NGC 381 in Cassiopeia
October 2025 - Nebula and Cluster of the Month
In October, the nights lengthen and observing becomes easier. The temperatures are not yet at their winter lows, yet the astronomical darkness reaches 13 hours by the end of the month. The brilliant winter constellations (at least I’m told that they’re brilliant; from my back garden they’re barely visible against the claggy, light-polluted sky) begin to make their presence known in the east as the month progresses.
Our object this month is a little-observed cluster in Cassiopeia. It sits very close to the galactic equator and culminates at midnight around the middle of October, when it reaches an altitude of 82° from mid-Britain.
It was discovered in 1783 by Caroline Herschel and included in her brother William’s class VIII (coarsely scattered clusters of stars) after he observed it on 3 November 1787. He described it as A forming cluster of pretty compressed stars. CH discovery 1783.
Herschel did not use the word ‘forming’ in a modern astrophysical sense. If so, the phrase would mean that the individual stars were still forming, and the object would probably still be swathed in the nebulosity that does actually give birth to clusters. Instead, he used it in the sense that he thought the stars may be beginning to gather together to form a cluster. The use of the word suggests that Herschel theorised that clusters formed by stars clumping together. It is best interpreted as meaning ‘loose’ or ‘poorly condensed’.
The cluster made its way into the NGC as number 381, its NGC description being a somewhat terse Cluster, pretty compressed
.
The sources are surprisingly consistent in the vital statistics of NGC 381. The greatest variation is with the Trumpler type, which can be summarised as III 1(or 2) m(or p). This means that there is agreement that there is no central condensation, but the cluster does appear detached from its background; it has a moderate range of brightnesses (or the stars are all of similar magnitude), and it has a medium richness (or it’s poor). All sources agree that the cluster contains 50 stars, so it can’t be poor by definition. It always amazes me that sources can quote a number of member stars and then give a Trumpler classification that blatantly ignores the number.
Deep images of the cluster, reaching around 20th magnitude, have revealed several hundred probable members.
One agreed vital statistic that I can’t understand at all is the magnitude of the brightest star in the cluster. It is usually given as magnitude 10.0 (or 10.9 in one instance). The Night Sky Observer's Guide adds that the brightest star is located in the chain of stars that heads north. My investigations suggest that none of the stars in that chain exceed magnitude 11, and most are around magnitude 12. The nearest tenth-magnitude star is the eclipsing binary OX Cas, which is given the optimistically exact magnitude of 9.996.
No star in this cluster reaches magnitude 10. The brightest stars are eleventh magnitude. Ho hum.
NGC 381 lies at a distance of around 2900—3300 light-years and is an intermediate-age cluster, its age usually being quoted within the range of 200—300 million years. At this age, it will be partially evaporated. Far, then, from a ‘forming’ cluster, it is a disintegrating cluster.
The cluster is fairly easy to find, being 1.6° to the north-east of γ Cas (the middle star in the ‘wonky wubbledoo’ of Cassiopeia).
Visually, it is immediately recognisable as a cluster. With an overall magnitude of 9.3, it is visible even in quite small telescopes. It is well detached from its background and stands out well. I observed it with my 12” Newtonian in October 2015. On that night, I described it as A small, reasonably compressed and moderately rich cluster of very faint stars. About 30 stars counted across about 7 or 8'. A chain of brighter stars (maybe not members) leads away to the north.
Most visual observers remark on this chain of stars. To my eyes (either when looking at the object or at an image of the object), it does not look to be physically associated with the cluster itself. A quick look at proper motion data suggests the same. In fact, several of the brighter stars within the cluster seem to have a fairly random scatter of proper motions.
When observing it, then, bear in mind that the cluster is beginning to break apart, and probably only the compressed small stars are actually cluster members.
| Object | RA | Dec | Type | Magnitude |
|---|---|---|---|---|
| NGC 381 | 01h 08m 20s | +61° 35’ | Open cluster | 9.3 |
If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Cluster of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.
