How can I modify a star system?

[moed]

Which HP allows me to modify a star system?

An example of which is let's say I wanted to add a nebula to a system that has none? Or an extra sun?

[Morgan]

There's two files you have to play with in order to modify a star system. "systemname#".py is where you would modify the locations of objects in the system (planets, asteroid fields, etc.) and "systemname#"_S.py is where you would define their parameters. Unfortunately there is no tool that makes system modding easier so you're stuck learning by examples in BC's stock systems and the ones available at BC Files, and you're also stuck with notepad and edit mode.

[Locke]

If you must edit by hand, do it with Notepad++.  Makes life so much easier when you have color-coded text and auto-complete for notation.

[Lurok91]

Which HP allows me to modify a star system?

An example of which is let's say I wanted to add a nebula to a system that has none? Or an extra sun?

As Nero said, not an HP but script - in scripts/Systems folder. 

useful tutorials here:
http://bc-central.net/forums/index.php/topic,40.40.html
and here:
http://bcs-tng.com/forums/index.php?PHPSESSID=a565d57c0bbd9bd5f87377c2700e57f3&topic=571.0

EditMode very useful as GUI allows you to 'see' where placing objects in system and saves parameters as script.
http://bc-central.net/forums/index.php/topic,3666.0.html