Explanation:
Galactic or open star clusters are relatively young.
These swarms of stars are born near the plane of the Milky Way,
but their numbers steadily dwindle as cluster members are
strewn through the Galaxy by gravitational interactions.
This
bright open cluster, known as M46, is around 300 million years
young and still contains a few hundred stars within its span of
30 light-years or so.
Located about 5,000 light-years away toward the
constellation
Puppis, M46
also seems to contain a contradiction to its youthful status.
In the lovely starscape,
the colorful, circular patch just below the center of M46
(also inset at upper left) is the planetary nebula NGC 2438.
Planetary nebulae are a brief,
final phase in the life of a solar-type star a few
billion
years old whose central reservoir of hydrogen
fuel has been exhausted.
In fact, old
NGC 2438
is estimated to be only 3,000 light-years distant and
moves at a different speed than M46 cluster members.
It likely represents a foreground object,
only by chance appearing
along our line-of-sight to young M46.
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