The fourth object in the Messier catalog (M4), is a globular cluster located in the constellation of Scorpius. It is one of the most visually unique and interesting globular clusters accessible to amateur astronomers.
As one of the closest globular clusters to our solar system, and also relatively near the galactic center (for a globular cluster), this summertime delight is a treat to observe.
As you follow along with this visual observing guide, note the features of this cluster through your own telescope, and learn how these unique features have been explained through extensive research of M4.
Read on to learn more about globular clusters in general, how to easily locate M4 and what you will see for yourself through various sizes of telescope.
About Globular Clusters
Globular clusters are often favorite targets for amateur astronomers due to their striking appearance at the eyepiece. As associations of hundreds of thousands of stars packed into a relatively tight space, these objects are differentiated from open clusters in key ways.
I previously wrote about M46, an open cluster in Puppis. While open clusters (also called galactic clusters) are made of relatively young stars born from the same molecular cloud and orbiting in the plane of our galaxy, globular clusters are an entirely different type of object.
In fact, globular clusters contain many of the oldest known stars and are therefore the oldest objects in our galaxy! These clusters do not orbit in the plane of our galaxy as open clusters do, rather, they are in a halo surrounding the galaxy.
As you might guess, the origin of globular clusters and their role in galactic evolution is an area of active research and remains unclear. Astronomers continue to explore big questions such as whether globular clusters originate within their host galaxy or if they are extragalactic objects. In some cases, astronomers believe that globular clusters in our galaxy may be the remnants of smaller galaxies that merged to form the Milky Way as we know it today. To me, that idea alone makes the observation of globular clusters an exciting pursuit.
Messier’s catalog contains 29 of the over 150 known globular clusters in the Milky Way. Certainly, there are more, as of yet undiscovered, as they are obscured from our view by dust and gas in the central regions of the galaxy itself.
Messier 4 – Vital Statistics
|Catalog Numbers||M4, NGC 6121|
|Type of Object||Globular cluster|
|RA (2000.0)||16h 23m 35s|
|Dec (2000.0)||-26° 31′ 33″|
|Apparent Size||36 arcminutes|
What is Messier 4?
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.
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.”
It is always surprising to me when I read that M4 was the only globular cluster that Messier was able to resolve through his “inferior” telescope.
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.
M13, for example, can reveal its peripheral members to a keen-eyed observer with a 3-inch telescope. Perhaps Messier only considered a cluster resolvable if he could identify stars near the core?
As we will see, M4 is an ancient 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.
When is M4 Visible?
This table shows when you can best see M4 from the mid-latitudes of the northern hemisphere. Times given are approximate local times for mid-month using a 24hr clock.
Times shown underlined are during the hours of darkness.
|March||00:30||05:00||09:30||observable before sunrise|
|April||22:30||03:00||07:30||observable before sunrise|
|May||20:30||01:00||05:30||Visible all night|
|June||18:30||23:00||03:30||Visible all night|
|July||16:30||21:00||01:30||Best before midnight|
|August||14:30||19:00||23:30||Best right after dark|
|September||12:30||17:00||21:30||Low in the SW after sunset|
M4 is best seen during May and June as it is visible all night. During July, it is well placed before midnight, having passed its highest point in the sky near sunset.
If you are an early morning observer, you can catch M4 in the southwestern sky before sunrise in March and April.
Seeing Conditions Needed to See M4
M4 does not rise very high for observers in the northern hemisphere and as a result, seeing conditions near the horizon can have a significant impact on how the cluster appears on a given night. When I observe, I make notes about the conditions during the observation.
I also rate the observing conditions on a scale of 1-5 (five being most ideal) based on
- Atmospheric stability (aka ‘seeing‘), and
- How transparent the atmosphere is
To estimate stability, look at bright naked-eye stars to see how much they are twinkling. Stars nearer the horizon always twinkle more due to the greater amount of atmosphere we’re looking at them through. I also take a look through the eyepiece and note how steady the stars are or how much they are scintillating in the eyepiece.
