Some say that,
The term “Northern star metric” is often used in the context of growth hacking to refer to a key performance indicator (KPI) that serves as a primary focus for a company’s growth strategy. This KPI is often used to measure the success of a company’s growth initiatives and can be used to guide decision-making and prioritize areas for improvement. The “Northern star” analogy comes from the idea that the North Star is a fixed point in the sky that can be used as a reference point for navigation, similarly, the Northern star metric provides a fixed point of reference for a company’s growth efforts.

You could also say that,
The purpose of the Northern star metric is to provide a clear and focused target for a company’s growth efforts. By identifying a key performance indicator (KPI) that is directly tied to the company’s overall growth strategy, the Northern star metric can serve as a guiding light for decision-making and help ensure that all growth initiatives are aligned with the company’s overall goals. This can help a company avoid spreading itself too thin and focus its efforts on the areas that will have the greatest impact on its growth. Additionally, by regularly tracking and measuring progress against the Northern star metric, a company can gain valuable insights into the effectiveness of its growth efforts and make adjustments as needed to improve its performance.

Regardless, your Norther Star represents your strategic objective. As with the actual northern star, it provides guidance on where you want to be hading. And it’s always at the same point in the sky so to say.