An Etymological Dictionary of Astronomy and Astrophysics
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A region of the → Hertzsprung-Russell diagram populated by evolving → low-mass to → intermediate-mass stars. These stars have an electron → degenerate core of carbon and oxygen surrounded by two burning shells of helium and hydrogen. The H and He-burning shells are activated alternately in the deep layers of the star. An extended and tenuous convection envelope, having a radius of 10⁴-10⁵ times the size of the core, lies above these shells. The loosely bound envelope is gradually eroded by the strong → stellar wind, which forms a dusty → circumstellar envelope out to several hundreds of stellar radii. The convective envelope, stellar atmosphere, and circumstellar envelope have a rich and changing chemical composition provided by → nucleosynthesis processes in the burning shells in the deep interior.

→ symptotic; → giant; → branch.

A giant star with spectral type O or B.

→ blue; → giant.

Qul, → giant; âbi, → blue.

An evolved star of spectral type O, B, or A; e.g. → Rigel, → Deneb.

→ blue; → supergiant.

An → evolved star which is more → luminous than normal → giant stars (→ luminosity class III) and between ordinary giants and → supergiants (class I). It is denoted by the symbol II. Examples are → Canopus and → Adhara.

→ bright; → giant.

A highly unstable, → very massive star lying just below the empirical upper luminosity boundary in the → H-R diagram (→ Humphreys-Davidson limit) with spectral types ranging from late A to M. Cool hypergiants very likely represent a very short-lived evolutionary stage, and are distinguished by their high → mass loss rates. Many of them also show photometric and spectroscopic variability, and some have large → infrared excesses and extensive circumstellar ejecta. The evolutionary state of most of these stars is not known but they are all → post-main-sequence stars (Humphreys, 2008, IAUS 250).

→ cool; → hypergiant.

A → giant planet composed mainly of → hydrogen and → helium with → traces of → water, → methane, → ammonia, and other hydrogen compounds. Gas giants have a small rocky or metallic core. The core would be at high temperatures (as high as 20,000 K) and extreme pressures. There are four gas giants in our solar system: → Jupiter, → Saturn, → Uranus, and → Neptune. Another category of gas giants is → ice giants. Ice giants are also composed of small amounts of hydrogen and helium. However, they have high levels of what are called "ices." These ices include methane, water, and ammonia.

→ gas; → giant.

A person or thing of unusually great size, power, importance. In astronomy, e.g. → giant star, → giant branch, → red giant, → asymptotic giant branch (AGB), → blue supergiant, → blue giant, → gas giant, → giant H II region, → giant impact hypothesis, → giant magnetoresistance (GMR), → giant molecular cloud (GMC), → giant planet, → Li-rich giant, → subgiant, → supergiant.

From O.Fr. géant, from V.L. *gagantem, from L. gigas "giant," from Gk. gigas (gen. gigantos), huge and savage monsters, children of Gaia and Uranus, who fought the Olympians but were eventually destroyed by the gods, probably from a pre-Gk. language. The Gk. word was used in Septuagint (the Greek translation of the Jewish Scriptures) to refer to men of great size and strength, hence the expanded use in Western languages.

Qul "an imaginary hideous demon, supposed to devour men and animals," Pers. word probably related to Skt. grábha- "a demon causing diseases, one who seizes," grahila- "possessed by a demon," from grah-, grabh- "to seize, take," graha "seizing, holding, perceiving," Av./O.Pers. grab- "to take, seize;" Mid.Pers. griftan; Mod.Pers. gereftan "to take, seize;" cf. M.L.G. grabben "to grab," from P.Gmc. *grab, E. grab "to take or grasp suddenly;" PIE base *ghrebh- "to seize."
Qulpeykar, from qul, as explained, + peykar "figure, form, body" (from Mid.Pers. pahikar "picture, image;" from O.Pers. patikara- "picture, (sculpted) likeness," from patiy "against" (Av. paiti; Skt. prati; Gk. poti/proti + kara- "doer, maker," from kar- "to do, make, build;" Av. kar-; Skt. kr-; cf. Skt. pratikrti- "an image, likeness, model; counterpart").
Qulâsâ, from qul + suffix of nature, relation -âsâ, → -aceous.
Kalân "great, large, big, bulky."

