O	Spectral type for the hottest blue stars, even hotter than B-type stars. O-type stars are rare and short-lived. [C95]
O Magnitude	The magnitude derived from observations at 11 microns. [H76]
OAO	Orbiting Astronomical Observatory [LLM96]
OB	Spectral type O or B - that is, hot and blue. [C95]
O Star	Stars of spectral type O are very hot blue stars with surface temperatures of about 35,000 K, whose spectra are dominated by the lines of singly ionized helium (see Pickering series). (Most other lines are from at least doubly ionized elements, though H and He I lines are also present.) O stars are useful because they are found in dust clouds and virtually define the spiral arms. Most O stars are very fast rotators. O stars have lifetimes of only 3 to 6 million years. [H76]
OB Association	A loose gathering of O and B stars that typically stretches over hundreds of light-years and contains a few dozen OB stars. [C95]
Oe	O-type stars with emissions in the Balmer lines. [JJ95]
Oef	Early O stars that show double emission lines in He II 4686. [H76]
Of	Peculiar O stars in which emission features at 4634-4641 from N III and 4686 from He II are present. They have a well-developed absorption spectrum, which implies that the excitation mechanism of the emission lines is selective, unlike that of Wolf-Rayet stars. The spectra of Of stars are usually variable, and the intensities of their emission lines vary in an irregular manner. Of stars belong to extreme Population I. All O stars earlier than 05 are Of. [H76]
O(f)	O-type stars in which NIII is present in emission and He II is weakly present in absorption or emission. [JJ95]
O((f))	O-type stars in which NIII is present in emission and He is strong in absorption. [JJ95]
OH	An interstellar molecule (the hydroxyl radical) first detected in 1963 at a wavelength of 18 cm. The four transitions that occur near 18 cm are caused by the splitting of the ground level. Galactic OH sources have been divided into three classes according to whether the OH emission is strongest in the main lines, particularly at 1665 MHz (Class 1), whether the emission and absorption are highly anomalous only in the satellite lines (Class 2) (Class 2a, 1720-line emitters; Class 2b, 1612-line emitters), or whether there is only absorption in all four lines (Class 3). [H76]
Oberon	Outermost satellite of Uranus, discovered by Herschel in 1787. P = 13.46 days (rotational and orbital); R 500 km. [H76]
Objective	(a) The lens or combination of lenses nearest the object in an optical instrument. The nearest lens to the object in a compound objective is often called the object lens. The large converging mirror in a reflecting telescope can also be described as the objective. [DC99] (b) The primary mirror of a reflecting telescope (or the primary lens of a refractor). [H76]
Objective Grating	A coarse grating placed in front of the telescope objective. [H76]
Objective Prism	A small-angle prism placed in front of a telescope objective to transform each star image in a field of stars into an image of its spectrum. [H76]
Oblateness	Ratio of the difference between the equatorial and polar radii to the equatorial radius. Oblateness usually is an indication of how fast the body is rotating. [H76]
Oblique Rotator	A stellar model in which the rotational and magnetic axes are not coincident. Magnetic stars are generally assumed to be oblique rotators of this kind. [H76]
Obliquity	In general the angle between the equatorial and orbital planes of a body or, equivalently, between the rotational and orbital poles. For the Earth the obliquity of the ecliptic is the angle between the planes of the equator and the ecliptic. [S92]
Observable Universe	The extent of the Universe that we can see with the aid of the largest telescopes. Its ultimate boundary is determined by the horizon size. [Silk90]
Observational Cosmology	The application of observational data to the study of the Universe as a whole. [F88]
Observer	(a) Anything in receipt of electromagnetic radiation. [c97] (b) Idealized person or piece of equipment, often hypothetical that measures relevant properties of a physical system. [G99]
Occam's Razor	(a) The notion that the simplest explanation of a problem is the preferred explanation, unless it is known to be wrong. [LB90] (b) Ockham's Razor: Entia non sunt multiplicanda ("Entities are not to be multiplied"). A doctrine formulated by William of Ockham in the fourteenth century. As used by physicists, it means that any hypothesis should be shorn of all unnecessary assumptions; if two hypotheses fit the observations equally well, the one that makes the fewest assumptions should be chosen. [H76]
