M4 - An Early Summer Globular

Jack Kramer

The sight of Scorpius rising is a tip-off that the summer Milky Way is just about in view. One of the attractions of summer is a wealth of globular star clusters. The main contribution of Scorpius certainly has to be M4 (NGC 6121), its brightest cluster, which is visible with the naked eye in very dark skies, about one degree west of the bright reddish star Antares.

At a distance of 7000 light years, M4 is perhaps the closest globular cluster. We are seeing it as it was 7000 years ago, near the dawn of recorded human history. Although it contains hundreds of thousands of stars and spans over 50 light-years, M4 is one of the smallest and sparsest globular clusters known. It is also one of the oldest objects for which astronomers can estimate age directly. Cluster white dwarfs appear to be at least nine billion years old - so ancient that they approach the age of our entire universe.

A particularly unusual aspect for a globular cluster is M4's vertically oriented central line of stars. This line is easily visible even in small telescopes, owing largely to the fact that it appears against the background of the relatively loose cluster. However, this central "line" is not single stars all in a row, but rather more like two rows in close proximity. This is shown only in large instruments; our scopes show what looks like just one line. Another feature shown in the photo here, courtesy of NASA, is a number of star "chains". These are lines of stars with similar magnitudes. The brightest of these chains are visible to amateur astronomers using larger telescopes in a dark sky.

I first observed M4 as a teenager in 1955 using a 31/2-inch reflector. My notes at the time indicate that it was resolved, even in that small scope. The problem with M4 is that at mid-northern latitudes it lies at such a low declination that it's well immersed in the horizon haze that especially predominates on warm summer nights. In light polluted skies, it becomes a difficult object to locate. Where there is a lot of skyglow to the south, the best bet is to aim the finder about where M4 is shown on a star atlas, then sweep for it using a wide-field eyepiece in your telescope. It'll first be noticed as a small hazy spot, then switch to a higher magnification with which it becomes resolved after prolonged viewing. A higher magnification eyepiece tends to darken the sky background, so this may provide a view with greater contrast.

Published in the July 2001 issue of the NightTimes