Transparency is simply a term to describe how easy it is for us to see out of the Earth’s atmosphere. Many things impact transparency such as humidity, dust, volcanic ash, moonlight, and of course scattered artificial light (light pollution). I make an estimate of the transparency based on both how dark the sky appears to my eye, as well as how close I can come to seeing the faintest naked-eye stars that I know are available at my location.
My suburban backyard is considered to be a Bortle 5 location – yet, on a given night, I may resolve many stars in M4, and on another night with poor seeing conditions and poor transparency I will experience greater difficulty. I share this as an example of why it is important to record the observing conditions when you make notes. This is especially true when using smaller telescopes.
To identify the Bortle rating for your location, I recommend visiting The Light Pollution Map website where you can zoom all the way in on the map and click on your exact location.
M4 is an easy target to see from most locations with optical aid. Even in a Bortle 8 location, M4 should be visible as a fuzzy spot in a small telescope, and with some experience, in a pair of 10 x 50 binoculars. As mentioned, M4 is somewhat low in the sky for Northern Hemisphere observers, so depending on how far north in latitude one is observing from, it can become increasingly challenging if one must look across the horizon through a lot of atmosphere, or it is sitting in the light dome of a nearby city.
How to Find M4
Popular atlas references:
- Sky and Telescope’s Pocket Sky Atlas: page 56
- Sky Atlas 2000: Chart 22
Finding M4 is rather easy, and is one of the first objects I learned to locate. It can be found approximately 1.3 degrees east of the red supergiant star Antares, and as a result, this is a cluster whose location you will forever be able to find by memory.
To begin, you will need to observe from a location with a clear southern horizon so that you can see the constellation of Scorpius. The SkySafari 6 chart below shows the situation for the central US at 11pm in late June. Note that the ‘spout’ of the easily recognized Teapot in Sagittarius points towards Antares in Scorpius.
For me, Scorpius resembles its namesake (the scorpion) far better than any other constellation. The red supergiant star Antares is the brightest in the constellation and I refer to it as the scorpion’s beating heart due to its reddish color and the way it scintillates in average seeing conditions.
Depending on how far north your observing location is, you may not be able to see the entire constellation, but you can use Antares as your guidepost because it is the brightest star in that region of the sky.
Zooming in a little, you can see just how close M4 is to Antares. They are separated by just over one degree, meaning they will comfortably fit inside the same wide-field eyepiece view.
To find M4, simply point your telescope with a low power eyepiece or binoculars at Antares, and move 1.25 degrees east. If you have a wide enough field of view, you may see both objects in the eyepiece.
It is really that simple to find, and while Antares has done us a favor in locating the cluster. However, it is important to now move Antares out of your field of view to observe and truly appreciate M4
What M4 Looks Like in a Telescope
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.
When observing globular clusters I like to make note of how well resolved a cluster appears – and I often reference the Shapley-Sawyer Concentration Class in my notes.
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.
|Did you know?|
In 2003 astronomers discovered an exoplanet in M4. The planet (PSR B1620-26 b), the first discovered in a globular cluster, is approximately 2.5 times the mass of Jupiter and orbits a binary star system. It has been nicknamed the “Genesis Planet” because it is believed to be over 12 billion years old.
Medium to large telescopes
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.
Compare this cluster to the Great Hercules globular cluster M13, or other globular clusters from Messier’s catalog and you will notice that it appears to have many fewer stars.
This is not just an appearance in your telescope, it is a fact!
Astronomers studying M4 have determined that its orbit takes it through the disc of the Milky Way at a distance of only approximately 16,000 light-years from the galactic nucleus (we are approximately 26,000 light-years from the center).
It is believed that M4 experiences tidal shocks every time it orbits through the galactic disc, which has resulted in the cluster repeatedly losing stars.
Now that you will never forget how to locate M4, I am hopeful that like me, you will visit this cluster many times.
Whether in a beautiful low power field with Antares, or at high magnification in a large telescope, M4 is always a unique sight among globulars.
Written by Dr. Alan Strauss