Occultation	(a) Eclipse of a star by another celestial body. [A84] (b) The cutoff of the light from a celestial body caused by its passage behind another object (see Eclipse ). (Strictly speaking, a Solar "eclipse" is a Solar occultation.) [H76]
Octave	The span over which the frequency doubles; e.g. Middle C is 262 cycles per second; the C one octave above it is 524 cycles per second. The observed electromagnetic spectrum covers a range of 17 decades (about 56 octaves) - from about 106 to about 1023 cycles per second. [H76]
Oersted	Unit of magnetic field strength. 1 Oersted corresponds to 1000 / 4 amperes per meter. [H76]
Olber's Paradox	(a) The puzzle of why the sky is dark at night. If the Universe extends infinitely in space, as it might, then the accumulated light from an infinite number of distant stars should seemingly cause the sky to be bright at all times, whether our sun is visible or not. This paradox, first posed in the eighteenth century, has been resolved by the big bang theory. In a Universe with a beginning, we can receive light only from that part of the Universe close enough so that light has had time to travel from there to here since the big bang (about 10 billion years ago). Thus, even if space extends infinitely far, only a limited region, and a limited number of stars, are visible to us. And the accumulated light from this limited number of stars is not sufficient to spoil the darkness of the night sky. [LB90] (b) A paradox formulated by the German astronomer Heinrich Olbers in 1826: Why is the sky dark at night? The amount of light we receive from a star decreases as the square of the distance from us. On the other hand, if we assume a uniform distribution of stars in space, the number of stars increases as the square of their distance from us, so the two factors should cancel out. In theory, then, the night sky should be a blazing mass of light, and obviously it is not. This self-contradictory statement is Olbers' paradox. In seeking to resolve it, astronomers noted that, besides the assumption of uniformity or homogeneity, Olbers made four other assumptions: (1) space is Euclidean; (2) the laws of physics that apply on Earth apply to the Universe as a whole; (3) the Universe is static (i.e., neither expanding nor contracting); (4) the Universe is spatially and temporally infinite. It is now known that all four of these assumptions are either incorrect or inaccurate. [H76] (c) A paradox formulated by the German astronomer Heinrich Olbers in 1826 that can be traced back to the writings of others, such as de Cheseaux, a century or more earlier. The paradox is: Why is the sky dark at night, if the Universe is infinite? We now know that several of the assumptions made by Olbers (explicitly or implicitly) are incorrect. [Silk90]
Old Inflation	The original (1981) Inflationary Universe model. see Inflationary Universe Model; New Inflation [LB90]
Old Stars	Stars that, according to contemporary stellar evolution theory, have an age comparable to that of the galaxy to which they belong. This is not an observational definition. [JJ95]
Old Thin Disk	The older part of the thin-disk population, ranging in age from about 1 to 10 billion years. The Sun and most other nearby stars belong to the old thin disk. The scale height of the old thin disk is about 1000 light-years. [C95]
Omega	The ratio of the average density of mass in the Universe to the critical mass density, the latter being the density of mass needed to eventually halt the outward expansion of the Universe. In an Open Universe, Omega is always less than 1; in a Closed Universe, it is always greater than 1; in a Flat Universe it is always exactly equal to 1. Unless omega is exactly equal to 1, it changes in time, constantly decreasing in an Open Universe and constantly increasing in a Closed Universe. Omega has been measured to be about 0.1, although such measurements are difficult and uncertain. see Critical Mass Density; Closed Universe; Flat Universe; Open Universe [LB90]
Omega Centauri	A bright globular cluster in the constellation Centaurus. [C95]
Omega Nebula	A bright HII region in Sagittarius. It is a double radio source. (also called Swan Nebula) (M17, NGC 6618)[H76]
One-Loop Process	Contribution to a calculation in perturbation theory in which one virtual pair of strings (or particles in a point-particle theory) is involved. [G99]
One-Standard-Deviation Uncertainty	An estimate of the uncertainty of a measurement which is specified so that the probability of the true value of the measured quantity lying within the uncertainty interval is two out of three. There is one chance in three that the true value lies outside the interval. [G97]
Omicron2 Eridani	A triple star that lies 16 light-years away and has the first white dwarf ever discovered. [C95]
