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(more about Power Words)
astronomy Scientific field dealing with celestial objects, space and the physical universe. People who work in this field call themselves astronomers.
black hole A region of space with a gravitational field so intense that no matter or radiation (including light) can escape.
density The measurement of the condensation of an object is obtained by dividing its mass by its volume.
fuel Any material that will release energy during a controlled chemical or nuclear reaction. Fossil fuels (coal, natural gas and oil) are a common type that release their energy through chemical reactions that occur when they are heated (usually to the point of combustion).
galaxy A massive group of stars linked together by gravity. The galaxies, which each include between 10 and 100 trillion stars, also include gas clouds, dust, and the remains of stars exploded.
gravity The force that attracts everything that has a mass or volume to anything else that has a mass. The larger the mass, the greater is its gravity.
infrared A type of electromagnetic radiation invisible to the human eye. The name includes a Latin term and means "below the red". Infrared light has wavelengths longer than those visible to humans. Other invisible wavelengths include X-rays, radio waves and microwaves. Infrared light tends to record the thermal signature of an object or the environment.
Mass A number that shows how much an object resists acceleration and deceleration – a measure of how much material is made from that object.
microwave Electromagnetic wave of shorter wavelength than normal radio waves but longer than infrared radiation (heat) and visible light.
Milky Way The galaxy in which the terrestrial solar system resides.
physicist A scientist who studies the nature and properties of matter and energy.
planet A celestial object orbiting a star is large enough that gravity crushes it into a round ball and has cleared other objects in its orbital neighborhood.
radiation (in physics) One of the three main ways to transfer energy. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which require materials to facilitate the transfer of energy, the radiation can transfer energy into a void space.
spectrum (plural: spectra) A range of related things that appear in any order. (in light and energy) Range of types of electromagnetic radiation; they range from gamma rays to X-rays, ultraviolet light, visible light, infrared energy, microwaves and radio waves.
star The basic building block from which galaxies are made. Stars develop when compact gravity clouds gas. When they become dense enough to sustain nuclear fusion reactions, stars emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
stellar An adjective that means or relates to the stars.
Sun The star in the center of the terrestrial solar system. It is a medium-sized star about 26 000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.
telescope It is usually a light-collecting instrument for bringing distant objects closer together through the use of lenses or a combination of mirrors and curved lenses. Some, however, collect radio emissions (energy from a different part of the electromagnetic spectrum) via an array of antennas.
universe The entire cosmos: all things that exist in space and time. It has been expanding since its inception at an event called the Big Bang, about 13.8 billion years ago (a few hundred million years ago).
white dwarf A very dense little star the size of a planet. This is what is left when a star whose mass is roughly equivalent to that of our sun has exhausted its hydrogen fuel and has collapsed.
radiography A type of radiation similar to gamma rays, but with slightly lower energy.
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