absolute zero
The lowest temperature attainable is –273°C (–459°F), known as zero kelvin (0 K). Essentially, this is the temperature at which atoms cease to exchange energy.
asteroid
A small rocky body, 1000 km (600 miles) or less in diameter, orbiting the Sun. Most are found in the asteroid belt between Mars and Jupiter but some have more elliptical orbits and swing past the inner planets.
atmosphere
A shell of gases around a planet or moon, of varying combinations and densities.
Big Bang
The event that marks the origin of the Universe 13.7 billion years ago: the entire Universe exploded into existence and has been expanding ever since.
billion
A thousand million: 109 or 1,000,000,000.
black hole
An extremely high concentration of mass, with an overwhelming gravitational field from which not even light can escape. Massive stars form black holes when they collapse at the end of their lives, and collapsing gas clouds form supermassive black holes at the centers of galaxies.
comet
A chunk of dust and ice, of the order of kilometers wide, left over from the formation of a planetary system. Comets orbiting the Sun are classified according to their orbital period: short-period comets complete their orbit in less than 200 years, while long-period comets can take millions of years.
constants of nature
The physical constants that help govern the behavior of nature, for example the value of the speed of light in a vacuum. Science cannot yet say why they have the values they do.
cosmic microwave background radiation
A faint glow of microwave radiation across the entire sky, believed to be the afterglow of the Big Bang itself, now cooled to –270°C (–454°F) or just 2.7 degrees above absolute zero.
cosmological distance ladder
A network of techniques that allow astronomers to measure and then estimate distances into the further reaches of the Universe.
The time between 300,000 years and approximately 1 billion years after the Big Bang when the first celestial objects were forming; before these objects formed, there was no light in the Universe.
dark energy
A theoretical energy that permeates all of space, or an unidentified force of nature, employed to explain the acceleration of the expansion of the Universe.
dark matter
A hypothetical form of matter thought to outweigh normal atoms by ten times. The effect of its gravity on visible matter infers its presence, but it cannot be detected by emitted radiation.
electron
A lightweight, negatively charged particle that usually “orbits” an atomic nucleus.
fission
The act of splitting an atomic nucleus to create two or more lighter atomic nuclei.
fusion
The act of joining two atomic nuclei in order to create a heavier nucleus.
galaxy
A gravitationally bound system of dust, gas and stars. Galaxies range in size from a few hundred to hundreds of thousands of light years across, and are classified according to their appearance. “The Galaxy” refers to our own Milky Way.
General Theory of Relativity
Albert Einstein’s theory that explains gravity as a distortion of space–time.
gravity
A physical force that appears to exert a mutual attraction between all masses, governed by the mass and distance apart of the objects.
inflation
A postulated moment of intense expansion of space–time shortly after the Big Bang.
kelvin
A scale of temperature that places zero as the coldest possible temperature (see absolute zero). To convert from kelvin to Celsius, subtract 273.
late bombardment
The final phase of the Solar System’s formation, when asteroids and comets pummeled the planets and moons. Our Moon’s craters were formed at this time.
The distance traveled by light in a vacuum during one year: 9.5 trillion km (5.9 trillion miles); it is used as a measure of astronomical distance to help to keep the vast numbers more manageable.
magnitude
A measure of the brightness of a celestial object.
moon
A moon is a natural satellite of a planet. There are more than 160 in the Solar System including “the Moon” which orbits Earth.
multiverse
The hypothesized multitude of universes somehow separate from our own.
neutron
A heavy, electrically neutral particle that can only exist inside an atomic nucleus.
nucleosynthesis
The process by which the chemical elements have been built up into their modern proportions.
orbit
The trajectory of one celestial body around another.
parallax
A trigonometric method of measuring the distance of a celestial object.
planet
A large object that orbits a star. By historical accident, the name is reserved for the larger bodies, but there is controversy over where the definition of a planet should end. Pluto has recently been reclassified as a dwarf planet.
principle of equivalence
The foundation stone of general relativity; it states that acceleration is indistinguishable from a gravitational field.
proton
A heavy, positively charged subatomic particle that is usually found inside an atomic nucleus.
quantum theory
A set of physical laws that describes the behavior of atoms and subatomic particles.
redshift
The lengthening of the wavelength of light from a distant object, caused by the expansion of the Universe.
The Sun and all the bodies in orbit around the Sun: planets and their moons, dwarf planets, asteroids and comets.
space–time
The hypothetical fabric of the Universe that, according to the General Theory of Relativity, is deformed by matter and produces gravity.
Special Theory of Relativity
Albert Einstein’s theory that allows observations from differently moving (but not accelerating) objects to be compared with one another.
star
A massive ball of hydrogen and helium bound together by gravity and shining for most of its life with light generated by nuclear fusion.
string theory
A possible way of unifying gravity with the other forces of nature, thus providing a single “theory of everything.” If successful it will supply a quantum theory of gravity.
Sun
The star at the center of our Solar System.
supernova
The total destruction of a star by a catastrophic explosion.
trillion
A thousand billion: 1012 or 1,000,000,000,000.
Universe
Everything that exists around us in space. There are estimated to be around 500 billion galaxies in our own Universe and there may be other universes somehow separate from our own.
wormhole
A hypothetical shortcut through the fabric of space–time.