Oort Cloud	(a) Home of most Solar System comets. [F88] (b) HI regions extending to more than 100,000 AU from the Sun, barely gravitationally bound, postulated as the birthplace of comets. [H76]
Oort's Constants	Parameters A and B that, to first order, characterize the rotation of our Galaxy in the neighborhood of the Sun. IAU Standard values (1985): A = +14.4 [1.2] km s-1 kpc-1; B = -12.0 [2.8] km s-1 pc-1. [BFM2002]
Oosterhoff Groups	Two groups of globular clusters which differ in the period of transition between Bailey type ab and type c variables, the ratio of type c to type ab stars, in the metallicity of RR Lyrae stars, and in the mean period of the ab variables. On the whole group I clusters have slightly weak metal lines whereas group II clusters have very weak metal lines. [H76]
Opacity	(a) A measure of the resistance of a medium to the transmission of visible light or other forms of radiation. A material that is opaque is said to have a high opacity; one that is transparent has low opacity. [LB90] (b) A measure of the ability of a gas to absorb radiation. Since the opacity at a given temperature depends on the number of particles per unit volume and since heavier elements contain more electrons than lighter elements, the opacity of a star will increase with increasing proportions of heavy elements. In stellar interiors, the heavier elements of the carbon group and the metals primarily determine the opacity. [H76]
Open Cluster	(a) A small, loose cluster of stars that typically contains several hundred members. The best examples are the Hyades and the Pleiades, both in the constellation Taurus. Open clusters line the Galactic plane, in contrast with globular clusters, which are members of the Galaxy's halo or thick disk. [C95] (b) A comparatively loose grouping (mass range 102-103 M) of Population I stars, strongly concentrated in the spiral arms or the disk of the Galaxy (in fact, open clusters give a good indication of where the spiral arms are). Unlike associations, open clusters are dynamically stable. Depending on their age, stars in open clusters "peel off" from the main sequence at different points (the higher the turnoff point, the younger the cluster). (Sometimes called Galactic Clusters ; NGC 188 is the oldest known open cluster in our galaxy.) [H76]
Open Inflationary Universe Theory	A version of the inflationary universe theory, proposed by Bharat Ratra and Jim Peebles in 1995, which produces an open Universe. The usual inflationary prediction of a flat universe is avoided by proposing an energy diagram for the inflation-driving field with a hill of just the right length, so inflation ends before the universe is driven to flatness. [G97]
Open Space	A space of infinite volume and without any boundary (in the cosmological context). [Silk90]
Open String	A type of string with two free ends. [G99]
Open System	a system communicating with the environment by the exchange of energy and matter.[D89]
Open Universe	(a) Any model of the Universe which does not contain enough matter to halt its expansion. [c97] (b) A homogeneous, isotropic Universe is said to be temporally open if gravity is not strong enough to eventually reverse the expansion, so the universe goes on expanding forever. It is said to be spatially open if it curves the opposite way from a closed universe, so that triangles would contain less than 180°, the circumference of a circle would be more than times the diameter, and the volume would be infinite. If Einstein's Cosmological Constant is zero, as is frequently assumed, then a universe which is temporally open is also spatially open, and vice versa. [G97] (c) Cosmological model in which the universe continues to expand forever; its space-time geometry is hyperbolic, or "open". [F88]
Ophiuchi	A reddened O9.5 V star 170 pc distant (a runaway star from the Sco-Cen association) with a high rotational velocity (396 km s-1). It is well known for its strong interstellar absorption lines in the visible part of the spectrum. [H76]
Opposition	(a) Occurs when the Earth comes directly between that planet and the Sun; it can thus only happen in relation to the superior planets and the asteroids. [A84] (b) A configuration of the Sun, Earth and a planet in which the apparent geocentric longitude of the planet differs by 180 degrees from the apparent geocentric longitude of the Sun. [S92]
Oppenheimer-Volkoff Limit	The limiting mass for a neutron star as the density approaches infinity. Beyond this mass all configurations are unstable. [H76]
Optical Chaos	in many nonlinear optical systems the output response varies in an unpredictable and uncontrollable fashion despite being governed by deterministic laws. Such optical chaos shows a rich and unexpected variety of phenomena which are only recently being discovered.[D89]
