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- Accretion Disk
- a disk of matter that forms when material is transferred to a gravitationally massive body, such a black hole. For black holes, the disks form outside the event horizons. For other objects, such as neutron stars or white dwarfs, the disks can extend down to the stellar surfaces. Friction and other forces heat the disks, which then emit a wide range of different types of electromagnetic radiation including x-rays.
- (protoplanetary) Accretion Disk
- accretion disks are not just found around black holes. When dust and gas orbit an object in a flattened disk-like shape, collisions within the disk cause most of the material to fall inward to create a central object. The remaining material moves outward, carrying with it all the angular momentum. In a protoplanetary accretion disk, a star will form at the center, while in the disk itself material can accumulate to form planets. Protoplanetary disks are a special type of accretion disk.
- Active Galaxy
- a galaxy with an unusually large amount of energy emitted from the core.
- Active Galactic Nucleus (AGN)
- the central, compact region of of an active galaxy, which is the source of most of the host galaxy’s emitted energy. The plural of nucleus is nuclei.
- Afterglow
- in gamma-ray bursts, afterglows are the emissions seen after a GRB, which can be seen in X-rays, optical, and radio. Afterglows can last for days or even weeks.
- Albedo
- the efficiency with which a particular surface reflects light. It is calculated by dividing the amount of light reflected by the surface by the amount of light that shines on it.
- Andromeda Galaxy
- A barred spiral galaxy that will collide with the Milky Way in ~4-5 billion years. It is the closest major galaxy to the Milky Way, and it is generally considered to be the largest and most massive galaxy within the Local Group.
- All Sky Survey
- a map created by taking images of every region of the sky, generally at a particular set of wavelengths, in order to detect and catalog the objects found.
- Altimeter
- an instrument to measure altitude.
- Altitude
- the height of an object above a reference plane, usually the surface of a planet. On Earth, the sea level is used as the reference plane.
- Amps (ampere)
- the standard international unit of electric current; equal to one coulomb per second where one coulomb is the charge corresponding to 6.241 x 1018 protons.
- Angle of Incidence
- the angle between a light ray incident on a surface and the line perpendicular to that surface.
- Angle of Reflection
- the angle between a light ray reflecting from a surface and the line perpendicular to that surface.
- Anglular Size
- the angle between the lines of sight to the two opposite sides of an object.
- Annihilate
- the process where a particle and its anti-particle collide, destroying both particles and leaving behind light energy (i.e. photons) in accordance with Einstein’s famous E = mc2 equation.
- Annular
- shaped like or forming a ring.
- Anode
- an electrode through which positive electric charge flows.
- Apex
- the point or tip of the cone; a peak.
- Apollo
- NASA’s Apollo program included the first manned mission to the Moon and ran from 1963 to 1972.
- Arcminute
- an angular unit of measurement. There are 360 degrees in a circle, 60 arcminutes in a degree, and 60 arcseconds in an arcminute. 1/60th of a degree.
- Arcsecond
- see arcminute.
- Argument
- the variable that is being passed to a function.
- Artificial Intellidence (AI)
- a system of hardware and software that mimics human intelligence.
- Asteroid
- a rocky object in space that can be a few feet to several hundred miles wide.
- Asterism
- a pattern of stars as seen on Earth’s night sky as opposed to a constellation, which is a defined area of the sky as seen from Earth.
- Astrobiology
- the science that tries to answer questions like: how do living organism originate? Is there life out in space, and what form does it have? What is the future for life on Earth and beyond?
- Astropheric Science
- the study of the atmosphere, its processes and the interactions between the atmosphere and other systems, especially the oceans. Atmospheric Science includes fields like meteorology (the study of weather systems) and climatology (the study of climates and climate change).
- Axion
- a hypothetical subatomic particle, introduced in the Peccei-Quinn Theory, that is also a candidate for dark matter.
- Atom
- the basic unit of an element composed of a nucleus made of protons and neutrons, and an outer cloud of electrons.
