An Etymological Dictionary of Astronomy and Astrophysics
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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.

An → ultraviolet photon with an energy between 11.2 and 13.6 eV, corresponding to the energy range in which the Lyman and Werner absorption bands of → molecular hydrogen (H₂) are found (→ Lyman band, → Werner band). The first generation of stars produces a background of Lyman-Werner radiation which can → photodissociate molecular hydrogen, the key → cooling agent in metal free gas below 10⁴ K. In doing so, the Lyman-Werner radiation field delays the collapse of gaseous clouds, and thus star formation. After more massive → dark matter clouds are assembled, atomic line cooling becomes effective and H₂ can begin to shield itself from Lyman-Werner radiation.

→ Lyman; → Werner band; → photon.

The → quantum of the → electromagnetic field, which mediates the interaction between charged particles. It is the mass-less → boson with zero → electric charge, which propagates with the → speed of light in vacuum. The energy of a photon is connected to its → frequency ν, through the formula E = hν, where h is → Planck's constant.

From phot-, variant of → photo- before a vowel + → -on a suffix used in the names of subatomic particles (gluon; meson; neutron), quanta (photon, graviton), and other minimal entities or components. The term photon was coined by Gilbert N. Lewis in 1926 in a letter to the editor of Nature magazine (Vol. 118, Part 2, December 18, page 874).

The time required for a photon created in the Sun's core to attain the → photosphere and leave the Sun. If the photons were free to escape, they would take a time of only R/c (a couple of seconds) to reach the surface, where R is the Solar radius and c the speed of light. The solar material is, however, very opaque, so that photons travel only a short distance before interacting with other particles. Therefore, photons undergo a very large number of → random walks before arriving at the surface by chance. The typical time is approximately 5 x 10⁴ years for a constant density Sun.

→ photon; → escape; → time.

→ Electromagnetic radiation in equilibrium in a → black body cavity. Photons can be treated as the simplest → ideal gas because all the particles move at the same velocity, the → speed of light. There are, nevertheless, two main differences. 1) Photons are → bosons and → Bose-Einstein statistics must be used. However, photons do not interact with each others so that no approximation is made by neglecting inter-particle forces. 2) Some photons scatter off the walls, with some being absorbed and new ones being emitted continually; so that no constraint can be placed on their number.

→ photon; → gas.

An effect occurring in the outer zones of → H II regions where the number of high-energy ultraviolet photons with energies well above the → ionization potential of hydrogen increases with respect to the number of → Lyman continuum photons. The effect is due to stronger absorption of weaker photons.

→ hard; → photon.

An intrinsic noise caused by the quantum nature of light. Same as → quantum noise.

→ photon; → noise.

A surface where if a photon is emitted from one of its points the photon follows a closed orbit and returns periodically to its departure point. Such a surface exists only near sufficiently → compact objects where the → curvature of → space-time is very important. In other words, a body can take a stable orbit around a → black hole provided that it moves with the → speed of light. However, only photons can have such a velocity; hence the term "photon sphere." For a non-rotating → Schwarzschild black hole, the photon sphere has a radius of R = 3GM/c² = 3 R_S/2, where G is the → gravitational constant, M is the mass, c is the → speed of light, and R_S is the → Schwarzschild radius. For a rotating, → Kerr black hole, the situation is much more complex due to the → Lense-Thirring effect. In that case circular paths exist for radii whose values depend on the rotation direction. More specifically, in the equatorial plane there are two possible circular light paths: a smaller one in the direction of the rotation, and a larger one in the opposite direction.

→ photon; → sphere.

The maximum → mass loss rate of a star when the → wind luminosity equals the total available → stellar luminosity. The mechanical luminosity of the wind at infinity is given by: L_wind = Mdot (v_∞²/2 + GM/R) = Mdot (v_∞²/2 + v_esc²/2). For L_wind = L_*, the mass loss rate is Mdot_max = 2L_*/(v_∞² + v_esc²). Following Owoki & Gayly (1997), Mdot_tir is the maximum mass loss rate when the wind just escapes the gravitational potential, with v_∞ tending toward zero. Mdot_tir is much larger than typical mass loss rates from → line-driven winds, where the driving lines become saturated with increasing density limiting the wind mass loss rates to about 10^-4 Msun yr^-1 in even the most luminous stars.

→ photon; tiring, from tire "to weary; become weary," → tired; → limit.

The plasma filling space before the → recombination epoch that mainly consisted of → cosmic microwave background radiation photons, electrons, protons, and → light elements.

→ photon; → baryon; → plasma.

The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems.

→ photon + → -ics.

The simultaneous emission of two photons whose sum of energies is equal to that of a single electron transition. The energy of each individual photon of the pair is not fixed, so that the spectrum of two-photon emission is continuous from the wavelength of that transition to infinity. In practice, there is a peak in wavelength distribution of the emitted photons. Two-photon emission is studied atomic physics with application in astrophysics, as it contributes to the continuum radiation from → planetary nebulae. It was recently observed in condensed matter and specifically in → semiconductors.

→ two; → photon; → emission.