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

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

The study of the universe and the objects in it through scientific investigation. The goal of astronomical research is to understand celestial objects and the nature and evolution of the cosmos (Fig. 1). Because much of contemporary astronomy uses the laws, mathematics, and methods of physics, the terms "astronomy" and "astrophysics" are usually used interchangeably. However, modern astronomy also uses techniques from many other scientific disciplines, including chemistry, geology, and biology, for which the terms astrochemistry, planetary science, and astrobiology are regularly used. Even astrometry, the measurement of the positions and motions of stars and other astronomical objects, has links to astrophysical investigations. See also: Astrobiology; Astrometry; Astrophysics; Biology; Chemistry; Geology; Physics

The study of the stars, galaxies, and other objects outside of the Solar System, with attention to physical causes. Astrophysics developed in the late 19^th century, taking over from the mere mapping of positions of objects that had been the traditional form of astronomy. In the late 20^th and early 21^st century, the terms “astronomy” and “astrophysics” are often used interchangeably, though the latter places more emphasis on the mechanisms driving cosmic phenomena stemming from the bulk behavior of particles according to fundamental interactions. Examples of astrophysical investigations are the study of the physics powering supernovae explosions, as well as the mechanisms producing electromagnetic emissions from the environments of black holes (Fig. 1). See also: Astronomy; Elementary particle; Fundamental interaction; Physics

A region of spacetime exerting a gravitational field so strong that neither matter nor radiation can escape. Black holes are extreme cosmic objects predicted by German-born U.S. theoretical physicist Albert Einstein's theory of general relativity. Within a boundary known as the event horizon, the escape velocity needed to overcome the gravitational attraction of the black hole exceeds the speed of light, meaning that nothing that crosses over the event horizon can ever leave. Black holes are therefore by definition invisible, but because of their powerful gravitational fields, they can be indirectly observed through the highly conspicuous effects they have on their cosmic environments. These effects include the gravitational intake of matter through accretion disks, a process that generates tremendous heat and light and is well-observed at scales from binary star systems to the cores of galaxies. In the absence of ongoing accretion, black holes should also theoretically cause severe localized warping of spacetime, gravitationally lensing light from luminous sources and distorting their appearance (Fig. 1). The merging of two black holes each with roughly 30 times the Sun's mass, detected in 2015 by the Laser Interferometer Gravitational-wave Detector (LIGO), opened a new and fruitful way of studying black holes, and revealed a mass range of stellar-mass black holes greater than had been thought to be possible. See also: Astronomy; Escape velocity; Gravitation; Gravitational lens; Gravitational radiation; Heat; Light; LIGO (Laser Interferometer Gravitational-wave Observatory); Relativity

A group of nuclear reactions that involve the interaction of protons (nuclei of hydrogen atoms, designated by ¹H) with carbon, nitrogen, and oxygen nuclei. The cycle involving only isotopes of carbon and nitrogen is well known as the carbon-nitrogen (CN) cycle. These cycles are thought to be the main source of energy in main-sequence stars with mass 40% or more in excess of that of the Sun. Completion of any one of the cycles results in consumption of four protons (4 ¹H) and the production of a helium (⁴He) nucleus plus two positrons (e⁺) and two neutrinos (ν). The two positrons are annihilated with two electrons (e⁻), and the total energy release is 26.73 MeV. Approximately 1.7 MeV is released as neutrino energy and is not available as thermal energy in the star. The energy E = 26.73 MeV arises from the mass difference between four hydrogen atoms and the helium atom, and is calculated from the Einstein mass-energy equation E = Δmc², where Δm is the mass difference and c² is the square of the velocity of light. Completion of a chain can be thought of as conversion of four hydrogen atoms into a helium atom. Because the nuclear fuel that is consumed in these processes is hydrogen, they are referred to as hydrogen-burning processes by means of the carbon-nitrogen-oxygen (CNO) cycles. See also: Solar neutrinos

A type of close binary star system containing a cool star transferring material to its hotter, high-density, degenerate white dwarf companion. The mass transfer results in a large range of observed variability, including cataclysmic events called outbursts, which can increase the brightness of the systems by 2– 10 magnitudes (a logarithmic scale with each magnitude being a factor of 2.5 in brightness) from quiescence, equivalent to a factor of 6–10,000 times in intensity. The specific behavior of each system and the extent and cause of the variability are related to whether the transferred material accumulates in an accretion disk surrounding the white dwarf, or whether it flows in a ballistic stream directly from the cool star to the white dwarf surface. Which of these processes will occur depends on the magnetic field strength of the white dwarf and the separation of the two stars in the binary, properties which are determined during the formation of the system (see illustration). See also: Magnitude (astronomy)

Light emitted by a high-speed charged particle when the particle passes through a transparent, nonconducting, solid material at a speed greater than the speed of light in the material. The emission of Cerenkov radiation—also spelled Cherenkov—is analogous to the emission of a shock wave by a projectile moving faster than sound, because in both cases the velocity of the object passing through the medium exceeds the velocity of the resulting wave disturbance in the medium. A common example of Cerenkov radiation is the blue glow observed in the water of a nuclear reactor, close to the active fuel elements, when electron particles travel through the water faster than the light that the electrons emit (Fig. 1); most Cerenkov light, however, is emitted in the ultraviolet and not the visible range. See also: Electron; Light; Motion; Sound; Speed; Ultraviolet radiation; Velocity; Wave motion

A powerful x-ray telescope in orbit around the Earth. Built by the National Aeronautics and Space Administration (NASA), it was launched and deployed by space shuttle Columbia on July 23, 1999. Named after the Indian-American astronomer Subrahmanyan Chandrasekhar, Chandra is unique because of its sensitivity and its extremely high precision mirrors. These features have made possible significant advances in astronomy—especially in relation to the life cycles of stars, the role of supermassive black holes in the evolution of galaxies, and the study of dark matter and dark energy.

Charged, energetic particles, mostly hydrogen and helium nuclei, that travel at nearly the speed of light through space and bombard Earth from all directions. Cosmic rays are generated in astrophysical environments, ranging from the Sun locally to the cores of galaxies many billions of light-years away. When cosmic rays reach Earth, they shatter molecules in the atmosphere, triggering so-called air showers of secondary particles that can propagate all the way to the ground if the original cosmic ray is sufficiently energetic (Fig. 1). See also: Atmosphere; Atomic nucleus; Galaxy; Helium; Hydrogen; Light-year; Molecule; Sun