January 2018 Feature – Messier 41 – An Open Star Cluster



Image of Charles Messier with galaxiesEighteenth Century French astronomer and comet hunter Charles Messier decided that he had had enough of those annoying fuzzy-looking clouds in the night sky, especially those that made he and his fellow comet hounds waste valuable observing time on in order to rule them out as being “not comets”.   Taking matters into his own hands, he began to catalog as many of these faint and fuzzy comet impostors as he could so that comet hunters could avoid them. Of course, what he didn’t know was that he was actually compiling a catalog of some of the finest deep-sky objects the modern-day backyard astronomer can observe with a small telescope. One of these faint and fuzzy objects is known to us as Messier 41, a lovely open star cluster located in the constellation of Canis Major which, according to at least one Greek myth, is the hunting dog of nearby Orion.   Right now, on these long winter nights, is the best time to observe Messier 41.


Graphic of Messier 41 Facing south in Canis Major


Messier 41, or, just M41, is well placed for observing during the months of December, January, and February.   On January evenings, preferably someplace where there is little light pollution, step outside on a clear moonless night and find the brightest star in our night sky, Sirius, the alpha star in Canis Major. M41 is located just 4 degrees to the south of Sirius. One degree upon the sky is about equal to the width of your pinkie finger held out at arm’s length (by comparison, the full moon is a half a degree wide).

M41 has a visual magnitude of +4.5, that’s bright enough to be detected with the unaided eye (magnitude +6 being the cutoff limit for naked eye observation) as a hazy patch, but its light is easily overpowered by light pollution and by the fact that it hangs relatively low in our sky (looking at objects low in the sky means that you are looking through more atmosphere and more atmosphere means more of the object’s light is going to be lost to you).   My recommendation is to wait and do your observing when Sirius is placed highest in the sky, which, in January, means going out and looking due south at around 11:00 P.M. or 12:00 A.M.

Start out by using binoculars with M41, if the sky is dark enough you should be able to pick it out within the same field of view of Sirius. With 7x binoculars you can expect to see a couple dozen of the stars within the cluster, 10x binoculars will of course show you many more.

Image of how to measure the sky distances using your hand

With a telescope, always use low power magnification with M41. Why? Because M41 has an apparent size of 38 arcminutes. Remember, one degree is about the width of your pinkie finger held out at arm’s length, astronomers then divide one degree up into 60 individual arcminutes (with each arcminute being further divided into 60 arcseconds). Recall that the full moon is about a half a degree across, or, we can say that the full moon is 30 arcminutes across. So, M41 has an apparent size a bit larger than that of the full moon and if you use higher magnifications, you won’t be able to see the cluster in its full glory.

A small telescope will reveal about 50 bright stars, you will also begin to notice that there is a strong concentration of stars in the center of the cluster.   Six to eight inch scopes will show many more fainter members. Notice the differences in star colors with the larger scopes, you are bound to see yellow, red, and blue stars scattered around. You might try slightly focusing and defocusing the eyepiece to help make these colors pop out.



Image of Messier 41 star clusterWhile Charles Messier was the first to include this cluster in any kind of official catalog, he was not the first to observe it.   While it is not clear, we think that Aristotle references M41 in his “Meteorlogica” back in 325 B.C., if so, then M41 may be one of the longest known deep-sky objects in history. Painting of Giovanni Batista Hodierna

We are a bit more certain that Italian astronomer, Giovanni Batista Hodierna discovered the cluster in 1654.

Messier 41 is an example of what is known as an open, or galactic cluster and contains about 100 or more individual stars. Open star clusters are defined as being a loose, irregular grouping of stars that all share a common and relatively recent origin.

M41 is thought to be somewhere between 190 and 240 million years old and is located some 2,300 light years from Earth. The light hitting your eye when you observe M41 left at time when, here on Earth, Ancient Greek civilization was flourishing,

Even though M41 has an apparent size of 38 arcminutes, as seen from here on Earth, it’s true size is about 25 light years across.

Astronomers have determined that Messier 41 is moving away from us at a speed of 23.3 km/second.



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