M4, Globular Cluster in Scorpius
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| M4 (NGC 6121) | Globular Cluster | 16h 23m 35s | -26° 31′ 33″ | Scorpius | 5.6 | 36.0 arcmins |
Overview
Located relatively close to the center of the Milky Way, M4 is currently believed to be the third closest globular cluster to our solar system, being only 7,200 light-years away from us.
Discovered by Swiss astronomer Phillips Loys de Cheseaux in 1746, M4 was cataloged by Charles Messier in early May 1784. Messier described a “Cluster of very small [faint] stars; with an inferior telescope, it appears more like a nebula; this cluster is situated near Antares & on its parallel.”
The cluster is 75 light-years across and has an unusual bar structure crossing its core that is visible in moderate telescopes. The total mass of this lovely cluster is about 6,700 suns
As we will see, M4 is an ancient – 12 billion years old – and beautiful object that is quite easy to locate near the star Antares and a worthy target for observers of all skill levels with even the most modest equipment.
Finder Charts
This first sky chart, from SkySafari 6, shows the sky looking south at 11 pm at the end of June. M4 is immediately next to the bright star Antares, which we can locate by using the Teapot ‘spout; in Sagittarius, as shown by the dashed orange line..
Stars on this chart are shown to magnitude 5.0 and the moon and planets are not shown.
This is shown in more detail in our second sky chart, which also shows stars to magnitude 5.0.
Antares (Alpha Scorpii), is one of the brightest stars in the sky, shining at magnitude 1.1. It’s red glow combined with that low magnitude makes it easy to mistake for the planet Mars; indeed – its name means ‘rival of Mars’.
Stars are shown to magnitude 7 in the chart below and we can see how close M4 is to Antares, which is just over 1°. The next bright star northwest from Antares, Al Niyat (Sigma Scorpii) shines at magnitude 2.9 and can be used to form a right-angled triangle with Antares and M4.
Use a wide field of view and move Antares to one edge of it to bring M4 into view at the other side.
Now you have this beautiful cluster in your eyepiece, what should you expect to see?
Individual Telescope Views
The following views will help you find M4 in different telescope types by presenting the images as your telescope will show them. The first image is with a black sky and white stars, the second picture is the same image but presented in inverse monochrome. Black stars on a white background is often easier to use at the telescope. Stars are shown to magnitude 10.0. Each image can be clicked on for a full-screen version.
Upright View – This is what your eyes see unaided and through a reflex or red-dot finderscope
Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal
Mirrored View – Refractors and Cassegrain models with a star diagonal show this view
Observation
M4 is an easy cluster to partially resolve, less condensed, and with fewer stars than most globular clusters. However, there are others in his catalog that are of similar brightness and more favorably situated higher in the sky for an observer at mid-northern latitudes.
M4 is not the brightest or most easily resolved of the Messier globular clusters, yet as you will see it is one of the most unique in its appearance.
Shining at magnitude 5.6 and visible as a fuzzy non-stellar object in binoculars, this cluster reveals its character in medium and large telescopes.
Regardless of the size of your telescope, it is critical to move Antares out of your field of view. The glare from Antares is significant and will make observing details in M4 more difficult.
In telescopes less than 4-inches (100mm), start with a magnification of around 100x. You should be able to resolve a few of the peripheral member stars.
If you have observed other globular clusters, you will immediately notice that there is something unusual about M4. While most globular clusters display some amount of central condensation (meaning they are brighter towards the center), you may just begin to note that there appears to be a brighter “central bar” running through the entire cluster, roughly north to south.
As a general guide, I find observing the 11th magnitude member stars of this bar to be a challenge on most nights in telescopes less than 4-inches. In telescopes of 4 to 6 inches, the 2.5 arcminutes long bar becomes more defined, using my 6-inch telescope I can resolve some of the bar’s stars.
If you have a night of good seeing and transparency, push the magnification up to 150x and you should start to resolve a greater number of M4’s peripheral stars. At this power, even if you have a telescope of 4-inches or less try using averted vision and see if you can resolve any of the stars in the central bar.
In medium to large telescopes, the central bar feature of M4 becomes obvious, and many of the dozen or so stars that make up the bar resolve at powers between 100x and 150x.
As you spend more time observing, begin to notice how many orange and red stars you see. This is an ancient cluster, even among globulars, and is believed to have an estimated age of ~13 billion years.
In larger telescopes, many stars will be resolved, although the core generally remains fuzzy. In my 12.5-inch Dobsonian, this is a strikingly beautiful cluster, mostly resolved and displaying notable arcs of stars in the cluster’s halo. The entire bar is resolved, and I note reddish stars throughout the cluster.
- Observation content from Dr. Alan Strauss
PDFs for Printing
Each of the star maps above is reproduced as a pdf below. Each star map has a number in [square brackets] beneath it which corresponds to the file number below. If you want image [M4-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.