Iron is one of the top six most abundant elements in the universe. Within a parent nebula [Parent Cloud] of a star system, there are typically large regions densely populated with metal atoms: primarily iron, with nickel and possibly trace amounts of other metal atoms. Metal atoms have higher atomic mass than most of the other atoms in the Parent Cloud, so these regions cause other atoms from the Cloud to pack around more tightly. (The lighter atoms are said to accrete around the dense clump of metal atoms.)
According to this academic paper, draft version March 31, 2009:
THE IRON ABUNDANCE IN GALACTIC PLANETARY NEBULAE
By G. Delgado-Inglada and M. Rodríguez
Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Apdo Postal 51 y 216, 72000 Puebla, Mexico.
A. Mampaso and K. Viironen
Instituto de Astrofísica de Canarias (IAC), C/Vía Láctea s/n, E38205 La Laguna, Tenerife, Spain
“more than 90% of [planetary nebulae] iron atoms are condensed onto dust grains.”
Opaque dust like one would expect from metal atoms, show up in Hubble images a brown cloudy parts of a nebula, like the seahorse shape in the image above [http://apod.nasa.gov/apod/ap141130.html]. Another example is given by the Eagle Nebula [http://apod.nasa.gov/apod/ap071209.html].
This Hubble image of the star-forming region, Westerlund 2, shows dark wisps as part of a larger nebula region and recently born stars produced from the region with a few remaining wisps of nebula on the edges.
Ten dense clumps of metal atoms are assumed to be present in the Parent Cloud that preceded our solar system. Clumps of iron in the parent nebula of a star form the heart of a protoplanet.