An Etymological Dictionary of Astronomy and Astrophysics
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The smallest → near-Earth asteroid (NEA) ever characterized in detail. 2015 TC25 is also one of the brightest NEAs ever discovered, since the object reflects about 60% of the incident sunlight. 2015 TC25 was observed using ground-based optical, → near-infrared and radar techniques during a close → flyby 126,000 km from the Earth in Oct. 2015. The observations suggest that its surface composition is similar to → aubrites, a rare class of high albedo → differentiated meteorites. 2015 TC25 is also a very fast rotator with a → rotation period of 133 seconds. There are clues that 2015 TC25 was chipped off by another impacting rock from its parent, → (44) Nysa, a large and bright → main belt asteroid that measures about 80 km in diameter (Reddy et al., 2016, arXiv:1612.00113).

Provisional name for an → asteroid designation whose orbit is not know.

The first → asteroid found to have an orbital → semi-major axis of less than one → astronomical unit. Discovered by the American woman astronomer Eleanor F. Helin (1932-2009) in 1982, it is the prototype of the → Aten asteroids.

Named after Aten, the Egyptian god of the solar disk.

The → emission line of → neutral hydrogen in → interstellar clouds. It lies in the → radio spectrum at → wavelength of about 21 cm, → frequency 1420 MHz. Its existence was predicted by Hendrik van de Hulst in 1944 and discovered by Harold Ewen and Edward Purcell in 1951.

Twenty-one (21); → twenty + → one; → centimeter; → line.

The → emission line of → neutral hydrogen in interstellar clouds. It lies in the radio spectrum at wavelength of about 21 cm, frequency 1420 MHz. Its existence was predicted by Hendrik van de Hulst in 1944 and discovered by Harold Ewen and Edward Purcell in 1951.

Twenty-one (21); → twenty + → one; → centimeter; → line.

The strongest spectral feature in the interstellar → extinction curve occurring in the → ultraviolet and centered at about 2175 Å. The feature is a strong function of the → metallicity of the gas. It appears slightly weaker in the → LMC extinction curve (metallicity about 50% solar), but essentially absent in the → SMC extinction curve (metallicity about 10% solar). Small → interstellar  → graphite  → dust grains or a mixture of graphite and → polycyclic aromatic hydrocarbons (PAHs) grains have been proposed in → dust models to explain the bump. However, at present the → carrier of the 2175 Å feature is basically unidentified.

→ angstrom; → bump.

A blue star of → apparent visual magnitude 4.96 lying in the → Orion constellation. It has other designations, including: ψ¹ Orionis, BD+01 1005, HR 1789, and HIP 25302. 25 Ori is a → main sequence star of → spectral type B1 V. It lies at a distance of 1,040 → light-years and has a → luminosity of 9,300 MSun, a radius of 5.5 Rsun, and a mass of about 10 Msun. 25 Ori is in fact a → Be star.

A group of nearly 200 low-mass → pre-main-sequence stars, concentrated within ~ 1 of the early → B star  → 25 Orionis, in the component a of the → Orion OB1 Association. The group also harbors the → Herbig Ae/Be star V346 Ori and a dozen other early-type stars. The velocity distribution for the low-mass stars shows a narrow peak at 19.7 km s^-1, offset ~ 10 km s^-1 from the velocity characterizing the younger stars of the Ori OB1b subassociation, and 4 km s^-1 from the velocity of widely spread young stars of the Ori OB1a population. This indicates that the 25 Ori group is a distinct kinematic entity. The low-mass members follow a well-defined band in the → color-magnitude diagram, consistent with an age of ~ 7-10 Myr (Briceno et al., 2007, ApJ 661, 1119).

→ 25 Orionis; → group.

A → main belt → S-type asteroid of size about 51 km. Its → semi-major axis is 2.198 → astronomical units and its orbital period 3.26 yr. It was discovered by C. H. F. Peters on October 8, 1887.

Name of an Iranian female deity, → Venus.

