An arc second is about the smallest angular size typically measurable from a good ground-based observing site. A deuterium atom's nucleus contains one proton and one neutron, rather than just a single proton like regular hydrogen. A device etched with many fine parallel lines or grooves that is used to spread light into its component wavelengths, or spectrum. Any part of the electromagnetic spectrum can be categorized according to the size of its wavelengths. The shorter its wavelength, the higher the light's energy.
Visible light is just a small piece of the electromagnetic spectrum.
Emission lines arise from gas that has been heated or energized by a power source, such as a bright young star or a supernova blast wave. Despite its low density, the IGM may harbor much more matter than all of the known stars and galaxies in the Universe, and yet it is extremely difficult to study directly. The far ultraviolet spectral region offers great promise for studying this elusive component of the Universe. The vast regions of space between the stars in a galaxy. Interestingly, even the densest parts of the ISM would be considered an excellent vacuum on Earth.