Alessi 1 (LeDrew 1) in Cassiopeia

October 2026 - Nebula and Cluster of the Month

In 1998, Glenn LeDrew was searching with 10x50 binoculars for the faint outer satellite galaxies of M31, NGC 147 and NGC 185, from a dark-sky site in Canada. His attention was drawn to a compact collection of ninth-magnitude stars. He was looking slightly too far north and east for the galaxies, but the diversion was significant. Intrigued by the group, he investigated it using Hipparcos/Tycho data. He studied colour-magnitude diagrams and proper motion data for the stars he had seen. In particular, he found six red giant candidates amongst them, four of which shared very similar proper motions.

Convinced by his findings, he began preparing a paper for the Monthly Notices of the RAS, announcing his discovery of a previously overlooked bright open cluster.

Five years later, in 2003, Bruno S. Alessi, André Moitinho and Wilton S. Dias, after having trawled through data from the Tycho-2 catalogue, published a paper called Searching for Unknown Open Clusters in the Tycho-2 Catalogue.¹

This paper included details of 11 previously unrecognised open cluster candidates. Although commonly known as the Alessi catalogue, this is not strictly speaking, a catalogue. The 11 candidates were numbered Alessi 1, 2, 3, 5, 6, 8, 9, 10, 11, 12 and 19. There have been further additions to the list more recently and although the final entry (so far) is listed as Alessi 190, this doesn't mean that there are 190 objects in the listing. In fact, there are only 34. Of the original 11 objects, 10 are now accepted as definitely genuine open clusters. Only Alessi 11 is generally rejected and considered to be an asterism.

Alessi 1 is the cluster of stars that had been noticed and investigated by Glenn LeDrew, who had now been beaten to publication.

Curiously, there are earlier reported discoveries of this cluster. Philipp Teutsch noticed it in Millennium Star Atlas material in 1997. In 1998, Eero Holmström in Finland independently found it with an 8” (20 cm) telescope, also whilst searching for NGC 147 and NGC 185.

The largest and most comprehensive of all the open cluster catalogues is the Unified Cluster Catalogue (UCC). This was first introduced in 2023 and contains, so far, about 14,000 objects, most of which are Milky Way open clusters. The UCC, using data derived from the massive Gaia database, utilises methods of determining the probability that a suspected open cluster actually is one. Two forms of evidence – kinematic and photometric – are given scores from A to D, with A being most convincing and D being least. These are combined, so a very likely cluster would have a score of A/A whereas a highly dubious one would be scored D/D. Intermediate scores such as A/B and B/C also occur. This is referred to as the 'C3 quality'.

For each, the letters can be interpreted as follows:

A

Excellent evidence

B

Good evidence

C

Weak evidence

D

Very poor evidence

Another measure used by the UCC is the calculated 'UCC Trust Index', or UTI. This calculation gives a result between 0 and 1. A certain open cluster about which there is no doubt, such as the Pleiades, for example, would have a UTI = 1.0. This is a very useful and easy-to-understand number that gives an immediate answer to the question of the likelihood of any cluster being genuine. It must be borne in mind, though, that this is still just an estimate of the probability.

For our cluster this month, Alessi 1 (also, with a nod to the earlier discoverer, known as LeDrew 1), has a C3 quality score of A/A, and a UTI of 0.84. In other words, there is very little chance that this is not a real open cluster.

The data from Gaia has revealed some fundamental facts about this ‘new’ open cluster. Alessi 1 contains 99 stars with a probability of membership greater than 50%, a core radius of 2.2 pc (~7 light-years) and a core density of 2.3 stars per parsec. It lies at a distance of 710 pc (~2,300 light-years), giving the brightest stars absolute magnitudes of 0.0 – 1.0.

The brightest stars of Alessi 1 are of 9th magnitude, making it an object that can be observed through larger binoculars, or, as LeDrew found, through standard 10x50s if you're in a good dark-sky location. I always reckoned that from averagely light-polluted sites, I could just reach 9th magnitude with my 10x50s.

Alessi 1 stretches across about 54' of sky – almost twice the diameter of the full moon, but the brighter stars are concentrated in an area less than half this diameter. If searching for it telescopically, a low power eyepiece would be needed. It is located, as you've probably worked out, near the remote satellite galaxies of M31, NGCs 147 and 185 in Cassiopeia. The stars ξ, ν and ο Cas form a triangle. Draw a line connecting ν and ο, then draw a line from ξ that crosses the first line at right angles. Now place a point as far from the first line as ξ is from it, but on the other side of the line. In other words, make a sort of kite shape. Alessi 1 is situated at this point. It's big and bright, so not easy to miss.

The visual impression is of a triangular, or funnel-shaped, cluster. The stars are bright, and the cluster contains a good number of stars of around 10th and 11th magnitude, which define the apparent shape. It stands out well against the fainter field stars. The brightest star in the field is HD 5083, magnitude 7.1, spectrum B8. I can't tell whether this is actually a member star or not, but my guess would be that it is not. Eventually, with more searching, the cluster takes on an elongated, much wider field than originally seen around the funnel-shaped central section.

The image was taken with a SeeStar S50. The field of view is about 40’ square, and north is to the upper left.

I find it fascinating that such an apparently obvious open cluster had to wait until 1998 before anyone spotted it, and that it then took data from the massive Gaia catalogue to confirm its reality. What else have we missed?

Patrick Maloney

Object	RA	Dec	Type	Magnitude
Alessi 1 (LeDrew 1)	00h 53m 27s	+49° 34’	Open cluster

Reference:

1. Searching for Unknown Open Clusters in the Tycho-2 Catalogue, Bruno S. Alessi, André Moitinho and Wilton S. Dias, Astronomy & Astrophysics, 410 no. 2, pp 565 – 575, November 2003.

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.