When Immanuel Kant proposed in 1755 the Nebular Hypothesis to explain how our solar system (and others) came into being, he probably had no idea how prolific his thoughts would be (ref. 1 pp 155-156). Indeed, the general tenants of his proposal have survived until today in spite of many modifications to it. The essence of the hypothesis is that an interstellar cloud (or nebula) collapses under its own gravity and forms a star system with attending planets.
As many current astronomy texts proclaim, we are the product of “stardust” (or starstuff, ref. 1 p157). The purpose of this article is to closely examine if this can indeed happen using observational data along with proven laws of math, physics and thermodynamics. Since observational data is involved, the hypothesis has matured into a theory since Kant’s day.
The observational data includes detailed images of various dust and gas clouds in our galaxy along with spectral and photometric studies of stars or other luminous objects in or near those clouds. As with most cosmological theories, it is important to realize that series of pictures taken at different times of large scale galactic structures such as nebulae and star clusters are essentially unable to show large scale changes because of the large time intervals required for significant movement. Nebulae are found in many sizes and shapes within our galaxy (ref. 2 p218). It is the dark nebulae that hold the most promise for the nebular theorist since their temperatures are presumed cold enough for gravity to act in a way to cause a general collapse to begin. In particular, a subset of dark nebulae called Bok globules (ref. 2 p526) are very interesting to the nebular theorist since their small size and higher density make them good candidates for incipient cloud collapse. Other observational objects believed to be evidence for the theory are stars which appear to have dusty disks surrounding them such as Beta Pictoris (ref. 3 p26). These are believed to be planetary systems in the final stage of formation with possible protoplanets imbedded in the disks. The nebular theorists contend that those star systems are in the planet accretion phase with the larger bodies acquiring mass from the dust and gas remains of the original nebula.
Beta Pictoris Dust Disk
Current attempts to explain the formation of a relatively homogeneous nebula into as eventual stellar-planetary system have two major hurdles to overcome. The first is how to get rotation and thus angular momentum started. The second involves the viability of the cloud collapse mechanism.