A conspicuous family of stars in the → Hertzsprung-Russell diagram composed of red, evolved stars with large sizes. → giant star; → red giant.

→ giant; → branch.

An → H II region emitting at least 10⁵⁰ → Lyman continuum photons per second, or about 10 times → Orion nebula. Such an H II region should be powered by at least one O3V star or by at least a dozen → O-type and tens → B-type stars. Our nearest giant H II region is → NGC 3603. Some other Galactic giant H II regions are: → Lagoon Nebula, M17, W31, W51A, and NGC 3576.

→ giant; → H II; → region.

A model for → Moon formation (initially put forward by Hartmann and Davis, 1975, Icarus 24, 504), according to which the → proto-Earth suffered a collision with another → protoplanet near the end of the → accretion process that ejected material into a → circumterrestrial disk, out of which the Moon formed. Also called → canonical model. The giant impact hypothesis is the leading theory for lunar formation. There are, however, some key observations that cannot be explained using this model. First, the Moon is a large fraction of the mass of Earth (~ 1%) and it is difficult to get enough mass into orbit to form such a massive Moon. Second, the Moon has a similar bulk composition to the Earth, but it is missing large amounts of more → volatile elements. The model does not properly explain Moon's distinctive composition. Finally, Earth and the Moon share virtually the same → isotopic ratios. It is therefore expected that the body that hit the Earth, often called → Theia, would have had a different isotopic ratio than the proto-Earth. In the canonical model, most of the mass of the Moon comes from Theia and so the Moon should have a different isotopic fingerprint than Earth, but it does not. The type of impact that formed the Moon in the canonical model is dictated by a very strong constraint, the → angular momentum of the Earth-Moon system. It is assumed that the angular momentum of the Earth-Moon system immediately after the Moon formed was the same as it is today. This assumption limits the velocity of the impact, the mass of the impacting bodies, and the angle at which the two bodies collided. It was found that only a grazing impact with a Mars-mass impactor at near the escape velocity can put enough mass into orbit to potentially form a lunar-mass Moon. This is why the canonical model is such a specific type of impact. However, the angular momentum of the Earth-Moon system could have been reduced over time by competition between the gravitational pull of Earth, the Moon and the Sun. Therefore, the Moon-forming collision could have been much more energetic than the canonical impact.
Simon Lock and Sarah Stewart (2017, J. Geophys. Res. Planets, 122, 950-982) have shown that such high-energy, high-angular momentum impacts can produce a different type of planetary object, → synestias. High-energy impacts vaporize a substantial fraction (~ 10%) of the rock of the impacting bodies and the resulting synestias can be huge, with equatorial radii of more than ten times that of the present-day Earth. Because the impact-produced synestia was so big, the Moon formed inside the vapor of the synestia surrounded by gas at pressures of tens of bars and temperatures of 3000-4000 K. Fragments of molten rock from the impact collided together and formed a lunar seed orbiting within the vapor of the synestia. The surface of the synestia was hot (2300 K) and the body cooled rapidly. The loss of energy led to the condensation of rock droplets at the surface of the synestia, and a torrential rock rain fell towards the center of the synestia. Some of this rain was revaporized in the hot vapor of the synestia, but some encountered the lunar seed, and the Moon grew. As the synestia cooled, more of the vapor condensed and the body contracted rapidly. After ten years or so, the synestia shrank inside the orbit of the Moon and the nearly fully-formed Moon emerged from the vapor of the synestia. The synestia continued to cool and became a planet within a thousand years or so of the Moon emerging from the structure. Without the tight constraint of the angular momentum, impacts that form synestias can put a lot more mass into the outer regions of the synestia than can be put into the disk in the canonical impact. This makes forming a large, lunar-mass Moon much easier. Moreover, because the Moon formed within the synestia, surrounded by hot vapor, it inherited its composition from Earth but only retained the elements that are more difficult to vaporize. The more volatile elements remained in the vapor of the synestia. When the synestia cooled and contracted inside the Moon's orbit, it took all the more volatile elements with it. This model can also help explain the isotopic similarity between Earth and the Moon. The Moon inherited its isotopic fingerprint from the vapor that surrounded it in the outer regions of the synestia. Energetic impacts that form synestias tend to efficiently mix material from the two colliding bodies, and the outer portions of the synestia in which the Moon formed would have had an isotopic composition that was similar to the rest of the synestia. Earth and the Moon therefore share a similar isotopic fingerprint which is made by a mixture of the isotopic compositions of both the bodies that collided.

