Dragonfly Cluster
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An open cluster is a loose congregation of tens to a few thousand stars that formed out of a single giant molecular cloud and thus share roughly the same age. Because the stars are bound to one another only weakly by gravity, the cluster gradually disperses into interstellar space over tens of millions of years. Observing open clusters therefore allows astronomers to trace stellar evolution and study the dynamical lives of star systems.
One of the most striking examples is NGC 457 in the constellation Cassiopeia. In both the literature and casual sky-watching circles it wears many nicknames — Owl Cluster, E.T. Cluster, Dragonfly Cluster, and Phi Cassiopeiae Cluster. All refer to the “pair of eyes” formed by two relatively bright stars: φ Cassiopeiae (5.0m) and HD 7902 (7.0m). William Herschel discovered the cluster on August 18, 1780, with a 6-inch reflecting telescope and cataloged it as VII 42. Modern measurements place it about 7,900 light-years from the Sun and give it an age of roughly 21 million years.
Observations
October 8, 2024, around 8:00 PM – Jaworzno, Poland
Conditions: urban sky, heavy light pollution
The easiest way to locate NGC 457 is to point a telescope about 2° southeast of δ Cassiopeiae (Ruchbah). With an apparent diameter of only 13′ — about half the Moon’s disk — the cluster is best viewed with a field of at least 1° at moderate magnification.
NGC 457 is a genuine gem of the northern autumn sky. Cassiopeia rides high at that season, so the cluster remains visible even from moderately light-polluted sites. A pair of 7 × 50 binoculars will reveal the twin “eyes” (φ Cas and HD 7902) surrounded by a faint mist of stars. A 4-inch (10 cm) telescope resolves a dozen of the brightest members, while an 8-inch (20 cm) instrument shows more than 50 stars of magnitude 9–13m — a pattern that explains the nickname Dragonfly Cluster.
As befits a youthful assembly, NGC 457 is dominated by blue-white B-type stars. One object, however, commands attention: the orange-red supergiant V466 Cassiopeiae, whose vivid hue lends the field extra color. Astronomers have identified roughly 60 confirmed cluster members; the few dozen remaining stars in the field are likely part of the Milky Way’s background.
Does φ Cassiopeiae truly belong to NGC 457? Proper-motion data remain inconclusive. If it does, the star would outshine Rigel in Orion, placing it among the most luminous stars known. For comparison, at the cluster’s distance the Sun would glimmer at a feeble 17.3m — far beyond the reach of amateur instruments.
Photo 1 Parameters:
- Total exposure time: 25 minutes (stack of 50 RAW frames at 30s each, using an appropriate number of dark, bias, and flat frames)
- Canon EOS 60D
- ISO: 1500
- Maksutov-Cassegrain telescope (100/1400), prime focus exposure
- A filter was used to reduce the effects of artificial light pollution and atmospheric glow
- Mount: equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system.
Further readings:
- Frinchaboy P.M., Majewski S.R., Muñoz R.R., Law D.R., et al., Open Clusters as Galactic Disk Tracers. I. Project Motivation, Cluster Membership, and Bulk Three-Dimensional Kinematics, The Astronomical Journal, vol. 136, no. 1, 2008, pp. 118–145
- Michael M., Binnewies S., Bildatlas der Sternhaufen & Nebel, Kosmos, Stuttgart, 2023, p. 309
- O’Meara S.J., The Caldwell Objects, Cambridge University Press, Cambridge, 2016, pp. 67–70
Marek Ples