It is known that Elul is Besulah which in its secular name is Virgo where did this constellation come from according to our Torah sources?
I see what you wrote in the comment section. Fascinating.
I think it possible to trace that legend a bit more. See here.
Thus, (also via this, in Geiger, where he draws connections to the Koran):
"Thus, when Shamhazai noticed a certain maiden whose name was Istahar, he gazed lustfully upon her and pleaded, "Do my bidding." She replied, "I will not do your bidding until you give me your wings and teach me the Explicit Name, which you go up to heaven upon uttering." So he gave her his wings and taught her the Name, whereupon she uttered it, went up to heaven, and was spared from corruption. The Holy One said: Since she shunned transgression, go and set her among the seven stars yonder. Thus, it came about that Istahar was set in the constellation of Draco." - from Jewish Gates
When the angels came to earth, and beheld the daughters of men in all their grace and beauty, they could not restrain their passion. Shemhazai saw a maiden named Istehar, and he lost his heart to her. She promised to surrender herself to him, if first he taught her the Ineffable Name, by means of which he raised himself to heaven. He assented to her condition. But once she knew it, she pronounced the Name, and herself ascended to heaven, without fulfilling her promise to the angel. God said, "Because she kept herself aloof from sin, we will place her among the seven stars, that men may never forget her," and she was put in the constellation of the Pleiades.
Finally, there is a connection to a Greek legend:
"Istahar's story is borrowed partly from the Greek writer Aratus (early third century B.C.). He tells how Justice, a daughter of Dawn, ruled mankind virtuously in the Golden Age; but when the Silver and Bronze ages brought greed and slaugter among them, she exclaimed: "Alas, for this evil race!" and mounted into Heaven, where she became the constellation Virgo. The rest of this story is borrowed from Apollodorus's account of Orion's attempt on the seven virgin Pleadies, daugters of Atlas and Pleione, who escaped his embraces transformed into stars. "Istahar," however, is the Babylonian Goddess Ishtar, sometimes identified with Virgo. Popular Egyptian belief identified Orion, the constellation which became Shemhazai, with the sould of Osiris." (The Sons of God and the Daughters of Men, From Hebrew Myths: The Book of Genesis, written by Robert Graves and Raphael Patai)
Read more there. The adaption of a Greek myth and a myth about Ishtar into Jewish midrashim is indeed quite fascinating. Thanks for sharing this riddle.