Yang-Mills Theories	Also known as gauge theories, these theories were invented in 1954 by Chen Ning Yang and Robert Mills. In 1973 David Gross, Frank Wilczek, and David Politzer showed that these theories possess a property called asymptotic freedom, just what was needed for a theory of how quarks bind to form protons and neutrons. The new theory, dubbed quantum chromodynamics or QCD, proposed that the color of the quarks acts as the charge of the Yang-Mills interactions. [G97]
Year	A period of time based on the revolution of the Earth around the Sun. The calendar year (see Gregorian calendar) is an approximation to the tropical year (see year, tropical). The anomalistic year is the mean interval between successive passages of the Earth through perihelion. The sidereal year is the mean period of revolution with respect to the background stars. (See Julian year; year, Besselian.) [S92]
Year (Besselian)	The period of one complete revolution in right ascension of the fictitious mean sun, as defined by Newcomb. The beginning of a Besselian year, traditionally used as as standard epoch, is denoted by the suffix ".0". Since 1984 standard epochs have been defined by the Julian year rather that the Besselian year. For distinction, the beginning of the Besselian year is now identified by the prefix B (e.g., B1950.0). [S92]
Year (Tropical)	The period of one complete revolution of the mean longitude of the sun with respect to the dynamical equinox. The tropical year is longer than the Besselian year (see year, Besselian) by 0s.148 T, where T is centuries from B1900.0. [S92]
Yellow Giant	A giant star with a spectral type of G. The nearest and brightest yellow giants are the two composing the double star Capella. [C95]
Yellow Spot (Macula Lutea)	An area a few millimeters across in the human retina. It has a high concentration of rods, giving high visual acuity and color vision but low sensitivity to dim light. [DC99]
Yellow Supergiant	A supergiant star with a spectral type of G. [C95]
Yerkes System	A spectral classification system for stars; see MKK system. [H76]
Ylem	(a) The word used by Gamow and his collaborators for the primordial material of the Big Bang. In most of his work Gamow assumed that the ylem consisted entirely of neutrons. In inflationary cosmology, the role of the ylem is played by the false vacuum. [G97] (b) Primordial state of matter - neutrons and their decay products (protons and electrons) - before the Big Bang. The term was taken from Aristotle and used for the -- theory. [A84]
Young Disk Cepheids	Population I Cepheids. [H76]
Young Thin Disk	A subpopulation in the thin disk whose stars range in age from 0 to 1 billion years old. The stars of the young thin disk have a scale height of 350 light-years and have very circular orbits around the Galaxy. [C95]
Ytterbium	A soft malleable silvery element having two allotropes and belonging to the lanthanoid series of metals. It occurs in association with other lanthanoids. Ytterbium has been used to improve the mechanical properties of steel. Symbol: Yb; m.p. 824°C; b.p. 1193°C; r.d. 6.965 (20°C); p.n. 70; r.a.m. 173.04. [DC99]
Yttrium	A silvery metallic element. It is found in almost every lanthanoid mineral, particularly monazite. Yttrium is used in various alloys, in yttrium-aluminum garnets used in the electronics industry and as gemstones, as a catalyst, and in superconductors. A mixture of yttrium and europium oxides is widely used as the red phosphor on television screens. Symbol: Y; m.p. 1522°C; b.p. 3338°C; r.d. 4.469 (20°C); p.n. 39; r.a.m. 88.90585. [DC99]
YY Ori stars	A subgroup of T Tau stars with inverse P Cyg type profiles in the CaII and H line emissions. [JJ95]