An Etymological Dictionary of Astronomy and Astrophysics
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A display of stellar properties using a plot of → effective temperature (or instead → color or → spectral type) along the abscissa versus → luminosity (or → absolute magnitude). The temperature is plotted in the inverse direction, with high temperatures on the left and low temperatures on the right. On the diagram the majority of stars are concentrated in a diagonal strip running from upper left to lower right, i.e. from high temperature-high luminosity → massive stars to low temperature-low luminosity → low-mass stars. This feature is known as the → main sequence. This is the locus of stars burning hydrogen in their cores (→ proton-proton chain). The lower edge of this strip, known as the → zero age main sequence (ZAMS), designates the positions where stars of different mass first begin to burn hydrogen in their cores. Well below the main sequence there is a group of stars that, despite being very hot, are so small that their luminosity is very small as a consequence. These are the class of → white dwarfs. These objects represent old and very evolved stars that have shed their outer layers to reveal a very small but extremely hot inner core. They are no longer generating energy but are merely emitting light as they cool (→ white dwarf cooling track). Stars with high luminosities but relatively low temperatures occupy a wide region above the main sequence. The majority of them have used up all the hydrogen in their cores and have expanded and cooled as a result of internal readjustment. Called → red giants, they are still burning helium in their cores (→ helium burning, → carbon burning). There are also stars with very high luminosities, resulting from their enormous outputs of energy, because they are burning their fuel at a prodigious rate. These are the → supergiants. They can be hot or cool, hence blue or red in color. Same as → H-R diagram.
See also:
→ asymptotic giant branch, → blue horizontal branch star, → extreme horizontal branch star, → field horizontal branch star, → Hayashi track, → horizontal branch, → post-asymptotic giant branch star, → red giant branch, → supra-horizontal branch star, → zero age horizontal branch star, → Humphreys-Davidson limit.

Named after the Danish Ejnar Hertzsprung (1873-1967) and the American Henry Norris Russell (1877-1957). However, the first H-R diagram was published not by Hertzpurung neither Russell, but by a PhD student of Karl Schwarzschild at Göttingen. The student was Hans Rosenberg (1879-1940), who in 1910 published the diagram for stars in the → Pleiades (Astronomische Nachrichten, Vol. 186 (4445), p. 71, 1910). Although Hertzpurung had a very preliminary diagram in 1908, his first proper diagram was published in 1911. Likewise, Russell published his version only in 1915 with the better and more numerous data then available (Nielsen, A.V., 1969, Centaurus 9, 219; Valls-Gabaud, D., 2002, Observed HR diagrams and stellar evolution, ASP Conf. Proceedings, Vol. 274. Edited by Thibault Lejeune and João Fernandes); → diagram.

A diagram showing the relative density of occurrence of stars at various → color-magnitude positions of the → Hertzsprung-Russell diagram for a given → galaxy.

Named after R. Hess who originated it in 1924: "Die Verteilungsfunktion der absoluten Helligkeiten in ihrer Abhängigkeit vom Spektrum". Probleme der Astronomie. Festschrift fur Hugo v. Seeliger. Springer, Berlin. p. 265; → diagram.

A type of graphical representation, used in statistics, in which frequency distributions are illustrated by rectangles.

Histogram, from Gk. histo-, a combining form meaning "tissue," from histos "mast, loom, beam, warp, web," literally "that which causes to stand," from histasthai "to stand," from PIE *sta- "to stand" (cf. Pers. ist-, istâdan "to stand;" O.Pers./Av. sta- "to stand, stand still; set;" Skt. sthâ- "to stand;" L. stare "to stand;" Lith. statau "place;" Goth. standan); → -gram.

Nemudâr, → diagram + sotuni "column-like," from sotun "column," from Mid.Pers. stun, from O.Pers. stênâ "column," Av. stuna-, Skt. sthuna- "column."

A three-dimensional image produced with the technique of → holography.

→ holo-; → -gram.

A plot of the → redshift of galaxies against their distance or against their → apparent magnitude.

→ Hubble; → diagram.

→ Hubble-Lemaitre parameter.

→ Hubble; → parameter.

The rate pf change of the → cosmic scale factor: H(t) = (dR/dt)/R. The Hubble parameter is a time-dependent quantity and therefore is not constant. The → Hubble-Lemaitre constant is the Hubble-Lemaître parameter measured today.

→ Hubble-Lemaitre law; → parameter.

A diagram where the surface → nitrogen  → chemical abundance of stars is plotted against their → projected rotational velocity (v sini).

I. Hunter et al., 2009, A&A, 496, 841; → diagram.

A device, which uses the energy of water flowing by gravity intermittently through a pipe to force a small portion of the water to a height greater than that of the source.

→ hydraulic; → ram.

The graphical record made by a → hygrograph.

→ hygro- + → -gram.

1) A measure of the distance by which a collision fails being frontal.
2) In → gravitational lensing, the distance of closest approach of the light path to the → lensing object.
3) In → rainbows, the displacement of the → incident from an axis that passes through the center of the water droplet.

→ impact; → parameter

→ inertial reference frame.

→ inertial; → frame.

A → reference frame or coordinate system in which there are no accelerations, only zero or uniform motion in a straight line. According to the special theory of relativity, it is impossible to distinguish between such frames by means of any internal measurement.

→ inertial; → reference; → frame.

A hypothetical planet, named Vulcan, that once was believed to exist between the Sun and Mercury.

→ intra-; → Mercury; → planet.

Within the molecule; occurring by a reaction between different parts of the same molecule.

→ intra-; → molecular.

A ratio representing the number of ionizing photons to the number of electrons in a nebular emitting region.

→ ionization; → parameter.

An energy schematic representing the → electronic states of a → molecule and the → transitions between them. The vertical axis shows energies whereas → energy states are grouped horizontally according to their spin → multiplicity. Radiation-less transitions are symbolized by usual arrows, while → radiative transitions are represented by wavy arrows. The vibrational ground states of each electronic state are indicated with thick lines and the higher → vibrational states with thinner lines.

Named after Aleksander Jablonski (1898-1980), a Polish physicist who was an expert in the field of luminescence and atomic optics; → diagram.

A version of the → H-R diagram displaying stellar gravities (→ gravity, log g) against the corresponding → effective temperatures (T_eff).

Named after the group of astrophysicists (W.-R. Hamann, W. Schmutz, U. Wessolowski) working at Kiel University (Germany), who introduced the diagram in 1980s; → diagram.

The basic unit of mass in the → International System of Units (SI) and → MKS versions of the → metric system, equal to 1,000 → grams. The kilogram was until 2019 defined as the mass of the standard (international prototype) kilogram, a platinum-iridium cylinder kept at the International Bureau of Weights and Measures (BIPM), at Sèvre, near Paris, France. Copies of this prototype are kept by the standards agencies of all the major industrial nations. A kilogram is equal to the mass of 1,000 cubic cm of water at 4°C (→ maximum density). According to the new (2019) definition, the kilogram is defined by taking the fixed numerical value of the → Planck constant (h) to be 6.62607015 × 10^-34 when expressed in the unit J.s, which is equal to kg m² s^-1, where the meter and the second are defined in terms of c and Δν Cs.

→ kilo-; → gram.

A metric unit of force which is equal to a mass of one kilogram multiplied by the standard acceleration due to gravity on Earth (9.80665 m sec^-2). Therefore one (1) kilogram-force is equal to 1 kg × 9.80665 m sec^-2 = 9.80665 → newtons.

→ kilogram; → force.