Optical Depth	A measure of the integrated opacity along a path through a layer of material, measured by the amount of absorption of a beam of incident light. The intensity ratio I / I0 = e-, where the optical depth = N l (N = column density, = cross section, and l is the path length). Optical depth 1 corresponds to the "visible" surface and occurs when the intensity is reduced by a factor e. [H76]
Optical Fibres	Glass and transparent plastics can be made into very thin wires or fibers. Typical dimensions are 10-50 µm. If a ray enters one end of a fiber at the appropriate angle, it will undergo total internal reflection and travel down the fiber without much loss through the sides. [McL97]
Optical Matching	The use of lenses or other optical devices to match the size of the image of the seeing disk, as it appears in the focal plane of the telescope, to the physical size of the CCD pixels. If the telescope yields 10 arcseconds per millimeter and the seeing is 1 arcsecond then the image is 0.1 mm in size. But a typical CCD pixel is 0.022 mm, five times smaller. [McL97]
Optical Pair	A pair of stars that appear close together on the sky as a result of perspective only, and that have no physical relation. [H76]
Optical Path Difference (OPD)	The difference in path length between the actual wavefront in an optical system and the equivalent spherical wavefront. [McL97]
Optical Soliton	a soliton is a wave pulse which propagates without changing shape or dispersion. It maintains its shape by a balancing between linear dispersion and nonlinear compression and occurs in many areas of wave physics where nonlinearities are important. Optical solitons are generated in nonlinear media excited by strong laser pulses.[D89]
Optical Window	A gap in Earth's atmospheric absorption spectrum through which visible light can pass down to the surface. The optical window includes the spectral region between the O3 cut-off at 2950 Å and the A band of O2 at 7600 Å. [H76]
Optics	The science of light. [F88]
Orbifold	A particular space used as a candidate for the compactified space of superstring theory. These six-dimensional orbifolds could be thought of as generalizations of a six-dimensional torus, but containing twenty-seven singular points. [P88]
Orbit	The path in space followed by a celestial body. [S92]
Orbital Elements	Seven quantities needed to establish the orbit of a celestial body (see elements of an orbit). [H76]
Orbital Velocity	Velocity required by a body to achieve a circular orbit around its primary: Vorb = (GM / r)1/2. [H76]
Orbiting Collision	A "collision" in which an ion and an atom approach each other very closely and spend a long time (several orbits of the atomic electrons) in close proximity. [H76]
Order	An integer (m) associated with a given interference fringe or diffraction pattern. In interference a bright fringe occurs for a path difference m; a dark fringe is produced if the path difference is (m + 1/2). A bright fringe is first order if it arises through a path difference of one wavelength (m = 1). Similarly, second order corresponds to m = 2, etc. [DC99]
Order of Magnitude	A factor of ten. Two orders of magnitude indicate a factor of 100, etc. (Not to be confused with astronomical `magnitudes'. [G97]
Order-of-Magnitude Estimate	An approximate estimate of the magnitude of something, accurate to within a range of 10 times too big to 10 times too small. For example, given that the population of the United States is 250 million, any estimate of the population lying between 25 million and 2,500 million would be an acceptable order-of-magnitude estimate. [LB90]
Order Parameter	a variable such as magnetisation used to describe the degree of order in a phase below its transition temperature. In a continuous phase transition the order parameter goes continuously to zero as the critical temperature is approached from below.[D89]
Orgueil Meteorite	A Type I carbonaceous chondritic meteorite that fell in France in 1864 and that has recently been found to contain amino acids. [H76]
Orion A	A radio continuum feature (an HII region) centered on the Trapezium, and excited by 1 Ori C. The Orion A molecular cloud, which lies beyond it, is a rich source of molecules CO, OH, HCN, and probably NO, HCO, and H2CO have been observed. [H76]
Orion Arm	(a) The spiral arm containing the Sun. It lies between the Sagittarius arm and the Perseus arm. [C95] (b) The spiral arm of the Milky Way on a spur of which the Sun is located (see Orion spur). It is about 600 pc across, and lies about 10.4 kpc from the galactic center, between the Sagittarius arm and the Perseus arm. The total density of interstellar gas in the Orion arm is about 1.5 atoms cm-3 (density of H I about 0.6 atoms cm-3). (also called the Local Arm)[H76]
Orion B	A radio continuum source (NGC 2024).