→ giant; → impact; → hypothesis.

A quantum mechanical phenomenon where the resistance of certain materials drops dramatically upon application of a magnetic field in certain structures composed of alternating layers of magnetic and nonmagnetic metals. The basis of the GMR is the dependence of the electrical resistivity of electrons in a magnetic metal on the direction of the electron spin, either parallel or anti-parallel to the magnetic moment of the layers. The 2007 Nobel Prize in physics was awarded to the French physicist Albert Fert (1938-) and German physicist Peter Grünberg (1939-) for the discovery of GMR.

→ giant; magneto- combining form of → magnet; → resistance.

A massive complex of → interstellar gas and → dust, consisting mostly of → molecular hydrogen, that typically stretches over 150 light-years and contains several hundred thousand → solar masses. Giant molecular clouds are the principal sites of star formation. → molecular cloud.

→ giant; → molecular; → cloud.

A planet much more massive than Earth. The solar system has four giant planets: → Jupiter, → Saturn, → Uranus, and → Neptune.

→ giant; → planet.

A high-luminosity star that has evolved off the → main sequence and lies above the main sequence on the → Hertzsprung-Russell diagram. A member of the → giant branch. → red giant.

→ giant; → planet.

A high luminosity star with absolute visual magnitude around -10, about 10⁶ times as luminous as the Sun. Hypergiant stars are evolved → massive stars belonging to the luminosity class Ia+ or Ia0. Their spectra show very broadened emission and absorption lines resulting from the high luminosity and low surface gravity which favor strong → stellar wind. See also → Humphreys-Davidson limit; → yellow hypergiant.

→ hyper-; → giant.

A member of the lesser mass group of → gas giants. Ice giants contain a higher quantity of materials that form ices at low temperatures, such as → water, → methane, and → ammonia. There are two ice giants in the Solar System, → Uranus and → Neptune.

→ ice; → giant.

A → giant star whose observed → lithium abundance is much higher (A(Li) ~ 2.95) than that predicted by stellar → evolutionary models. Standard evolutionary models predict severe → depletion of surface Li → abundance, which is as low as 1.4 → dex in K giants, a factor of about 80 lower than the maximum value of about 3.3 dex observed in → main sequence stars. Observations confirm model predictions showing much less Li compared to model predictions in most → red giant branch (RGB) stars (Kumar et al., 2018, J. Astrophys. Astr. 39, 25 and references therein).

→ lithium; → rich; → giant.

A variable → supergiant star with typical periods of the order of 10 to 100 days and amplitudes less than a few tenths of a magnitude. PVSGs are thought to be pulsating → g modes, caused by a density inversion, arising from an → opacity bump, most likely from Fe, H, and/or He.

→ periodical; → -ly; → variable; → supergiant.

A star in a short-lived evolutionary stage evolving from the → asymptotic giant branch toward higher → effective temperatures. The majority of low and intermediate mass stars (1 to 8 → solar masses) are believed to pass through this stage on their way to becoming → planetary nebulae.

→ post-; → asymptotic giant branch.

A certain star of spectral type K or later that occupies the upper right portion of the → H-R diagram. Red giants are evolved stars that have exhausted their hydrogen fuel in the core. They may have a → luminosity up to 1000 times greater than → main sequence stars of the same → spectral type. Red giants belong to the → luminosity class III or II (bright giants). They are luminous because of their great size, but have a relatively low surface temperature. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolution. When a main sequence star has converted approximately 10% of its hydrogen to helium, nuclear reactions in the core stop (→ Schönberg-Chandrasekhar limit). The → hydrostatic equilibrium is no longer maintained, and the core contracts while the outer layers expand and cool. This process produces the low surface temperature and large size (from 10 to 100 times that of the Sun) that characterize the red giant. In the core the temperature continues to rise. When it approaches 100,000,000 K helium will begin to fuse into carbon. → helium flash. Prominent bright red giants in the night sky include → Aldebaran and → Arcturus.

→ red; → giant.