<span>Atoms and ions, at least within the quantum mechanical model do not have boundaries that are sharply defined at which the electron distribution becomes zero. When this occurs it can be very difficult to measure an atomic radius. The distance between the electrons in quantum mechanics is not set and would be problematic in determining the precise radius.</span>
Stars form from an accumulation of gas and dust, which collapses due to gravity and starts to form stars. Stars are typically classified by their spectrum in what is known as the Morgan-Keenan or MK system.
