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AAS

Overview page. Subjects: Astronomy and Astrophysics.

Abbreviation for either American Astronautical Society or American Astronomical Society.

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Abbé Georges Édouard Lemaître

Overview page. Subjects: Astronomy and Astrophysics.

(1894–1966) Belgian astronomer and cosmologist

Lemaître was born at Charleroi in Belgium. After serving in World War I, he studied at the University of Louvain in Belgium from...

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Abd al-Ramān al-Ṣūfī

Overview page. Subjects: Astronomy and Astrophysics.

(903–86)

Arabastronomer (Latinized name Azophi), born in modern Iran. His Book of the Fixed Stars (c.964) contained a star catalogue and atlas based on Ptolemy's*Almagest, revised...

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Abell Catalogue

Overview page. Subjects: Astronomy and Astrophysics.

A catalogue of 2712 rich clusters of galaxies published in 1958 by the American astronomer George Ogden Abell (1927–83) from inspection of the Palomar Observatory Sky Survey photographs....

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Abell cluster

Overview page. Subjects: Astronomy and Astrophysics.

A cluster of galaxies listed in the Abell Catalogue. To appear in the catalogue, a cluster must satisfy selection criteria which include containing more than 50 galaxies and having a dense...

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Abell radius

Overview page. Subjects: Astronomy and Astrophysics.

A radius of about 2 megaparsecs within which at least 50 galaxies of a particular range of brightness must be found if the cluster is to qualify as an Abell cluster.

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aberration

Overview page. Subjects: Astronomy and Astrophysics.

The small apparent difference between the observed direction of a star and its true direction (see diagram). It is due to the combined effect of the observer's motion across the path of...

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ablation

Overview page. Subjects: Astronomy and Astrophysics.

The wearing away of the outer layers of a body by melting, erosion, vaporization, or some other process due to aerodynamic effects as the body moves at high speed through a planetary...

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ablation age

Overview page. Subjects: Astronomy and Astrophysics.

The period of time since the outer glassy layers of a tektite solidified following ablation during its re-entry into the Earth's atmosphere. The ablation ages of known tektites vary from...

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ablation shield

Overview page. Subjects: Astronomy and Astrophysics.

The heat shield on a spacecraft to protect it during re-entry. The intense heat generated by friction during the high-speed entry of the craft into the Earth's atmosphere burns away...

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abort

Overview page. Subjects: Astronomy and Astrophysics.

A premature end of a space flight due to danger to the crew, the mission, or the environment, such as an accident or systems failure. This can occur during launch or during a mission. The A...

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abort sensing and implementation system

Overview page. Subjects: Astronomy and Astrophysics.

(ASIS)

The automatic detection system added to the Atlas rocket in 1959 to create near-perfect reliability for crewed space flights. ASIS could sense and signal an impending...

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absolute magnitude

Overview page. Subjects: Astronomy and Astrophysics.

(M)

1 The brightness that a star would have if it were at a distance of 10 parsecs in perfectly clear space without interstellar absorption. Absolute magnitude is...

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absorption

Overview page. Subjects: Astronomy and Astrophysics.

The transfer of energy from a photon to an atom or molecule. If the energy of the photon is just sufficient to raise an electron from one energy level to another, the result is an...

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absorption edge

Overview page. Subjects: Astronomy and Astrophysics.

A limiting feature in a series of absorption lines from a single element (e.g. hydrogen), representing the wavelength at which the element becomes ionized. In the Balmer series of hydrogen,...

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absorption line

Overview page. Subjects: Astronomy and Astrophysics.

A dark feature in the spectrum of a star, formed by cooler gas in the star's outer layers (the photosphere) that absorbs radiation emitted by hotter gas below. The Fraunhofer lines in the...

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Acamar

Overview page. Subjects: Astronomy and Astrophysics.

The star Theta Eridani, magnitude 2.9. It is a double star, consisting of an A5 subgiant and an A1 dwarf, magnitudes 3.2 and 4.3, 161 l.y. away.

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acceleration of the Universe

Overview page. Subjects: Astronomy and Astrophysics.

The expansion rate of the Universe, measured by the Hubble parameter, would normally be expected to be slowing down, as the gravitational force between objects opposes the expansion. That...

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acceleration, secular

Overview page. Subjects: Astronomy and Astrophysics.

A continuous and non-periodic change in orbital velocity of one body around another, or the axial rotation period of a body.

An example is the axial rotation of the Earth. This is...

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accretion disk

Overview page. Subjects: Astronomy and Astrophysics.

A structure that forms around a compact object (e.g. a white dwarf, neutron star, or black hole) when matter flows towards it. Accretion disks are found in interacting binary stars, and are...

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