a binary system was discovered within the cluster, cataloged as PSR J1807-2459. A 1993 study identified variable stars in the region, with one RR Lyrae variable (period 0.57 days) confirmed as belonging to the cluster.

Reader's Guide

NGC 6544 is significant as a small, relatively nearby globular cluster that lies very close to the galactic plane, a region where such clusters are rare. Its unusual prograde orbit and high proper motion raise questions about its origin, and the possible association with the newly discovered cluster VVV CL160 suggests a complex dynamical history. The presence of a millisecond pulsar in a binary system adds to its astrophysical interest, as such objects are tracers of stellar evolution and binary interactions in dense environments. For observers, its location near the Lagoon Nebula makes it easy to find, though its southern declination favors observers in the Southern Hemisphere and medium temperate latitudes. It can be glimpsed with 10×50 binoculars as a misty star, and telescopes of 200mm or more resolve dozens of dim stars. The best viewing period is between June and October.

Did You Know?

• NGC 6544 is located less than 1 degree southeast of the Lagoon Nebula (Messier 8).
• It is one of the closest globular clusters to the galactic plane, at only 100 parsecs from it.
• A millisecond pulsar in a binary system, PSR J1807-2459, was discovered within the cluster in 1999.
• In 2021, a low-luminosity globular cluster named VVV CL160 was found with a similar location and trajectory, possibly associated with NGC 6544.