A star (S/45) or possibly the planet Jupiter (MR/435). Its name is simply the noun form of the adjective alcarin(qua) “glorious”, or possibly a feminine form as suggested by Vyacheslav Stepanov (many star names on MR/435 seem to be gendered).
Quenya
alcarin
proper name. Glorious
alcarinquë
proper name. Glorious
-inqua
glorious
-inqua adjectival ending, seen in alcarinqua "glorious" (WJ:412) from alcar "glory". Etymologically, -inqua means "-full", like "glory-full" in this case. A variant *-unqua is implied in WJ:415 (only referred to in archaic form -unkwā). "The forms using u were mainly applied to things heavy, clumsy, ugly or bad", whereas -inqua (in the same source derived from -inkwā) is neutral.
alcarin
glorious, brilliant
alcarin adj. "glorious, brilliant" (shorter form of alcarinqua, q.v.) (PE17:24), hence Alcarin masc. name (or title) "the Glorious", title taken by Atanatar II of Gondor, also name of one of the Kings of Númenor (Appendix A).
alcarinqua
radiant, glorious
alcarinqua adj. "radiant, glorious" (AKLA-R [there spelt "alkarinqa"], WJ:412, VT44:7/10), "glorious, brilliant" (PE17:24), noun Alcarinquë, "The Glorious", name of a star/planet (SA:aglar - there spelt "Alkarinquë", but the Silmarillion Index has "Alcarinquë". The celestial body in question seems to be Jupiter, MR:435). Cf. also Alcarin, q.v.
alcarin(qua)
adjective. glorious, brilliant, glorious, brilliant, [ᴹQ.] radiant
Alcarinquë
Alcarinquë
Alcarinquë (or Alkarinque) is a Quenya name meaning "The Glorious", likely derived from the Quenya adjective alcarinqua ("glorious").
Tar-Alcarin was the 17th ruler of Númenor (LotR/1035, UT/222). His name is simply the shortened form of alcarin(qua) “glorious”. Alcarin “Glorious” was also a sobriquet for Atanatar II, the 16th king of Gondor (LotR/1038, 1045).