Original attempt: sub lux astri lucens
Your choice of vocabulary is good, and the word order is natural.
What you are missing is some inflection.
There is no freedom of choice when it comes to case, number, and gender here.
Rather than giving you the full answer, I will give you a list of specific questions, as I believe this to be most useful for learning.
Please leave a comment below if you are unsure of the answers.
What should the case of lux be?
It comes with the preposition sub.
What should the case of astri be?
It describes the owner of the light.
What should the case and gender of lucens be?
It modifies the star.
Participles behave just like adjectives when it comes to choosing the correct case, gender, and number.
Updated attempt: sub luce astri lucentis
This is perfect!
(See the comments below for the discussion leading to this.)