M46 is an open star cluster located in Puppis about 10 degrees East and slightly North of the bright star Sirius. This is a rich group of about 150 stars between magnitude 10 and magnitude 13. The total population of stars is probably about 500. M46 is about 1,500 light-years away, although some estimates place it further away. The brightest stars are blue giants that are 100 times brighter than our sun. The group occupies a space about 30 light-years in diameter. The magnitude 11 planetary nebula NGC 2438 is superimposed on the cluster, and is actually located a substantial distance from the star cluster, and has a different motion in space. The central star of the planetary nebula has a surface temperature of about 75,000 degrees K, and is one of the hottest stars known.
Charles Messier discovered the M46 star cluster in 1771. His crude telescope did not resolve the cluster well, and he thought it contained a little nebulosity. The nebula NGC 2438 was first noticed years later by Sir William Herschel, (who had a better telescope than Messier did). The star cluster M47 is only 1.5 degrees away from M46, and can be seen in the same field of view in a wide field telescope.
This image is an RGB color composite CCD image taken with a Takahashi FCT-150 refractor from my backyard in Scottsdale, Arizona. An ST-8E was used at prime focus at f7. Data acquired on two nights was combined to make this image.
M46 (NGC 2437)
RA: 07h 41m 59.5s Dec: -14d 49' 40"
March 9 & 11, 2004
Image by Sid Leach
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