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Collinder 161

NGC 2451

Composite Open Cluster in Puppis

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Proper Name	None
Messier Number	None
NGC/IC Number	NGC 2451 (comprising the component clusters NGC 2451A and NGC 2451B)
Other Designations	Collinder 161
Constellation	Puppis
Right Ascension	7h 45m 24s
Declination	-37° 58' 0"
Distance	636 light years / 195 parsecs (NGC 2451A) c.1,200 light years / c.400 parsecs (NGC 2451B)
Magnitude	Apparent: +2.70
Diameter	Apparent: 1° 48'
Number of Stars	c. 800 (NGC 2451A) c. 4,300 (NGC 2451B)
Optimum Visibility	January / February
Notes	The apparent cluster designated Collinder 161 or NGC 2451 is in fact a pair of open clusters widely separated in space, but overlaying one another as seen from Earth. NGC 2451A is a cluster of about eight hundred stars some 640 light years from the Sun, while NGC 2451B is a more populous cluster of some four thousand stars lying approximately twice that distance from Earth (at an estimated 1,200 light years).

As seen from Earth, Collinder 161 appears as a single cluster, but it is actually composed of two unrelated open clusters on the same line of sight. Perhaps counterintuitively, the prominent orange star near the centre of this image (c Puppis) actually belongs to the more distant of the component clusters, being a particularly luminous member of NGC2451B. Imagery provided by Aladin sky atlas

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