A metal (from Greek μέταλλον métallon, “mine, quarry, metal”) is a material (an element, compound, or alloy) that is typically hard, opaque, shiny, and has good electrical and thermal conductivity. Metals are generally malleable — that is, they can be hammered or pressed permanently out of shape without breaking or cracking — as well as fusible (able to be fused or melted) and ductile (able to be drawn out into a thin wire). About 91 of the 118 elements in the periodic table are metals (some elements appear in both metallic and non-metallic forms). The meaning of “metal” differs for various communities. For example, astronomers use the blanket term “metal” for convenience to collectively describe all elements other than hydrogen and helium (the main components of stars, which in turn comprise most of the visible matter in the universe). Thus, in astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. In addition, many elements and compounds that are not normally classified as metals become metallic under high pressures; these are formed as metallic allotropes of non-metals.