An Etymological Dictionary of Astronomy and Astrophysics
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Chemical element; symbol He; atomic number 2; atomic weight 4.0026; melting point below -272°C at 26 atmospheres pressure; boiling point -268.934°C at 1 atmosphere pressure.

Helium, from Gk. helios "sun;" cognate with Persian hur "sun", variant xor; Mid.Pers. xvar "sun;" Av. hû-, hvar- "sun;" Skt. surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun; PIE *sawel- "sun." The element was discovered by spectroscopy during a solar eclipse in the Sun's chromosphere in 1868 by the French astronomer Pierre-Jules-Cesar Janssen (1824-1907).

The relative amount of helium with respect to another → chemical species, usually → hydrogen, in an astronomical object.

→ helium; → abundance.

The stage in the evolution of a star, after the exhaustion of hydrogen, when the star produces its energy by the fusion of helium into carbon and oxygen.

→ helium; → burning.

The sudden onset of → helium burning in the core of an → intermediate-mass star that has exhausted its hydrogen and has become a → red giant. With a → degenerate core, the temperature increases but the pressure does not. Therefore, the core cannot expand and cool, so the temperature continues to rise. When it approaches 100,000,000 K, helium will begin to fuse into carbon in the → triple alpha process. The helium flash ends the giant star's ascent of the → red giant branch. However, the violent ignition of helium in the core does not increase the star's luminosity. On the contrary, the energy released in the helium flash expands and cools the core and ultimately results in a reduction in the energy output. On the → H-R diagram the star moves down from red giant branch to the → horizontal branch, a stable state with steady helium burning in the core.

→ helium; → flash.

1) The normal component of → liquid helium (⁴He) existing between the superfluid transition point (→ lambda point about 2.17 K) at 1 atmosphere of pressure and its boiling point of 4.2 K.
2) In stellar and interstellar plasma spectroscopy, neutral helium.

→ helium.

1) A → superfluid, colder component of → liquid helium (⁴He), occurring when → helium I is cooled below the → lambda point.
2) 2) In stellar and interstellar plasma spectroscopy, ionized helium.

→ helium.

A stage in the evolution of an → asymptotic giant branch star, when all the helium in the core is fused into carbon and oxygen. No more fusion takes place in the core, and as a result the core contracts. The core contraction generates a sufficient temperature for fusing the surrounding layers of helium. Since helium shell burning is unstable, it causes → helium shell flashes.

→ helium; → shell; → burning.

A violent outburst of energy that occurs periodically in an → asymptotic giant branch star. It occurs when helium is being burnt in a thin shell surrounding the inner dense core of carbon and oxygen. → Helium shell burning is unstable, producing energy mainly in short intense flashes. The shell flash causes considerable expansion of the star followed by collapse, thus setting up deep convection. As a consequence, the → convective zone in the outer part of the star goes deeper and may → dredge-up carbon to the surface. See also → late thermal pulse; → very late thermal pulse; → AGB final thermal pulse.

→ helium; → shell; → flash.

An → evolved star which has lost most or all of its hydrogen-rich envelope, leaving just a core of helium.

→ helium; → star.

A wavelength calibration of astronomical spectra using a helium-argon light source.

→ helium; → argon; → calibration.

A comparison light source containing the known spectral lines of helium and Argon.

→ helium; → argon; → lamp.

A → helium flash event that occurs during the → post-AGB phase. Some of the central stars of planetary nebulae (→ CSPN) experience a final → thermal pulse after having achieved a → white dwarf configuration and begun their descent along a → white dwarf cooling track of nearly constant radius. During such a pulse, most of the hydrogen remaining in the star at pulse onset is incorporated into the helium-burning convective shell and completely burned. Following the pulse, the star swells briefly to → red giant dimensions (Iben et al. 1983; ApJ 264, 605).

→ late; → helium; → flash.

The state of helium (⁴He) below its boiling point of 4.2 K. Its normal form is called → helium I, but converts into superfluid → helium II below 2.17 K (→ lambda point). Liquid helium is colorless and transparent so that it is impossible to see the surface of the liquid with the naked eye. Helium was first liquefied in 1911 by the Dutch physicist Heike Kamerlingh Onnes (1853-1936), Physics Nobel Prize 1913.

→ liquid; → helium.

The helium element created in the → early Universe, around 3 minutes after the → Big Bang, when the temperature dropped to 10⁹ degrees; in contrast to the helium being synthesized in stars. Based on observations of helium → emission lines in → H II regions of metal-poor dwarf galaxies (→ metal-deficient galaxy), the primordial ⁴He → chemical abundance (by mass) is estimated to be Y_P = 0.24672 ± 0.00017. Moreover, using observations of a near-pristine → intergalactic cloud, a value of 0.250 ^+0.033_-0.025 has been reached (Cooke & Fumagalli, 2018, Nature Astronomy, 2, 657). The theoretical He abundance predicted by → Big Bang nucleosynthesis is 0.24709 ± 0.00017 (Pitrou et al., 2018, arXiv:1801.08023).

→ primordial; → helium.