An Etymological Dictionary of Astronomy and Astrophysics
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An → OB association consisting of several dozen → hot stars of → spectral types O and B (→ O star, → B star). The Orion OB1 association consists of several subgroups, first divided by Blaauw (1964) into four subgroups. The subgroups differ in age and gas and dust content:
Orin OB1a, which contains the stars to the northwest of → Orion's Belt stars. Within this group lies another subgroup, known as the → 25 Orionis group;
Orion OB1b, containing the group of stars located in and around the Belt;
Orion OB1c, including the stars around → Orion's Sword; and
Orion OB1d, which contains the stars in and close to the → Orion Nebula (including the → Trapezium cluster).

→ Orion; → OB association.

Same as → Orion Arm.

→ Orion; → → spur.

Three prominent stars in the central regions of the constellation → Orion that align to form the "belt" of the mythological Hunter. They are → Alnitak (ζ Ori), → Alnilam (ε Ori), and → Mintaka (δ Ori). The easternmost star Alnitak is separated from the middle one, Alnilam, by 1°.36, and the westernmost Mintaka has an angular distance of 1°.23 from Alnilam. Their distance is between 800 and 1,300 → light-years from Earth. They probably formed inside the same → molecular cloud less than 10 million years ago.

→ Orion; → belt.

An astronomical → asterism in the constellation → Orion forming an almost vertical line beneath → Orion's Belt. From north to south, the most prominent objects in the Sword are the cluster NGC 1981, the star → 42 Orionis, the famous → Orion Nebula, and the Sword's brightest star → Iota Orionis (→ Hatsya). None of these objects is particularly bright in itself, but their proximity to one another and the nebulosity across much of this region makes the Sword stand out clearly in the night sky.

→ Orion; → sword.

Same as → Orion Arm.

→ Orion; → Cygnus; → arm.

A meteor shower that appears to emanate from the constellation → Orion. It peaks between October 14 and 20 with about 20 meteors per hour.

→ Orion + → -ides.

In elementary geometry, pertaining to or involving right angles or perpendiculars.
Of a system of real functions, defined so that the integral of the product of any two different functions is zero.
Of a linear transformation, defined so that the length of a vector under the transformation equals the length of the original vector.
Of a square matrix, defined so that its product with its transpose results in the identity matrix.

→ ortho- + gonia "angle," related to gony "knee;" L. genu "knee;" Mod.Pers. zânu "knee;" Av. žnav-, žnu- "knee;" Skt. janu-; PIE base *g(e)neu-, see below, + → -al

Ardâkonj, from ardâ-, → ortho-, + konj "angle, corner, confined place" (variants xong "corner, angle," Tabari kânj, Kurd. kunj, Hamadâni kom), maybe from the PIE base *g(e)neu-, as above, and related to Mod.Pers. zânu "knee" (Av. žnu-), Skt. kona- "angle, corner," Gk. gony, gonia, L. cuneus "a wedge," Albanian (Gheg dialect) kân "angle, corner," Albanian (Toks) kënd "angle, corner."

A set of functions, any two of which, by analogy to orthogonal vectors, vanish if their product is summed by integration over a specified interval.

→ orthogonal; → function.

Perpendicular lines.

→ orthogonal; → line.

Math.: An → isogonal trajectory where the family of curves are cut at right angles.

→ orthogonal; → trajectory.

Two non-zero vectors which are perpendicular, i.e. their → scalar product is zero.

→ orthogonal; → vector.

1) The property of → orthogonal functions.
2) The following conditions satisfied by the → Fourier series:
∫ cos (mx) sin (nx) dx = 0 (summed from -π to +π) for all m, n
∫ cos (mx) cos (nx) dx = 0 (summed from -π to +π) for all m ≠ n, = 2π for m = n = 0, = π for (m = n) > 0
∫ sin (mx) sin (nx) dx = 0 (summed from -π to +π) for m ≠ n, = π for (m = n) > 0.

→ orthogonal; → -ity.

Two non-zero vectors that are → orthogonal and have magnitude 1.

→ orthogonal; → vector.

The state of any quantity when the value of that quantity is continually changing so that it passes through maximum and minimum values.

Verbal noun of → oscillate.

Same as → pulsation mode.

→ oscillation; → mode.

The contact between two osculating curves or the like.

Verbal noun of → osculate

An approximate empirical criterion for the stability of a → galactic disk against its collapse to form a bar. The disk is stable if the following relation holds: T/|W| < 0.14, where T is the rotational → kinetic energy and |W| is the absolute value of the gravitational → potential energy. While the → Toomre criterion applies only to small linear perturbations, the Ostriker-Peebles criterion describes global modes.

Ostriker & Peebles, 1973, ApJ 186, 467; → criterion.

A → Lindblad resonance expressed by: Ω_p = Ω + κ/m.

→ outer; → Lindblad resonance.

A note of lesser intensity and higher frequency than the fundamental note, and superimposed upon the latter to give a note of characteristic quality. Overtones whose frequencies are an integral multiple of the fundamental are said to form a harmonic series. The fundamental with a frequency f₁ is the first harmonic. The frequency 2f₁ is the first overtone and so on.

→ over-; → tone.

The combination of → oxygen with a substance, or the removal of → hydrogen from it.

Verbal noun of → oxidize.