In vishnu Purana and bhagavathaa purana They explained about how Dhruv got the position of fixed star by his severe penance at age of five (5).
Dhruv gets insulted by his stepmother Suruchi
Dhruv’s father Uttaanpaad had 2 wives, Suruchi and Suniti. Suruchi had a son Uttam. Suniti had son Dhruv. Uttaanpaad loved Suruchi more than Suniti. One day Uttam was playing in the lap of his father, Dhruv also wanted to play in his fathers lap but Suruchi pulled him far and told Dhruv that he is not born to her to get that love from uttaanpaad.
Dhruv soon came to his mother and told what all happened with him. Suniti explained Dhruv not to worry about these things instead guides him to possess higher knowledge like his grandfather Swayambhu Manu who is the Son of Brahma.
Dhruv after listening to mothers advise he headed into the dense forest.
Dhruv meets the Seven Sages
According to Visnu purana 7 sages guided him. According to Bhagavat purana Narada muni guided him .
So Dhruv happens to see seven sages or Narada muni according to their respective puranas but both of them guided the same. Dhruv then narrates entire story of what all happened with him and asks for help to attain something very high position that no one has attained that position till now.
Then seven sages (1.Marichi, 2. Atri, 3. Angira, 4. Pulastya, 5. Pulaha,6. katru, 7. Vashista) or Narada Muni each given their advice and assured that only lord Vishnu will help him to attain such position. Dhruv as he is too small, unknown of how to meet him and make his wish to fulfill asked sages to what to do further to please Lord Vishnu. They told him to only concentrate on Lord, chanting a mantra no matter what happens around till lord himself comes to you.
Dhruv’s severe Penance
Dhruv then sat under Cadamba tree on the bank of river Yamuna in Madhuvan and started to recite the mantra. Dhruv ate roots and tubers in his first month of sadhana. In the second month he ate dried leaves and in the third month he was only on water. In the fourth month he ate nothing , he was just breathing, later on Dhruv stopped breathing.
Then Lord arrived in front of Dhruv. Seeing the Lord Himself arrived before him, Dhruv’s eyes filled with tears. Lord Vishnu told Dhruv that he was born in a Brahmin family in his previous birth and was a friend of a prince and wished to enjoy the same luxuries like your friend. You had in fact wished to be a prince and are born as a prince in this birth and got a place in the lineage of Swayambhu Manu. But this honour has no value for a devotee like you.
Dhruv attains a Fixed star
Then Lord Vishnu granted him a special boon and made him base of all the planets and all the constellations by granting him a fixed star position that is far above the Sun, the Moon, the planets, constellations, Saptarishis and all the gods who fly about in divine aircraft.
Besides it, Dhruv will stay for a complete Kalpa whereas even the gods do not stay in their position more than a single Manvantara. Dhruv’s mother Suniti too will come to stay with him as a bright star for the same period of time. There she will stay on an aircraft. Thus receiving a boon from Lord Vishnu, Dhruv occupied a fixed position in the northern sky.