Neutron stars are remnants of exploded stars. As more matter falls into a neutron star its mass increases; and as its mass increases its gravity increases. A point will be reached where gravity would have grown so much that not even light could escape, thus a black hole forms. Most neutron stars discovered today are in the form of radio pulsars. They are called radio pulsars because they emit radio waves. We can simply connect a radio telescope to a loud speaker and hear a pulsar. Pulsars sound like someone persistently knocking. Click here and listen to a slow knocking pulsar. Click here and listen to a fast knocking pulsar. So in short, we can hear a pulsar knock; and if matter continues to fall into this pulsar a black hole will eventually form. Moslems say that this is what Allah says. The Quran describes a star by "The one who knocks" and says that it is "The one who makes a hole".
[Quran 86.1-3] And the heaven and the "Knocker" (Tarek in Arabic) How could you know about the "Knocker"? The piercing star (Thakeb in Arabic).
The Arabic word "Thukb" means a hole; "Thakeb" means the one who makes the hole. The Quran is describing a knocking star that makes a hole.
How could an illiterate man who lived 1400 years ago have known that a star can make a hole?
God swears by celestial bodies that are invisible, that move, that sweep:
(Quran 81.15-16) I swear by those that are invisible (Khunnas خنس), that move, that sweep.
Satan is invisible and is described in Arabic as "Khannas" (Quran 114.4). The Quran uses the same word "Khunnas" to describe the celestial bodies that are invisible. All celestial bodies have gravity and can sweep everything in their way but the only ones that are invisible are blackholes.(Our sun is a very small star. Actually our solar system (including the atoms that you are made of) came from a dying star 100 times more massive than our sun. Some stars are even bigger than our solar system, so you can imagine how large stars are.