A new → interstellar object of → cometary nature. Orbital and morphological properties of this body show that this is the first certain case of an interstellar comet, and the second-known interstellar minor body identified in the → Solar System (after → 1I/'Oumuamua). 2I/Borisov's motion shows significant systematic deviation from what is expected for a → parabolic orbit and is consistent with an enormous orbital → eccentricity of 3.14 ± 0.14. Images show an extended → coma and a faint, broad → tail. The → color index of 0.63 ± 0.03 is essentially the same as measured for the native Solar System → long-period comets. 2I/Borisov will reach its → perihelion on 7 December 2019, when it will be 2 → astronomical units (AU) from the Sun and also 2 AU from Earth. By January 2020 it will be at its brightest in the southern sky. It will then begin its outbound journey, eventually leaving the Solar System forever (Guzik et al., 2019, Nature Astronomy, 14 October).

Named after the amateur astronomer Gennady Borisov, from MARGO observatory, Crimea, who on 30 August 2019 discovered an object with a comet-like appearance.

Same as → Tarantula Nebula.

30 (→ thirty), the object number in the catalog of Johann Elert Bode (1747-1826) Allgemeine Beschreibung und Nachweisung der Gestirne nebst Verzeichniss, compiled in 1801; → Dorado.

An → Apollo asteroid with unusual properties. Phaethon was discovered on October 11, 1983 using the → Infrared Astronomical Satellite. It is dynamically associated with the → Geminid meteor shower. Whereas most established → meteor shower parents are clearly → cometary in nature, the orbit of Phaethon is that of an → asteroid . Its → semi-major axis, → eccentricity, and → orbital inclination are a = 1.271 AU, e = 0.890, and i = 22.2, respectively, and the → Tisserand parameter with respect to → Jupiter is T_J = 4.5 (while the conventional → comets have T_J < 3). The → perihelion distance is remarkably small, q = 0.14 AU. Phaethon is roughly 5 km in diameter. Not to be confounded with → Phaeton.

Named after the Greek myth of Phaethon, son of the Sun god Helios, due to its close approach to our Sun.

A catalog (Bennett, 1962) based on the original 3C survey (Edge et al., 1960) made at Cambridge at 159 MHz using a complex → interferometer system. This → survey was preceded by the 2C survey made with the same instrument at 81 MHz with a resolution two times poorer. The 2C catalog contained 1936 sources, but owing to the poor resolving power, it became clear at an early stage that many of these sources were not real, and were due to blends of two or more sources in the primary antenna beam. Moreover, except for the strongest sources, the determination of the flux density and angular coordinates was poor. The 3C survey contained only 471 sources and was considerably more reliable. Nevertheless, because of the relatively poor primary resolving power, there were still large errors in the positions and flux densities. In particular, it was frequently uncertain in which → lobe of the → interference pattern a particular source was located, and this introduced large positional uncertainties. In order to reduce these uncertainties an additional survey was made at 178 MHz using a large parabolic cylinder antenna. The narrow E-W beam of this antenna eliminated nearly all of the lobe ambiguities of the original 3C catalog. The data from the two surveys were combined to form the then most reliable radio source catalog - the Revised 3C or 3CR Catalogue. The same parabolic cylinder antenna was later used together with a smaller moveable antenna as an aperture synthesis instrument to make a complete high-resolution survey of the northern sky (the 4C survey), which contains about 2000 sources (NASA/IPAC Extragalactic Database, NED).

3, for the third "version;" C, for "catalog;" R, for "revised;" → catalog.

A blue star with → apparent visual magnitude 4.59 in → Orion's Sword. Also known as → c Orionis, HD 37018, HR 1892, and HIP 26237. More specifically, it is a → main sequence star of → spectral type B1 V, lying in the → H II region→ NGC 1977. 42 Orionis is approximately 900 → light-years away based on parallax. It is the major source for ionizing photons in NGC 1977. 42 Ori is a → mutiple star system. The primary star, Aa, of magnitude 6.3 has a → companion companion Ab at a separation of 0.16'', and a more distant companion B of 7.5 magnitude at 1.6'' separation. An irradiated → circumstellar disk near 42 Ori has been detected by Bally et al. (2012) in the → HST image using Hα filter (F658N). They identified a bent → protostellar jet HH1064 from Parenago 2042 (the Spindle) in NGC 1977 with numerous → bow shock features. They argue that the arc feature in the Hα Spindle is centered on the star and its brightened side of the arc is facing toward 42 Ori, suggesting that it may be a → proplyd (Kim et al., 2016, arXiv::1606.08271).

