An Etymological Dictionary of Astronomy and Astrophysics
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The reciprocal of the → dispersive power of a substance. Also known as constringence.

→ Abbe sine condition; → number.

A → Besselian day number denoted by C or D.

→ aberration; → -al; → day; → number.

The ratio of the flow velocity to the → Alfvén speed in a medium.

→ Alfvén wave; → number.

The ratio of the flow velocity to the → Alfvén speed in a medium.

→ Alfvén wave; → number.

A number, → real or → complex, that is a → root of a → non-zero polynomial equation whose → coefficients are all → rational. For example, the root x of the polynomial x² - 2x + 1 = 0 is an algebraic number, because the polynomial is non-zero and the coefficients are rational numbers. The imaginary number i is algebraic, because it is the solution to x² + 1 = 0.

→ algebraic; → number.

The total number of → protons and → neutrons in the → nucleus of an → atom (symbol A). For example, Oxygen-16 has a mass number of sixteen, because it has eight protons and eight neutrons.

→ atomic; → mass; → number.

The number of → protons in an → atomic nucleus (symbol Z). Same as → Z-number. The atomic number is written as a subscript to the left of the → chemical element name. For example, the most common isotope of oxygen is shown as ₈¹⁶O, which has 8 → protons and its → mass number (A) is 16.

→ atomic; → number.

→ Avogadro constant.

Named after Amedeo Avogadro (1776-1856), whose law allowed other physicists to calculate Avogadro's number; → constant.

In quantum mechanics, a quantum number that distinguishes the different shapes of the orbitals.

Azimuthal, adj. from → azimuth; → quantum number.

1) The difference between the total number of → baryons and the total number of → antibaryons in a system of → subatomic particles. It is a measure of → baryon asymmetry and is defined by the quantity η = (n_b - n_b^-)/n_γ, called the → baryon-photon ratio, where n_b is the → comoving number density of baryons, n_b^- is the number of antibaryons, and n_γ is that of photons. The value of η for the → cosmic microwave background radiation (CMBR) has been very well determined by the → WMAP satellite to be η = (6.14 ± 0.25) x 10^-10. The baryon number is assumed to be constant. The photons created in stars amount to only a small fraction, less than 1%, of those in the CMBR.
2) A property of an → elementary particle represented by a → quantum number. It is equal to +1 for a baryon and -1 for an antibaryon. → Bosons, → leptons, and → mesons have a baryon number B = 0. → Quarks and → antiquarks have baryon numbers of B = +1/3 and -1/3, respectively. The baryon number is → conserved in all observed types of particle-particle interaction.

→ baryon; → number.

Any of the five quantities denoted by A, B, C, D, and E used in conjunction with → Besselian star constants for the reduction of a star's → mean catalog place to its → apparent place.

→ Besselian; → day; → number.

A → numeral system that has 2 as its base and uses only two digits, 0 and 1. The positional value of each digit in a binary number is twice the place value of the digit of its right side. Each binary digit is known as a bit. The decimal numbers from 0 to 10 are thus in binary 0, 1, 10, 11, 100, 101, 110, 111, 1000, 1001, and 1010. And, for example, the binary number 11101₂ represents the decimal number (1 × 2⁴) + (1 × 2³) + (1 × 2²) + (0 × 2¹) + (1 × 2⁰), or 29. In electronics, binary numbers are the flow of information in the form of zeros and ones used by computers. Computers use it to manipulate and store all of their data including numbers, words, videos, graphics, and music.

→ binary; → number; → system.

An ordinary number such as 0, 1, 2, or 3, as opposed to an → ordinal number such as 1st, 2nd, or 3rd. Cardinal numbers can be → zero or → positive and are used for counting the things that are assumed to be not divisible.

→ cardinal; → number.

Any number of the form u = a + bi, where a and b are → real numbers and i imaginary, i.e. i² = -1.

→ complex; → number.

A whole number which is the product of whole numbers other than itself and 1. The opposite of prime number. → compound number.

→ composite; → number.

A quantity expressed as the sum of two or more quantities of differing units. For example 5 hours and 15 minutes or 4 meters and 20 centimeters. → composite number.

→ compound; → number.

Number theory: An → integer N if there exists a → right triangle with → rational sides so that the area of the triangle is N. For example, the number N = 6, because of the 3-4-5 triangle.

→ congruent; → number.

The conjugate of a → complex number, expressed by ū = a - bi. The complex number and its conjugate have the same real part. Same as → complex conjugate.

→ conjugate; → complex; → number.

1) Crystallography: The number of nearest neighbors of an atom or ion in a → crystal lattice. A large coordination number indicates that the structure is more closely packed.
2) Chemistry: The number of atoms, ions, or molecules surrounding a central atom or ion in a complex.

→ coordination; → number.

Unit of the analog-to-digital conversion system of a CCD apparatus. For example a 16 bit system may use a maximum of 65536 DN. The acronym ADU, for analog-to-digital unit, is also used.

→ data; → number.