Radar astronomy measures the reflections of transmitted radio waves to determine an object’s size, position, velocity, and surface characteristics. Radio astronomy is like optical astronomy but it uses the VHF, UHF, and microwave portions of the electromagnetic spectrum rather than the infrared and visible portions. Radio waves penetrate the dust that hides the centers of galaxies and obscures regions where stars are forming. They can also detect hydrogen gas that constitutes 85% of the known mass of the universe.