The basics of the Universe in terms and definitions

# A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
There are 23 names in this directory beginning with the letter A.
The study of how life on Earth could have arisen from inanimate matter. It should not be confused with evolution (the study of how living things change over time), biogenesis (the process of lifeforms producing other lifeforms) or spontaneous generation (the obsolete theory of complex life originating from inanimate matter on an everyday basis).

A process by where the atmosphere melts away and removes the surface material of an incoming meteorite.

Absolute Magnitude
Absolute magnitude is a measure of a star's absolute brightness. It is defined as the apparent magnitude the star would show if it were located at a distance of 10 parsecs, or 32.6 light years.

Absolute Zero
The lowest temperature possible, equivalent to -273.15°C (or 0° on the absolute Kelvin scale), at which point atoms cease to move altogether and molecular energy is minimal. The idea that it is impossible, through any physical process, to lower the temperature of a system to zero is known as the Third Law of Thermodynamics.

Accretion Disk
Diffuse material orbiting around a central body such as a protostar, a young star, a neutron star or a black hole. Gravity causes the material in the disc to spiral inwards towards the central body with great speed, and the gravitational forces acting on the material cause the emission of x-rays, radio waves or other electromagnetic radiation (known as quasars).

A stone meteorite that contains no chondrules.

Albedo Feature
Albedo feature is a large area on the surface of a reflecting object that shows a contrast in brightness or darkness (albedo) with adjacent areas.

Alpha Particle
A particle of 2 protons and 2 neutrons (essentially a helium nucleus) that is emitted by an unstable radioactive nucleus during radioactive decay. It is a relatively low-penetration particle due its comparatively low energy and high mass.

Am Star
Am star is a chemically peculiar star belonging to the more general class of A-type stars. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals.

Angular Momentum
A measure of the momentum of a body in rotational motion about its centre of mass. Technically, the angular momentum of a body is equal to the mass of the body multiplied by the cross product of the position vector of the particle with its velocity vector. The angular momentum of a system is the sum of the angular momenta of its constituent particles, and this total is conserved unless acted on by an outside force.

Anthropic Principle
The idea that the fundamental constants of physics and chemistry are just right (or “fine-tuned”) to allow the universe and life as we know it to exist, and indeed that the universe is only as it is because we are here to observe it. Thus, we find ourselves in the kind of universe, and on the kind of planet, where conditions are ripe for our form of life.

A large accumulation of antiparticles - antiprotons, antineutrons and positrons (antielectrons) - which have opposite properties to normal particles (e.g. electrical charge), and which can come together to make antiatoms. When matter and antimatter meet, they self-destruct in a burst of high-energy photons or gamma rays. The laws of physics seem to predict a pretty much 50/50 mix of matter and antimatter, despite the observable universe apparently consisting almost entirely of matter, known as the “baryon asymmetry problem”.

Apoapsis is the point of furthest excursion, or separation, between two orbiting objects.

Apparent Magnitude
Apparent magnitude is a measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower the value of its magnitude.

Appulse is the closest approach of one celestial object to another, as viewed from a third body.

Asterism is a pattern of stars recognized on Earth's night sky. It may form part of an official constellation, or be composed of stars from more than one.

Asteroid (Minor Planet)
A solid body orbiting the Sun that consists of metal and rock. Most are only a few miles in diameter and are found between the orbits of Mars and Jupiter, too small and far away to be seen easily in a small telescope. A few venture closer to the Sun and cross Earth’s orbit.

Astrometric Binary
Astrometric binary is a type of binary system where evidence for an unseen orbiting companion is revealed by its periodic gravitational perturbation of the visible component. See also spectroscopic binary.

Astronomical Unit
Astronomical unit, or AU, is the approximate distance between the midpoints of the Earth and the Sun.

The basic building block of all normal matter, consisting of a nucleus (which is itself composed of positively-charged protons and zero-charged neutrons) orbited by a cloud of negatively-charged electrons, so that the positive charge is exactly balanced by the negative charge and the atom as a whole is electrically neutral. Atoms range from about 32 to about 225 picometres in size (a picometre is a trillionth of a metre). A typical human hair is about 1 million carbon atoms in width.

Autumnal Equinox
Autumnal equinox is the point in the year when the Sun appears to cross the celestial equator, while generally trending southward at each zenith passage. It represents the moment when the North Pole of the Earth begins to tilt away from the Sun.

Averted Vision
Viewing an object by looking slightly to its side. This technique can help you detect faint objects that are invisible when you stare directly at them.

Azimuth is an angular measurement of an object's orientation along the horizon of the observer, relative to the direction of true north. When combined with the altitude above the horizon, it defines an object's current position in the spherical coordinate system.