M73 - Asterism in Aquarius | Astrophotography Pictures and Tips
Updated: Feb 20
Messier 73 is either an asterism or an open star cluster in the constellation Aquarius, but it is for now considered to be just an asterism. M73 is made up of only 4 stars located about 2,500 light-years away. M73, along with M40 and M24, is one of the 3 Messier objects that is actually not considered a deep sky object.
Object Designation: M73, NGC 6994
Also known as: N/A
Constellation: Aquarius
Object Type: Asterism
Distance: 2,500 light-years
Magnitude: 9.0
Discovery: Charles Messier on October 4, 1780
The M73 asterism is a very easy target for beginner astrophotographers, although it is, of course, one of the least exciting objects in the Messier catalog to photograph. Being in Aquarius, M73 is a good Summer astrophotography target.
M73 Astrophotography with a RASA 8 and Color Camera
September 2023
I decided to shoot both M72 and M73 in one time as they both are very close to one another. Using the RASA 8 and a cropped sensor camera, the M72 globular cluster and the M73 asterism can both be seen in the same field of view, meaning we captured 2 Messier objects in one go and can check these two off our list! 🥳
I decided to do 5-minute exposures and just shoot for 10 hours to see if any IFN is present in this section of the sky. I was not disappointed as a lot of space dust became visible after stacking the data! Can you spot the asterism on the full image below? If not, scroll down to see a cropped version of it! Hint: It is somewhere in the top left quadrant.
M73 might be difficult to spot. It is the tiny set of bright stars you see around the upper middle, slightly to the left. This picture was taken using our telescope installed at Utah Desert Remote Observatories!
GEAR USED:
Camera: ZWO ASI2600MC
Telescope: Celestron RASA 8
Mount: 10Micron GM1000 HPS
Guiding: ZWO ASI 220MM Mini
Accessories: Celestron Focuser / PrimaLuceLab Eagle 5S
Processing: Pixinsight, with R-C Astro plugins. Final touches in Skylum Luminar Neo
ACQUISITION DETAILS:
Total Exposure Time: 10 hours
Exposure Time per frame: 300 seconds
Filters: N/A
Gain: 100
How to Locate M73
The four stars making up the M73 asterism are located about 2,500 light-years away in the constellation Aquarius. They form a "Y" shape that is easily distinguishable at long focal lengths.
Messier 73 appears dim and small when observed or photographed with a telescope.
Visually, the asterism is one of the most difficult Messier objects to find, due to how small it is and the fact that well, it's just 4 stars. It has an apparent size of 2.8 arc minutes.
The asterism itself has a magnitude of 9.0, but the individual stars have fainter magnitudes (10.48, 11.32, 11.90, and 11.94). This makes the object very difficult to see, especially if the telescope you are using is small.
It is somewhat possible to spot using a pair of binoculars, but it is tricky as you won't be 100% sure you are actually looking at the right pattern of stars.
Attached you can see a drawing of the M73 asterism done by DeepSkySketch. The telescope used was a Sumerian Alkaid 10" Newtonian telescope paired with a 7mm eyepiece.
M73 is located at the very edge of the constellation Aquarius. It is very close to M72 which is much easier to find as it is a compact globular cluster. The easiest way to find M73 is to either first look for M72, and go about 1.5 degrees east, or to slowly look around while going from the last bright star of Aquarius towards the constellation Capricornus.
The best time to observe and photograph the M73 Cluster is in August.
Messier 73 Asterism Information
Discovery
M73 was discovered between the night of October 4th and the morning of October 5th 1780 by Charles Messier.
Charles Messier had previously taken a look at the nearby globular cluster M72, which had recently been discovered by Messier's friend Pierre Méchain.
Upon confirming the discovery of M72, Messier stumbled on M73 and accidentally believed that it was a deep sky object, due to what he thought was some nebulosity.
Messier wrote the following about M73:
"Cluster of three or four small stars, which resembles a nebula at first sight, containing a little nebulosity: this cluster is situated on the same parallel as the preceding nebula (M72): its position was determined from the same star Nu Aquarii".
Several astronomers observed M73 in the years following Messier's discovery, and were surprised to not be able to see any nebulosity and also questioned whether these four stars could in fact be a cluster or not.
Messier 73: Asterism or Cluster?
As of today, M73 is considered to be an asterism, meaning a set of stars that make a specific shape (in this case, a small "Y" shape) but aren't related in any way.
For decades there has been a debate on whether M73 was an asterism or an open cluster of stars. The latest studies confirmed that M73 could not be a cluster, and was in fact an asterism. This is because the four stars are not related in any way, and only appear to be close because of our line of sight from Earth. In reality, they are located at completely different distances, are moving in different directions, and have no gravitational link.
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How to Process Messier 73
Processing M73 is very simple, mostly because there is not much to do besides stretching the image, taking care of the noise, and playing with the colors.
If you are looking to get the best out of your image, you can try revealing the faint IFN dust present in the field of view of your image. This requires some advanced techniques but is doable if your data comes from a dark site and is many hours long.
Below you can see what a single 5-minute shot looked like. As you can see, no IFN is visible at all and both M72 and M73 are difficult to spot.
If you'd like to learn how to process images with IFN in them (or ISM, another type of space dust), we strongly suggest taking a look at our 4K video walkthrough that shows all the techniques you can use to process such images. It also comes with high resolution raw data!
Messier 73 FAQ
What is M73?
M73 is an asterism made up of 4 stars.
In which constellation is Messier 73 located?
M73 can be found in the constellation Aquarius.
How big is M73?
Messier 73 has an apparent size of about 2.8 arcminutes.
How far is Messier 73?
M73 is believed to be around 2.500 light-years away from Earth.
How long should my exposure time be when photographing M73?
We recommend taking 30-second exposures if you include the M72 globular cluster and want to play it safe in case of winds or tracking issues. If your goal is to get the IFN in the background, we suggest doing either 5 or 10-minute exposures.
Should I use a filter to image M73?
M73 is just a set of stars, and you should not use filters to capture it as you will want the true color coming from the stars.
What equipment do I need to photograph M73?
You can capture this object with any telescope, but the larger the better!
Final Thoughts
Messier 73 is small and not impressive when compared with some of the other Messier entries. It is still nice to capture to check it off your Messier list though! You can also look at the bright side and try to challenge yourself to reveal the IFN dust present around the target.
Have you imaged M73? If so, upload your picture in the comments! We'd all love to see your work :)
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