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Credit & Copyright: Dan Bartlett
Explanation:
Which of these is
not a comet?
You guessed it -
the one on the right is a globular star cluster.
The diffuse greenish coma of periodic comet
10P/Tempel 2
is at left in the frame.
In fact the globular star cluster is Messier 30,
also known as M30, or the 30th entry in astronomer
Charles
Messier's
catalog of things which are not comets.
The well-known 18th century astronomer
kept a list of objects he observed, now his famous Messier
Catalogue of Nebulae and Star
Clusters,
which did not move from night to night against
the background stars and so were not the comets he was hunting for.
So the famous comet hunter would get the correct answer too, even
though his telescope would show both 10P/Tempel 2 and distant
star cluster as similar looking
faint and fuzzy objects in his field of view.
Recorded on July 29, this modern telescopic image
captures periodic comet Tempel 2
as it briefly swept close on the sky to M30.
While the periodic comet's faint, narrow,
orbital dust trail seems to pierce
the globular star cluster, Tempel 2 was a mere 3.5 light-minutes away.
Messier
30
is some 28,000 light-years distant.
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A service of: LHEA at NASA / GSFC
& Michigan Tech. U.
Based on Astronomy Picture
Of the Day