U GEMINORUM STARS - Type of variable star displaying sudden and unpredictable increases in brightness, sometimes referred to as dwarf novae. Brightness may increase by as much as 2–5 magnitudes in hours, followed by a slower fading over days to weeks. U Geminorum stars are a binary system consisting of a white dwarf and a red giant that has filled its Roche-lobe. Roche-lobe overflow causes matter to fall onto an accretion disk around the white dwarf creating a hot spot at the point of impact. Thus, the light from a U Geminorum star has at least four sources: the two stars, the accretion disk and hot spots on the accretion disk. Sudden increases in brightness probably reflect increases in the amount of material falling onto the white dwarf from the accretion disk.
The causes of sudden increases in accretion rate are not fully understood. One possibility is that sudden increase in the transfer of material from the companion star to the accretion disk causes the disk to collapse onto the white dwarf. Another, preferred, possibility is that the accretion disk can only accumulate a finite amount of gas before becoming unstable and instability causes an abrupt increase in the amount of matter falling onto the white dwarf.
ULTRA HIGH ENERGY COSMIC RAYS (UHECR) - Extremely energetic subatomic particles (mostly protons, but also some heavier atomic nuclei) with energies >109 MeV. The record holder so far is a UHECR with an energy of 6 × 1014 MeV! UHECRs are observed only in cosmic ray showers, because of their extremely low numbers. The source of UHECRs is a mystery, as is the mechanism that accelerates particles to these energies. They have sufficient energy to escape the typical magnetic field of a spiral galaxy, and most UHECRs are probably of extragalactic origin. Possible sources include active galactic nuclei, dormant quasars with associated supermassive black holes and galaxy mergers.
UNCERTAINTY PRINCIPLE - Principle central to quantum mechanics, which states that two complementary parameters (such as position and momentum, energy and time, or angular momentum and angular displacement) cannot both be known to infinite accuracy. The more you know about one, the less you know about the other. It can be illustrated in a fairly clear way as it relates to position vs. momentum. For example, to see an electron, one has to fire photons at it. The photons bounce off and return, allowing one to see the electron. Low-energy photons will not impart much momentum to the electron (higher certainty in momentum), but yield a very fuzzy picture (a higher uncertainty in position). High-energy photons (x-rays or γ-rays) provide a very clear picture of the electron's location (high certainty in position), but impart a great deal of momentum to the electron (higher uncertainty in momentum). In a more generalized sense, the uncertainty principle tells us that the act of observing changes the observed in fundamental way.
UNIT CELL - Smallest repeating unit of a crystalline solid that can be used to describe the entire structure. Unit cells are like templates which can be copied to produce an entire crystal.
UNIVERSAL GRAVITATIONAL CONSTANT (G) - Constant with value of 6.670 × 10-11 N m2/kg2.
UNIVERSE - That which contains and subsumes all the laws of nature, and everything subject to those laws; the sum of all that exists physically, including matter, energy, physical laws, space, and time. Also, a cosmological model of the universe.
URACIL - One of the five nitrogen-containing bases occurring in nucleotides.
UREILITE (URE) - Member of a subgroup of primitive achondrites named for Novo Urei, a village in the Mordova Republic, Russia, where several meteorites fell in late 1886. It has been reported that one stone recovered by local peasants was broken apart and eaten! The report doesn't reveal why this happened - perhaps the freshly fallen meteorite smelled good, or perhaps because it was shaped like a loaf of bread, which some ureilite are. However, not all of the stones were eaten, and Novo Urei became the type specimen of one of the best-represented achondrite groups.
There are two subgroups of ureilite. Main group ureilites are composed mainly of coarse-grained olivine (Fa5-25), pigeonite (En75Wo15) and orthopyroxene (En80-90), set in a dark carbonaceous matrix of graphite and diamond, Ni-Fe metal, and troilite. Polymict ureilites consist of a mixture of different lithologies. Besides clasts from main group ureilites, they contain magmatic inclusions, dark carbonaceous clasts, chondritic fragments of different origins, and various other inclusions. This suggests a surface or regolith origin for the polymict ureilites, an assumption supported by the values for noble gases that have been implanted into the regolith by the solar wind. However, both the origin and the formation history of the ureilites remain enigmatic. Their mineral and oxygen isotopic compositions suggest that they are residues of partial melting, and therefore represent primitive achondrites that probably formed on several parent bodies. On the other hand, REE patterns and other chemical characteristics indicate that ureilites are highly fractionated igneous rocks that formed in different regions of the same parent body; probably a moderately differentiated C-class asteroid that was disrupted by an impact event and then rapidly cooled. An impact history would also explain the occurrence of high-pressure minerals such as diamond and londsdaleite that are formed by intense shock metamorphism. Even this theory isn't without its problems though. Recently, a ureilite from the Libyan Sahara, named DaG 868, was found to contain diamonds, but paradoxically, appears to be nearly unshocked.