The apparent change in direction of a celestial source of light caused by an observer's component of motion perpendicular to the impinging rays. In astronomy, aberration manifests, for example, as a star appearing aberrantly from what its true position on the sky would be because of the motion of the Earth. The star in question appears to move in the same direction of motion as an observer on the Earth, an effect which would not occur if Earth were stationary with respect to the star. Because all astronomical bodies are in motion relative to one another, aberration is experienced universally, but is only subjectively relevant, based on the observer. For humankind as observers, aberration has played a historically significant role in astronomy and physics, specifically in the development of theories regarding light, electromagnetism, and relativity. See also: Astronomy; Astrophysics; Electromagnetism; Light; Motion; Physics; Star

An increase in the expansion rate of the universe, attributed to dark energy. In 1998, astronomers presented evidence that the universe's expansion is not slowing down, as would be expected due to the gravity from the abundances of matter strewn throughout the cosmos. Instead, the observations of distant, exploded stars revealed that cosmic expansion is accelerating (Fig. 1). Since this initial discovery, multiple other lines of evidence, including observations of the afterglow of the big bang, known as the cosmic microwave background, and in the clustering of galaxies over cosmic history have made the case for this surprising result more secure. See also: Big bang theory; Cosmic background radiation; Matter (physics); Physics; Universe

Technologies to optimize the performance of optical systems wherein controllable optical elements maintain a sharply focused image in the presence of wavefront aberrations. The practical development of adaptive optics started in the late 1960s. Main applications have been to compensate for the effects of atmospheric turbulence in ground-based astronomical telescopes (Fig. 1) and to improve the beam quality of lasers for use in communications, imaging, microscopy, and optical fabrication. A distinction is made between active optics, in which optical components are modified or adjusted by external control to compensate slowly changing disturbances, and adaptive optics, which applies to closed-loop feedback systems employing sensors and data processors, operating at much higher frequencies. See also: Astronomical observatory; Astronomy; Astronomical imaging; Atmosphere; Control systems; Laser; Optical communications; Optical microscope; Optics; Telescope

The efficiency of a surface in reflecting light. White surfaces have albedos close to 1; black surfaces have albedos close to 0. Because this efficiency can vary with the direction to the light source, the direction to the observer, and the color (wavelength) of the light, there are several types of albedos in common use.

Propagating oscillations in electrically conducting fluids or gases in which a magnetic field is present.

A book that contains astronomical or meteorological data arranged according to days, weeks, and months of a given year and may also include diverse information of a nonastronomical character. This article is restricted to astronomical and navigational almanacs.

The third brightest star in the sky, apparent magnitude −0.3, and the brightest in the southern constellation Centaurus. It is the closest star to the Sun at a distance of 1.35 parsecs (2.59 × 10¹³ mi or 4.16 × 10¹³ km), and its light takes more than 4 years to reach the Earth. It has an unusually large proper motion across the sky of 3.7 seconds of arc per year. See also: Centaurus

A prominently located constellation in the northern sky (see illustration), named for the daughter of Cassiopeia in Greek mythology: When Cassiopeia bragged that her daughter Andromeda was more beautiful than Poseidon's daughters, the Nereids, Poseidon created Cetus, the sea monster. (In some versions of the myth, Cassiopeia boasted of her own beauty.) The situation required Andromeda's sacrifice. However, Perseus saved Andromeda by showing Cetus the head of Medusa, turning Cetus to stone. See also: Cassiopeia; Perseus

The closest spiral galaxy to the Milky Way Galaxy. The name of the Andromeda Galaxy (Fig. 1) derives from its appearance in the constellation of Andromeda. The galaxy is also known as M31 and NGC 224, based on its entries in two widely used catalogs. See also: Andromeda; Astronomical catalogs; Constellation; Messier Catalog; Milky Way Galaxy

α Scorpii, a cool supergiant star of spectral type M1Ib, whose red color stands out in the midsummer sky. With an effective temperature of approximately 6000°F (3600 K), Antares resembles Betelgeuse, the brightest of the red supergiants, and would fill the solar system beyond the orbit of Mars if it replaced the Sun. Antares has a distance of 185 parsecs (600 light-years) from the Sun, and its angular diameter of about 0.045 arc-second has been measured by interferometric and lunar occultation methods. Red supergiants of this type originate as stars with mass at least 20 times that of the Sun. Such stars quickly evolve through successive stages of thermonuclear fusion of heavier and heavier elements in their cores with lighter elements undergoing fusion in concentric shells surrounding the hot core. Eventually, the core builds up with iron, the most tightly bound of all atomic nuclei. The fusion of iron nuclei absorbs rather than releases energy, and the supergiant star implodes in a type II supernova explosion. This fate is likely for Antares in less than a million years. Prior to this dramatic event, Antares will have shed up to 50% of its mass through a stellar wind of material blown away from the star into the surrounding interstellar medium. See also: Betelgeuse; Scorpius; Spectral type; Stellar evolution; Supergiant star; Supernova