236 gloSSary Photosynthesis: A process that converts carbon dioxide and water into organic compounds and oxygen using the energy of sunlight. The emergence of photosynthetic organisms led to a rise in the oxygen level in the Earth's atmosphere. It is vital for present life on Earth. Planetesimal: One of the small solid bodies that build up in a protoplanetary disk, which in turn accumulate to build up planets. Plate tectonics: Slow, large-scale motions of plates of the Earth's (or another rocky planet's) crust. Proton: A positively charged subatomic particle found in the nucleus of every atom. The number of protons determines each element; e.g., hydrogen atoms have one proton, helium atoms have two protons, carbon atoms have six protons. Protoplanetary disk: The rotating disk of dust and gas that surrounds a newborn star, out of which planets form. Protostar: An early phase in the formation of a star, after it has fragmented out of a gas cloud but before it has contracted enough for nuclear fusion to begin. A dusty cocoon, which blocks visible light but allows infrared and microwave radia- tion to escape, surrounds a protostar. Pulsar: A rapidly rotating neutron star that emits beams of radiation (usually radio waves), similar to the beams of a lighthouse, which are detected as pulses as the beams sweep past the Earth. radial velocity: The velocity of an object toward or away from the observer. Also see Dopplershift. radiometric dating: A technique for dating materials, based on a comparison of radioactive isotopes and their decay products, given known decay rates. red dwarf: A star with a much lower mass than the Sun (about 10­50 percent of a solar mass) but a higher mass than