20 October 2016 | Sailboat of Cassiopeia

vdB 4,NGC 225

Credit: Antonio Sánchez | Click image to enlarge

Floating in the tranquil starfields of the Milky Way flowing through the constellation of Cassiopeia, NGC 225 is a sparkly treasure trove of stellar sapphires. The overall shape of the open cluster has led to the popular name of the Sailboat Cluster being bestowed upon it! This cluster is located a little over 2000 light years away, meaning it appears to us as it looked 2000 years ago as this is the amount of time its light has taken to reach Earth.

Prominently visible in this telescopic image are many brilliant blue stars, which are usually indicative of a young stellar population and this is indeed corroborated by a young age of less than 10 million years for the cluster. It contains a number of stars belonging to a category known as Herbig Ae/Be, a type of pre main sequence early type star, which are the precursors of large supergiants such as Betegeuse or Rigel. The majority of the bright stars visible in the cluster haven’t entered the main sequence, the main part of stellar evolution, which is also the phase the Sun is currently on.

This suggests that the cluster has experienced a recent burst of star formation, which is evidenced by the brilliant azure reflection nebula vdB4 to the right of NGC 225. The central illuminating star V594 Cassiopeiae also drives a small outflow nebula, which is visible in hydrogen-alpha exposures. This nebula consists of two lobes to the south of the star, which is suggestive of multiple episodes of mass loss. This star is also catalogued as MWC 419 and previous studies have shown it is a definite member of the cluster by comparing its proper motion with the other cluster member stars.

MWC 419 has a magnitude of 10 and the reflection nebula was discovered in 1960 by George Herbig. It is also one of 158 reflection nebulae catalogued by the astronomer Sidney van den Bergh as part of the vdB catalogue, which was published in 1966. vdB4 and its illuminating star are very likely to be associated with the dusty dark nebula LDN 1302 to the north. This is part of a group of molecular clouds and there is considerable dust absorption in the region. This is visible as faint streams of dust around the cluster, which attenuate and absorb the blue light of stars that are further in the distance, thereby making them appear more red than they actually are. This phenomenon of interstellar reddening also affects the light of certain galaxies, many of which are brighter than they appear to us from our vantage point in the Milky Way.

One of many clusters to be found in Cassiopeia, the Sailboat Cluster was discovered in 1783 by the astronomer Caroline Herschel who also mistakenly discovered it a second time in 1784. This gem of a cluster is bright with a magnitude of 7 and can easily be observed with binoculars or a telescope.

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