Orion Molecular Cloud 1	(OMC-1) -- Centered approximately 1' north-west of the Trapezium, it contains the Becklin-Neugebauer and Kleinmann-Low infrared sources. [H76]
Orion Molecular Cloud 2	(OMC-2) -- An infrared and molecular emission complex about 12' northeast of the Trapezium, centered on a cluster of infrared sources. [H76]
Orion Nebula	(a) A large cloud of gas and dust giving birth to young stars in the constellation Orion and visible to the naked eye. It is an HII region 1500 light-years away. [C95] (b) An HII region about 500 pc distant, barely visible to the naked eye in the center of Orion's sword. It is undoubtedly a region where stars are being born; young O stars and many T Tauri variables are associated with it, and its members are extreme Population I. Probably no more than 20,000 years old. It is also an X-ray source (3U 0527-05 and M42, NGC 1976). [H76]
Orion Spur	That part of the local spiral arm in which the Sun is embedded. (The Sun is on an inner edge of the Orion spur.) [H76]
Orionis	Betelgeuse
Orionis	Rigel
BM Orionis	A peculiar eclipsing binary (B2-B3) in the Trapezium, with a flat-bottomed light curve suggesting a total eclipse. The spectrum of the secondary has never been seen. [H76]
FU Orionis	A newly formed star, probably a pre-main-sequence star (cF5-G3 Ia) presently near the top of its Hayashi track. In 1936 it suddenly appeared in the middle of a dark cloud, and rose by 6 magnitudes in the photographic band. Its lithium abundance is 80 times that of the Sun. It has developed a reflection nebula. [H76]
1 Orionis	see Trapezium
2 Orionis	A 21.03-day O9.5 Vp spectroscopic binary tentatively identified with 2U 0525-06. [H76]
YY Orionis	An extremely young star (younger than T Tauri) in the Orion Nebula. YY Orionis stars are very young, late-type, low-mass stars in the gravitationally contracting stage in which the star is still accreting matter from the protostellar cloud. [H76]
Ortho-Hydrogen	Molecular hydrogen in which the two protons of the diatomic molecule have the same direction of spin. It is a higher energy state than the para form. Terrestrial H2 is 75% ortho-hydrogen, 25% parahydrogen. [H76]
Orthonormal Tetrad	A set of four mutually orthogonal unit vectors at a point in spacetime, one timelike and three spacelike, which give the directions of the four axes of a locally Minkowskian coordinate system. [H76]
Orthorhombic Crystal	A crystal in which the atoms are arranged in a rectangular solid, for which each of the three principal lengths are different. Such a crystal provides a simple example of spontaneous symmetry breaking, since the rotational invariance of the underlying physical laws is broken by the randomly chosen orientation of the crystal. Inside such a crystal there are three distinct speeds of light, depending on which of the three axes the light is following. [G97]
Ortho-Spectrum	Spectrum of triplet (l = l). [H76]
Oscillating Universe	Cosmological model in which the Universe is "closed" and its expansion is destined to stop, to be succeeded by collapse and "then" (if ordinary temporal terms may be said to apply) by a rebound into a new expansion phase. [F88]
Oscillator Strength (f-value)	A measure of the probability that a transition represented by an electronic oscillator will occur. It is independent of the physical conditions under which the atom is radiating. [H76]
Osculating Elements	A set of parameters (see elements, orbital) that specifies the instantaneous position and velocity of a celestial body in its perturbed orbit. Osculating Elements describe the unperturbed (two-body) orbit that the body would follow if perturbations were to cease instantaneously. [S92]
Osculating Orbit	The path that an orbital body (e.g., a planet) would follow if it were subject only to the inverse-square attraction of the Sun or other central body. In practiced secondary bodies, such as Jupiter, produce perturbations. [H76]
Osmium	A transition metal that is found associated with platinum. Osmium is the most dense of all metals. The metal is used in catalysts and in alloys for pen nibs, pivots, and electrical contacts. Symbol: Os; m.p. 3054°C; b.p. 5027°C; r.d. 22.59 (20°C); p.n. 76; r.a.m. 190.23. [DC99]
Out-Gassing	(a) The absorbed gases released from the interior walls and components of a vacuum chamber which has already been "roughed-out". [McL97] (b) Ejection of the gases locked in the interior of a planet so that they become part of the planet's atmosphere. [H76]
	Additional clock pulses in both the horizontal and vertical directions in excess of the actual number of real pixels in order to sample the electronic offsets (bias level) in the system. [McL97]
Overshoot	A condition that obtains when the momentum of a particle carries it past its equilibrium point. [H76]
Overstability	A form of instability which, when it sets in, sets in as oscillations of increasing amplitude. [H76]
Overtone	See harmonic overtone. [H76]
OVLA	Optical Very Large Array. [LLM96]
OVV	Optically Violent Variables.
Owl Nebula	A planetary nebula (M97, NGC 3587) in the constellation of Ursa Major, approximately 600 pc distant. [H76]
Oxygen	The most abundant metal in the universe, and the third most abundant element overall, after hydrogen and helium. Oxygen has atomic number eight and is produced by massive stars-those born with over eight Solar masses-which eject the element into the Galaxy when they explode. [C95]
Oxygen Burning	The stage when a star fuses Oxygen into Silicon and Sulfur. It occurs only in stars born with over eight Solar masses. [C95]
Oxygen-Rich Giants	A collective designation for giants showing metal oxide molecules - thus M, MS and S stars. [JJ95]
Ozone Layer	A layer in the lower part of Earth's stratosphere (about 20-60 km above sea level) where the greatest concentration of ozone (03) appears. This is the layer responsible for the absorption of ultraviolet radiation. [H76]