An Etymological Dictionary of Astronomy and Astrophysics
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The antiparticle of a proton, identical in mass and spin but of opposite (negative) charge.

From → anti- + → proton.

Grammar: A word opposite in meaning to another.

From → anti- + (syn)onym, → synonym.

Pâdcem, from opâd-, → anti-, + cem, → meaning.

1) Self-governing; independent; subject to its own laws only; having autonomy.
2) Pertaining to an autonomy, or a self-governing community.
3) Biology: Existing and functioning as an independent organism. Growing naturally or spontaneously, without cultivation (Dictionary.com).

Adjective from → autonomy.

1) Independence or freedom, as of the will or one's actions.
2) The condition of being autonomous; self-government or the right of self-government (Dictionary.com).

→ auto-; → -nomy.

The → baryon number compared with the number of photons in the → Universe. The baryon-photon ratio can be estimated in a simple way. The → energy density associated with → blackbody radiation of → temperature  T is aT⁴, and the mean energy per photon is ~kT. Therefore, the number density of blackbody photons for T = 2.7 K is: n_γ = aT⁴/kT = 3.7 x 10² photons cm^-3, where a = 7.6 x 10^-15 erg cm^-3 K^-4 (→ radiation density constant) and k = 1.38 x 10^-16 erg K^-1 (→ Boltzmann's constant). The number density of baryons can be expressed by ρ_m/m_p, where ρ_m is the mass density of the Universe and m_p is the mass of the → proton (1.66 x 10^-24 g). → CMB measurements show that the baryonic mean density is ρ_m = 4.2 x 10^-31 g cm^-3 (roughly 5% of the → critical density). This leads to the value of ~ 2 x 10^-7 for the number density of baryons. Thus, the baryon/photon ratio is approximately equal to η = n_b/n_γ = 2 x 10^-7/3.7 x 10² ~ 5 x 10^-10. In other words, for each baryon in the Universe there is 10¹⁰ photons. This estimate is in agreement with the precise value of the baryon-photon ratio 6.14 x 10^-10 derived with the → WMAP. Since the photon number and the baryon number are conserved, the baryon-photon ratio stays constant as the Universe expands.

→ baryon; → photon; → ratio.

A fundamental constant, first calculated by Bohr, for the intrinsic → spin magnetic moment of the electron. It is given by: μ_B = eħ/2m_e = 9.27 x 10^-24 joule/tesla = 5.79 x 10^-5 eV/tesla, representing the minimum amount of magnetism which can be caused by the revolution of an electron around an atomic nucleus. It serves as a unit for measuring the magnetic moments of atomic particles.

→ Bohr; magneton, from → magnet + → -on.

A system for counting rotations of the Sun based on the mean → synodic rotation period of the Sun. Initially, Lord Carrington determined the solar rotation rate by watching low-latitude → sunspots. He defined a fixed solar coordinate system that rotates in a sidereal frame exactly once every 25.38 days. This means that the solar rotation period, as viewed from the Earth, is assumed to be constant. However, the synodic rotation rate varies during the year because of the changing speed of the Earth in its orbit and the mean synodic period is about 27.2753 days. Carrington rotation number 1 began on November 9, 1853.

Named for Richard C. Harrington (1826-1875), British astronomer, who initiated the system; → rotation.

The American physicist Arthur Holly Compton (1892-1962), the Nobel Prize in Physics 1927, who made important contributions to the study of X- and cosmic rays.
→ Compton catastrophe, → Compton effect, → Compton equation, → Compton era, → Compton recoil, → Compton scattering, → Compton shift, → Compton suppression, → Compton wavelength, → Comptonization.

In a compact, steady radio-source where the density of relativistic electrons and the density of synchrotron radiation due to these electrons are very large, the radio photons should be transformed into X-ray and gamma-ray photons through inelastic Compton scatterings onto the relativistic electrons. Thus the radio photons should rapidly disappear and only gamma-ray photons should be observed. This phenomenon does not take place if the radio source is in relativistic expansion.

→ Compton; → catastrophe.

Increase in the wavelength of an → X-ray or → gamma ray  → photon when it collides a → free → electron. The photon transfers part of its energy to the electron, the electron recoils, and the photon itself is scattered at a reduced energy.

→ Compton; → effect.

Theoretical equation which gives the change in the photon wavelength due to the → Compton effect.

→ Compton; → equation.

A period in the early evolution of the Universe, before t = 10^-23 sec when the radius of curvature of the Universe was less than the → Compton wavelength of typical particles.

→ Compton; → era.

The change of direction undergone by the electron in the → Compton effect. The scattered photon and the collided electron move in different directions from that of the incident photon.

→ Compton; → recoil.

Scattering of a → photon due to the → Compton effect.

→ Compton; → scattering.

Of the → Compton effect, the amount of increase in the wavelength of an energetic photon upon its collision with an electron.

→ Compton; → shift.

In → gamma ray → spectroscopy, a technique to reduce the contribution of gamma rays generated by → Compton scattering.

→ Compton; → suppression.

The quantum wavelength of a particle with a highly relativistic velocity. The Compton wavelength is given by h/mc, where h is Planck's constant, m is the mass of the particle, and c the light speed. For an electron, the Compton wavelength is about 2.4 × 10^-10 cm, intermediate between the size of an atomic nucleus and an atom.

→ Compton; → wavelength.

The change in the → spectrum of → electromagnetic radiation due to → scattering from → electrons. When → photons and electrons coexist in the same volume of space, their → collisions can → transfer energy from photons to electrons (→ Compton effect) or from electrons to photons (→ inverse Compton effect).

Verbal noun of → Comptonize; → -tion.

The verb describing the → Camptonization process.

→ Compton; → -ize.

Emission undergone → Comptonization.

→ Comptonize; → emission.