Clyde Tombaugh’s Two Southern Open Clusters (1938)

*Clyde Tombaugh at his family’s farm with his homemade telescope in 1928, two years before he discovered Pluto.*

17 March 2019

The two southern clusters discovered by Clyde Tombaugh in 1938 may be very small and they may be very faint – but what a delight to observe the two southern clusters discovered by the extraordinary young man who discovered Pluto.

He published their discovery in the Astronomical Society of the Pacific under the title, Two New Faint Galactic Star Clusters, writing: “In the course of the Trans-Neptunian planet search survey with the 13-inch Lawrence Lowell Telescope, I recently came upon two faint, open, galactic star clusters. The two clusters are actually within two-thirds of a degree of each other.”

In 1957, the German astronomer Hans Haffner, who was working as Director of the Boyden Observatory in Bloemfontein, South Africa, published his discovery of 26 new open clusters in the southern Milky Way (Puppis and Canis Major, to be precise) but his Haffner 1 and 2 were indeed Tombaugh 1 and 2.

Tombaugh discovered 5 clusters in all, one of which – Cluster No. 3 – he found to be IC 166. Tombaugh 4 lies in Cassiopeia and Tombaugh 5 lies in Camelopardalis.

Our two southern Tombaugh clusters lie in the rich star fields of Canis Major. I have included Tombaugh’s notes regarding the two clusters – it is always enjoyable to compare what one is seeing through the telescope with what the discoverer of the object saw.

Tombaugh 1

RA 07 00 30.2 Dec -20 34 08

Mag –

Size 6.1′

Dist 9,784 l-y

Note: Cluster No. 1 has an angular diameter of about 5 minutes of arc. The individual stars, of which I counted about thirty, are of the 14th and 15th magnitudes. It is hard to understand how this one has been missed, although the numerous stars of the sky field would tend to obscure the cluster.1

Tombaugh 2

RA 07 00 30.2 Dec -20 34 08

Mag –

Size 6.1′

Dist 43,248 l-y

Note: Cluster No. 2 has an angular diameter of only 2 minutes of arc. About twenty-five stars are shown, ranging from the 16th to the 17th magnitudes. This cluster is evidently more remote than the other one.