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A common kind of celestial object made of rock and ice that typically moves in a closed orbit around a star, such as the Sun, and appears as a hazy streak when viewed in Earth’s skies. Physically, a comet is a small, solid body, termed the nucleus,…

One of the brightest comets of the twentieth century and among the most observed comets in human history. Comet Hale-Bopp could be seen with the naked eye in the night sky between February and May 1997 over the heavily populated Northern Hemisphere;…

One of the 88 areas into which the night sky is divided and named according to an imagined pattern of stars representing a mythic character, creature, animal, or object. A constellation is a "picture" drawn on the heavens by human observers, using…

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 study of the origin, development, and fate of the universe, including the origin of galaxies, the chemical elements, and matter itself. As a modern scientific discipline, cosmology incorporates astronomy, physics, and chemistry, among many other…

An entity that comprises the majority of the mass-energy of the universe and is responsible for its accelerating expansion. The term “dark energy” derives from the inference that it is nonluminous and does not interact with normal matter. According…

Particles or objects that exert a gravitational force but do not emit any detectable visible light or other electromagnetic radiation of any kind. Dark matter is the dominant form of matter in our Galaxy and in the universe. Astronomers have…

A space probe mission that made the first visits to the two most massive bodies in the asteroid belt, Ceres and Vesta. A primary objective of the Dawn mission was to learn more about the origin of the solar system by mapping objects that recorded…

After a decades-long search, astronomers have detected the signature of a molecule predicted as the first type to have ever formed in the universe. Called helium hydride and symbolized HeH+, this positively charged ion consists of a helium atom that…

Researchers recently announced the discovery of the farthest object ever identified within our solar system. Designated 2018 VG18 and nicknamed Farout, the object is located about 120 astronomical units (AU) from the Sun, or 120 times the average…

The third planet from the Sun and the largest of the four inner, or terrestrial, planets. The Sun is an average-sized, middle-aged star situated in the Milky Way Galaxy. So far as is known, Earth (Fig. 1) is unique in the solar system in having…

The occultation (obscuring) of one celestial body by another. Solar and lunar eclipses take place at syzygies of the Sun, Earth, and Moon, when the three bodies are in a line. At a solar eclipse, the Moon blocks the view of the Sun as seen from the…

The sixth-largest moon of Saturn, known for its bright icy surface, active geysers, and astrobiological potential. Enceladus is the most reflective astronomical body in the solar system because it is covered in fresh, smooth ice. The sources of this…

In a breakthrough for astronomy and physics, the Event Horizon Telescope (EHT) has obtained a direct image of a black hole. The first black hole captured in this manner is a supermassive one at the heart of a nearby galaxy, Messier 87. Further study…

Planets beyond our solar system. For millennia, humankind has known about most of the major planets in our solar system because these worlds are visible to the naked eye. Exoplanets, however, were not indisputably known until 1995, when astronomers…

Postulated entities beyond Earth with a level of intelligence and comprehension at least equal to that of present-day humans. Whether alien life—and especially intelligent alien life—exists is one of the profoundest mysteries in science.…

A space telescope that viewed the universe in a specific band of far-ultraviolet light. The Far Ultraviolet Spectroscopic Explorer (FUSE) mission launched in June 1999 from Cape Canaveral, Florida, and lasted until October 2007, ending with the…

A celestial source of millisecond-duration, highly dispersed pulses of radio waves emanating from cosmological distances. Fast radio bursts (FRBs), first discovered in 2007, are distributed uniformly across the sky and are thought to have been…

Researchers have reached a key new milestone in the continuing search for worlds like Earth that could support life elsewhere in the universe. For the first time, water vapor has been detected in the atmosphere of an exoplanet that resides within…

One of the large self-gravitating aggregates of stars, gas, dust, planets, rocks, and dark matter that occurs by the hundreds of billions in the universe, collectively containing most of the cosmos' visible matter. Everywhere we look beyond our own…

How the diverse array of galaxies that are now observed originated and evolved into their present form. Deep telescope observations of the cosmos have found evidence for the first galaxies arising within several hundred million years after the big…

The study of gamma rays of cosmic origin. Gamma rays are the highest energy form of light. They comprise a vast spectral domain extending from an energy of 0.01 MeV (megaelectronvolts, corresponding to a wavelength of 10−10 m, the adopted boundary…

Attempts to unify three fundamental interactions—strong, electromagnetic, and weak—into one interaction at some very high energy. Also referred to as Grand Unified Theory, grand unification theories (GUTs) posit that three out of the four recognized…

The late stage in stellar evolution in which the pressure in a star is insufficient to maintain the star at a stable size. During gravitational collapse, material in an aged star falls inward under its own gravitational attraction. Depending on the…

The most famous of comets, associated with many important events in history. A comet is a small body, consisting of rock and ice, whose orbit takes it near enough to the Sun that its ice begins to sublimate, oftentimes generating a bright atmosphere…

A large, space-borne astronomical observatory that studied the “cold” universe in the far infrared and submillimeter portions of the electromagnetic spectrum. The Herschel Space Observatory was a major mission in the European Space Agency (ESA)…

The study of the universe as revealed by the high-energy, invisible forms of light, x-rays and gamma rays, and through cosmic rays and neutrinos. High-energy forms of radiation and particle emissions are produced through the heating and acceleration…

The largest visible-light observatory ever placed into space. The Hubble Space Telescope's orbit, some 612 km (380 mi) above Earth's surface (Fig. 1), keeps it beyond almost all of Earth's atmosphere, at a location where its view of the heavens is…

A theory motivated by considerations from elementary particle physics and problems in standard big bang cosmology, which asserts that the early universe underwent a rapid period of exponential expansion. Inflationary theories predict that the scale…

The field of astronomical observations specializing in detecting photons from the infrared portion of the electromagnetic spectrum. Infrared astronomy plays an important role particularly in the study of cooler astrophysical objects, ranging from…

Researchers Chi Ma of Caltech and Alan Rubin of the University of California at Los Angeles reported in the journal American Mineralogist (September 2019) the discovery in a meteorite of the first natural occurrence of the iron-carbide mineral…

A space telescope mission dedicated to detecting exoplanets. The tenth mission launched under the National Aeronautics and Space Administration's (NASA's) Discovery Program, Kepler was the agency’s first mission dedicated to the search for planets…