Sun letters:
When a consonant letter (targeted letter) with “Shadda” preceded by the definite article ال, and that letter is in the third position, the following rule must be adhered:
    1) The targeted letter (with Shadda) will be pronounced   twice,
        a) with sukoon (pause),
        b) then with its one vowel (Fatha, Kasra, or Dama)
    2) The targeted letter absorbs the letter ل and the letter ل will not be pronounced but written only.

In other words, when a targeted letter has “Shadda” and preceded by ال, the letter ل will not be pronounced and the targeted letter will be pronounced twice; with sukoon and then with its one vowel.

Sun letter example:
The Arabic word has the letter ل written but not pronounced In this example the is called a sun letter and it is pronounced twice.    

Every letter that belongs to the Sun Arabic letters (Sun letters) must adhere to the above rule and the Sun letters are as follows:

Notice if a sun letter is preceded by the definite article ال, and it is not in the third position, it will be a Moon litter like: ; see sura 001, verse 007.
The Arabic letter is neutral letter and it is neither a Sun Nor a Moon letter.