A type of emission nebulae composed of very hot gas (about 10^4 K), mainly ionized hydrogen, created by the ultraviolet radiation of massive stars. H II regions originate when O or early-type stars, b
orn in giant molecular clouds, start heating up the cold gas, causing it to become ionized and 'glow'. The effective temperatures of the exciting stars are in the range 3 x 10^4 to 5 x 10^4 K, and throughout the nebula hydrogen is ionized. Helium is singly ionized, and other elements are mostly singly or doubly ionized. Typical densities in the H II region are of the order 10 to 10^2 cm^-3, ranging as high as 10^4 cm^-3. Internal motions occur in the gas with velocities of order 10 km s^-1. The spectra of H II regions are mainly composed of strong H I recombination lines and forbidden lines such as [O III], [O II], [N II].
The asteroids found on the outer edge of the main asteroid belt in a 2:3 orbital resonance with Jupiter. The group is not an asteroid family since the members are not physically related. The group con
sists of asteroids with semi-major axes between 3.70 AU and 4.20 AU, eccentricities less than 0.30, and inclinations less than 20°. It is dominated by D- and P-type asteroids.
A hill is a landform that extends above the surrounding terrain, in a limited area. Hills often have a distinct summit, although in areas with scarp/dip topography a hill may refer to a particular sec
tion of scarp slope without a well-defined summit (e.g. Box Hill). A hillock is a small hill. [Wikipedia]
A qualitative assessment of the relief of multiyear ice formed as a result of non-uniform melting of initially level ice and smoothing of ice ridges and hummocks. It can be assessed as smoothed multiy
ear ice, moderately hillocky multiyear ice and strongly hillocky multiyear ice.