Accretion disk: A flattened cloud of material that accretes onto a central gravitating mass (star or black hole).

Accretion-induced collapse (AIC): The collapse of a white dwarf, initially formed in a binary system, into a neutron star.

Acoustical peaks: Peaks in the temperature fluctuations of the cosmic microwave background as functions of the multipole moment l.

Active galactic nucleus/nuclei (AGN): Active accretion of mass onto the supermassive black hole at the center of a galaxy. Such black holes appear to be a feature of most galaxies, but in most cases, the rate of accretion is minimal. When the accretion rate is high, the AGN releases energy well in excess of the host galaxy starlight, all of it originating near the center of the galaxy, hence the name.

Adaptive optics (AO): A technology used to improve the resolution of ground-based telescopes by compensating for atmospheric distortions in real time.

Adiabatic fluctuations: Adiabatic fluctuations are fluctuations in the density of dark matter, ordinary matter, and radiation with the ratio of the density of matter components to the density of the radiation components held spatially constant.

AGN (quasar) winds: Outflows or jets generated by quasars or AGN. See also Jets.

Akari: Japanese satellite for infrared astronomy.

Allen Telescope Array (ATA): ATA-42 is an array of 42 6-meter-diameter radio telescope dishes at Hat Creek Observatory, California.

Arecibo: A 305-meter radio telescope in Puerto Rico, operated by the National Astronomy and Ionosphere Center (NAIC).

Asphericity: Deviation from a spherical shape.

Asteroseismology: The study of the internal structure of stars through observation of surface vibrations.

Astro2010: The National Research Council’s decadal survey of astronomy and astrophysics published in 2010 as New Worlds, New Horizons in Astronomy and Astrophysics.

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