Number 42 in → Flamsteed designation; Orionis, genitive of → Orion.

An → S-type asteroid about 34.4 × 11.2 × 11.2 km in size, which is the second-largest → near-Earth asteroid after 1036 Ganymed. It belongs to the group of → Amor asteroids. Eros was discovered on 13 August 1898 by Gustav Witt in Berlin and Auguste Charlois at Nice. It was the first asteroid orbited by an Earth probe (in 2000).

Named after the Gk. god of love Eros.

The largest member of a rare class of → asteroids with a high surface → albedo, classified as an → E-type. It has an albedo of about 0.5, and is irregularly shaped, 113 × 67 × 65 km. Nysa has an → aphelion of 2.782 → astronomical units and a → perihelion of 2.064 AU. The → eccentricity is 0.15 with an → inclination of 3.70 degrees. The → orbital period is 1,378 days, or 3.77 years. It rotates in 6.42 h. The apparent magnitude ranges from 8.83 to 12.46. Nysa's orbit makes it the largest E-type asteroid inside Jupiter's resonance orbits. Nysa was discovered by Hermann Goldschmidt on May 27, 1857.

Named after the birthplace of Dionysus in Greek mythology.

The second brightest → globular cluster, after → Omega Centauri. It is seen on the sky near the → Small Magellanic Cloud in the southern constellation → Tucana. Its apparent size is 31', about the size of the full Moon. It is about 13,000 → light-years away and contains several million stars in a volume only about 120 light-years across.

47, the object number in the catalog of Johann Elert Bode (1747-1826) Allgemeine Beschreibung und Nachweisung der Gestirne nebst Verzeichniss, compiled in 1801; → Tucana.

A → trans-Neptunian object located in the → Kuiper belt; provisional designation 2014 MU₆₉ and nicknamed Ultima Thule. Its overall dimensions are about 36 × 18 × 10 km. NASA's → New Horizons spacecraft conducted a close → flyby of the object on January 1, 2019 just 3,500 km above the object's surface. The images it took show that the object has in fact two distinct lobes. The individual lobes have approximate dimensions of 21 × 20 × 9 km and 15 × 14 × 10 km, respectively. At a → heliocentric distance of 44 → astronomical units, 486958 Arrokth is the most distant object ever visited by a spacecraft. It has an → orbital period of 298 years, a small → inclination (~ 2.45°), and a low → eccentricity (0.042).

Arrokoth is a Native American term meaning "sky" in the Powhatan language of the Tidewater region of Virginia and Maryland; → asteroid designation.

A → binary star in the → constellation  → Cygnus lying at a distance of 11.4 → light-years. It consists of a pair of → K-type  → dwarf stars. The more massive component (61 Cygni A) is a K5 type with a mass of 0.70 Msun and the other one (61 Cygni B) a K7 type of 0.63 Msun. The components of this → visual binary orbit each other in a → period of about 659 years. 61 Cygni was the first star whose distance was measured using its → annual parallax. It was done by the German astronomer Friedrich Bessel in 1838 who derived a parallax of 0".314. Recent more precise observations with the → Hipparcos satellite give a value of 0".286. Same as → Bessel's star and → Piazzi's Flying Star.

61 Cygni, according to the → Flamsteed designation; → Cygnus.

A large, bright → main belt asteroid. With a diameter of 136 km, it is the innermost of large → asteroids and the largest member of the → Flora family. It revolves around the Sun with an → orbital period of 3.27 years between a → perihelion of 1.858 → astronomical units and an → aphelion of 2.546 AU. Flora was discovered by J. R. Hind on October 18, 1847.

The name Flora was proposed by John Herschel, from Flora, the Latin goddess of flowers and gardens, wife of Zephyrus (the personification of the West wind), and mother of Spring.

A → Kuiper belt object discovered in 2004. It has a large satellite named Vanth. Orcus is similar to → Quaoar in size and may be classified as a → dwarf planet.

Named for the ancient Roman god of the underworld, identified with the Greek Pluto, or Hades. The name was chosen because it shares a similar size, orbit, and orbital inclination to that of Pluto.