A cumulene is a hydrocarbon with three or more cumulative (consecutive) double bonds. They are analogous to allenes, only having a more extensive chain. The simplest molecule in this class is butatriene (H2C=C=C=CH2), which is also called simply cumulene. Unlike most alkanes and alkenes, cumulenes tend to be rigid, comparable to alkynes, which makes them appealing for molecular nanotechnology. Polyynes are another kind of rigid carbon chains. Cumulenes are found in regions of outer space where hydrogen is rare (see astrochemistry). Cumulenes containing heteroatoms are called heterocumulenes; an example is carbon suboxide.