I am scientist and I want to create a graphic/chart to display a list of two different star types with some subcategories (some subcategories also have subcategories). Is there some sort of design principle I should follow when wanting to create something for this?
Graphics do not lift the scientific value, but it can help reading.
Belonging to a category can be visualized by letting the item be inside the area of that category. Your indented list already uses that idea. Do not try any wonky structure tree which forces one to follow a string to see which belongs to which.
Colors can be useful to visualize the areas. But the data content of an area is text, so do not obscure the readability. Faint colors can have easy to read black texts on them, but you cannot have many colors because faint ones easily look the same.
The shortest way to go on is simply to colorize your list. It's also space-effective, all texts can be horizontal and it doesn't create anything which could be considered to be a false illusion of size or importance.
If the writing order presents something it should be mentioned clearly and exactly in the drawing and also in the text.
Coloring example with zero decorations:
The black (or colored) outlines of the areas are useful if there's needed grayscale copies or one wants to help those who cannot see colors. Do not try to use any flashy outline geometries, they cause distraction from the content. A slight rounding is acceptable.
Not asked: Check if the numbering of the categories and items could be removed or at least use something easier than romans like xvii. Of course changing it can be difficult if your text already uses them.