M46 is an open star cluster in the constellation Puppis, with a population of up to 500 stars. It is a striking object because of the embedded planetary nebula (NGC2438) visible as a small, colored circle in the bottom left of the image.
A planetary nebula occurs when a star with an initial mass of up to 8 solar masses consumes sufficient fuel that its fusion reactions diminish, allowing the inexorable, inward force of gravity to crush downward until abruptly halted by the resistive force of electron degeneracy. This produces a white star (see open cluster NGC1502 for more details on white dwarfs), and a "rebound" or outward explosion of the star's outer atmosphere which we see as a "planetary nebula." These objects have nothing to do with planets, but were so named because they appeared to astronomers who first observed them through a telescope to be similar to a planet. We are witnessing the expanding shell from that explosion, illuminated by the hot white dwarf star at its core. It probably is not a true member of the star cluster, but rather a foreground object that happens to be in the same line of sight from our perspective.
Open Star Cluster M46 (and Planetary Nebula NGC2438)
Date Taken:February 17, 2005
Location Taken: Conditions of Location:FWHM 2.8
Equipment Used:Takahashi TOA-130 5" apochromat refractor telescope, SBIG ST-10XME CCD camera, Optec TCF-S focuser, Astrodon RGB filters.
Processing Used:9x5 minutes luminance, 3x5 minutes red, green and blue (total exposure 90 minutes) processed in Maxim DL and Photoshop.
Distance from Location:5,400 light years
Constellation:Puppis (the "stern")
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