Stephenson 1 - Open Cluster
Stephenson 1 is a sparse open star cluster grouped around the star “Delta 1 Lyrae”. It is therefore sometimes also called the “Delta Lyrae Cluster”.
It is located in the constellation Lyra and is around 1,220 light-years away from Earth. With an angular extent of 20' and a total apparent magnitude
of 3.8mag, it is even visible in binoculars.
Its true nature was first suspected by Charles B. Stephenson in 1959, but disproved in 1966. However, further follow-up observations (photometry)
showed that "Stephenson 1" is indeed a star cluster with 33 members. With the help of GAIA it could be shown that as many as 97 stars belong to the
cluster. While “Delta 2 Lyr”, at a distance of 770 light-years, is much closer to us than “Stephenson 1” and therefore cannot belong to it, “Delta 1 Lyr”,
at 1,170 light-years, is at about the same distance - however, the deviating radial motion (-26km/s) also speaks against membership here.
The total mass of “Stephenson 1” is given as 590 solar masses. Using the known distance, the absolute brightness can be determined as M= -4.0mag,
which corresponds to a luminosity of 3,600 suns. The brightest stars shine about 100 times brighter than the sun.
The age of the cluster is given as 50 million years. It is therefore still very young. However, due to the low density of stars, it is very likely that it will
disintegrate in the next few hundred million years and will only survive a few orbits around the center of the Milky Way, if at all.
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I used to look at Stephenson 1 often with my former 8” Dobsonian - so it was nice to visit the cluster with more than twice the aperture. The cluster
appears most beautiful at a magnification of 120x. There are many bright stars to be seen. The second brightest star “Delta 2 Lyr” shines deep yellow
and offers a great color contrast to the whitish stars of the cluster.
A wonderful object for small magnifications! :)))
