Barnard's Star is a relatively well-studied astronomical object, and has likely received more attention than any other M dwarf star given its proximity and favourable location for observation near the celestial equator. It has also been the subject of some controversy. For a decade from the early 1960s onward, an erroneous discovery of a planet or planets in orbit around Barnard's star was accepted by astronomers. It is also notable as the target for a study on the possibility of rapid, unmanned travel to nearby star systems. Research has focused on stellar characteristics, astrometry, and refining the limits of possible planets.
A binary star is a stellar system consisting of two starsorbiting around their center of mass. For each star, the other is its companion star. Recent research suggests that a large percentage of stars are part of systems with at least two stars. Binary star systems are very important in astrophysics, because observing their mutual orbits allows their mass to be determined. The masses of many single stars can then be determined by extrapolations made from the observation of binaries.
Binary stars are not the same as optical double stars, which appear to be close together as seen from Earth, but may not be bound by gravity. Binary stars can either be distinguished optically (visual binaries) or by indirect techniques, such as spectroscopy. If binaries happen to orbit in a plane containing our line of sight, they will eclipse each other; these are called eclipsing binaries.
The components of binary star systems can exchange mass, bringing their evolution to stages that single stars cannot attain. Examples of binaries are Algol (an eclipsing binary), Sirius, and Cygnus X-1 (of which one member is probably a black hole).
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latincometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Comet Hyakutake (Japanese: 百武彗星 Hyakutake suisei, IPA[çakɯtakesɯiseː]; formally designatedC/1996 B2) is a comet that was discovered in January 1996, and passed very close to the Earth in March of that year. It was dubbed The Great Comet of 1996, and was one of the closest cometary approaches to the Earth in the previous 200 years, resulting in the comet appearing very bright in the night sky and being seen by a large number of people around the world. The comet temporarily upstaged the long-awaited Comet Hale–Bopp, which was approaching the inner Solar System at the time, although Hyakutake was at its brightest for only a few days.
Scientific observations of the comet led to several notable discoveries. Most surprising to cometary scientists was the discovery of X-ray emission from the comet, the first time a comet had been found to be emitting X-rays. This emission is believed to be caused by ionised solar wind particles interacting with neutral atoms in the coma of the comet. The Ulysses spacecraft also unexpectedly crossed the comet's tail at a distance of more than 500 million km from the nucleus, showing that Hyakutake had the longest tail yet known for a comet.
Hyakutake is a long-period comet. Before its most recent passage through the Solar System, its orbital period was about 15,000 years, but the gravitational influence of the giant planets has now increased this to 72,000 years.
