what are these clouds?
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H).
Molecular cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H). 
difference between solar nebula theory and nebular hypothesis The widely accepted modern variant of the nebular theory is the solar nebular disk model (SNDM) or solar nebular model. It offered explanations for a variety of properties of the Solar System what is this hypothesis? The Nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System. what does it say about rocky planets? According to the solar nebular disk model, rocky planets form in the inner part of the protoplanetary disk, within the frost line, where the temperature is high enough to prevent condensation of water ice and other substances into grains. does it say anything about protostars? Yes, Gas in the central part of the nebula, with relatively low angular momentum, undergoes fast compression and forms a hot hydrostatic core.  The core gradually grows in mass until it becomes a young hot protostar. when was this type of star discovered? The modern picture of protostars, summarized above, was first suggested by Chushiro Hayashi in 1966 is there anything particular about their evolution? Star formation begins in relatively small molecular clouds called dense cores.  As the dense core accrues mass from its larger, surrounding cloud, self-gravity begins to overwhelm pressure, and collapse begins.
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Molecular cloud [SEP] Introduction
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H). This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas. Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H is carbon monoxide (CO). The ratio between CO luminosity and H mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.