|M25 (IC 4725)||Open Cluster||18h 31m 47s||-19° 07′ 00″||Sagittarius||4.59||29.0 arcmins|
This pleasing little cluster was first discovered in 1745 by Philip Loys de Chésaux and later seen by Messier himself in June 1764. Messier described it as “a cluster of faint stars between the head and the tip of the bow of Sagittarius”. A fitting and succinct description!
You may have noticed that it has a catalog number of IC (Index Catalog) and not an NGC (New General Catalog) that we’re so used to seeing for objects such as this. And that’s because William Herschel didn’t include it in his General Catalog originally. Nobody is sure why, and the conundrum is confounded by the fact Herschel’s father was known to observe it in 1783!
The table above also gives this cluster’s magnitude as 4.59, making it easily observable with the naked eye under a moderately dark sky.
Lying about 2,000 light-years away, the cluster is thought to contain as many as 600-1000+ stars in an area of just 15 light-years. We can see between thirty and forty of these well resolved in a small telescope covering a half-degree of sky, which means they easily fit in a moderate magnification field of view.
Astronomers have dated the cluster from the main types of stars within it – spectral type M and G giants – to about 70 million years old.
This cluster is highest in the sky around 10 pm in August, which is what the first sky chart shows below. There are no planets shown on any of the charts so they can be used at any time of year. The charts all from SkySafari 6.
This first chart shows you where M25 is located. This first view shows the cluster about one-third of the way towards the overhead point (zenith) when looking south. This is from mid-latitudes of the US, if you are further north than this, M25 will be lower in the sky. Stars are shown to magnitude 5.0.
Note how the cluster sits over the top of the handle and lid of the Teapot asterism in Sagittarius. Note also the two marked stars, we shall use these to find M25.
We now zoom in to show stars to magnitude 7.0. Use the two stars marking the right hand edge of the Teapot’s lid to point you in the direction of the M25. It is roughly as far on again from Kaus Borealis as it is from Kaus Media. Look for the bright stars Polis (Mu Sagittarii) and 21 Sagittarii to help locate it.
This last star chart shows the star hop from 21 Sagittarii to M25. The blue circles are a 1° field of view and stars are shown to magnitude 9.0.
In reality, M25 is bright enough to identify visually if you’re looking in the correct part of the sky, so star hopping shouldn’t be needed.
Add U Sagittarii to the diagrams
Now you have these relatively close galaxies in your eyepiece, what should you expect to see?
The following views will help you find M25 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 13.0 and the larger ring is a 1° field of view. 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
As noted under the image at the top of this page, the defining view of this cluster is two lines, or arcs, of stars running left to right in the image, and roughly east/west through your eyepiece. South of those two arcs, we can see another loose patch of stars in a grouping.
What we can also see is a Cepheid variable star, U Sagittarii, sitting on the eastern edge of the northernmost arc of stars. At its brightest, it is the brightest star in the cluster, shining at magnitude 6.4. However, it dims every 6.7 days to magnitude 9.1, making it blend in more with the other stars we see. Can you pick it out when you look through your telescope? (U Sagittarii is identified in chart [M25-4], above).
With larger scopes or stronger magnification, see how many of the stars you can resolve. Also, look for the wispiness of nebulosity in and between the brighter star lanes.
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 [M25-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.