Initially, this gas was probably so Blank 2 , perhaps over a region a few light-years in diameter that gravity alone may not have been sufficient to pull it together and start its Blank 3 . Instead, the formation may have been triggered by a Blank 4 , such as the impact of a shock wave from the explosion of a nearby star (a supernova). Once the collapse started, gravity enabled it to continue to take a its appropriate shape, orbital and spin