One calling you out, perhaps

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Possible Answers: UMP.

Last seen on: NY Times Crossword 11 Jan 22, Tuesday

Random information on the term “UMP”:

During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations.

In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in .templatequote{overflow:hidden;margin:1em 0;padding:0 40px}.mw-parser-output .templatequote .templatequotecite{line-height:1.5em;text-align:left;padding-left:1.6em;margin-top:0}

[…] The two types of stellar populations had been recognized among the stars of our own galaxy by Oort as early as 1926.

Baade noticed that bluer stars were strongly associated with the spiral arms and yellow stars dominated near the central galactic bulge and within globular star clusters. Two main divisions were defined as

with another newer division called

they are often simply abbreviated as Pop. I, Pop. II, and Pop. III.

Between the population types, significant differences were found with their individual observed stellar spectra. These were later shown to be very important, and were possibly related to star formation, observed kinematics, stellar age, and even galaxy evolution in both spiral or elliptical galaxies. These three simple population classes usefully divided stars by their chemical composition or metallicity.

UMP on Wikipedia