Extrapolated into the past, these observations show that the universe has expanded from a state in which all the matter and energy in the universe was at an immense temperature and density. Physicists do not widely agree on what happened before this, although general relativity predicts a gravitational singularity (for reporting on some of the more notable speculation on this issue, see cosmogony).
The term Big Bang is used both in a narrow sense to refer to a point in time when the observed expansion of the universe (Hubble's law) began — calculated to be 13.7 billion (1.37 × 1010) years ago (±2%) — and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and expansion of the universe, as well as the composition of primordial matter through nucleosynthesis as predicted by the Alpher–Bethe–Gamow theory.
From this model, George Gamow in 1948 was able to predict, at least qualitatively, the existence of cosmic microwave background radiation (CMB). The CMB was discovered in 1964 and further corroborated the Big Bang theory, giving it an additional advantage over its chief rival, the Steady State theory.
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).
The Cat's Eye Nebula (NGC 6543) is a planetary nebula in the constellation of Draco. Structurally, it is one of the most complex nebulae known, with high-resolution Hubble Space Telescope observations revealing remarkable structures such as knots, jets and sinewy arc-like features.
Modern studies reveal several mysteries. The intricacy of the structure may be caused in part by material ejected from a binary central star, but as yet, there is no direct evidence that the central star has a companion. Also, measurements of chemical abundances reveal a large discrepancy between measurements done by two different methods, the cause of which is uncertain.
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.
