An Etymological Dictionary of Astronomy and Astrophysics
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→ North Pole Star; → Polaris.

→ north; → star.

A luminous, hot, blue star whose spectrum is dominated by the lines of hydrogen, atomic helium, and ionized helium; also known as O-type star. This is the earliest → spectral type and the only → main sequence star in which ionized helium is present. The → effective temperatures of these stars range from about 30,000 K to 50,000 K, their luminosities from 50,000 to 1,000,000 times that of → solar luminosity, and their masses from about 20 to 100 → solar masses. The hottest O-type stars display high ionization emission features such as N III and He II, → Of star. They are divided into subtypes O2, the hottest, to O9.7, the coldest. O-type stars are relatively rare, for each star of 100 solar masses there are 10⁶ stars of solar mass. They are relatively short-lived since they spend only a few million years on the main sequence. The brightest O-type star in the sky visible with naked eye is → Alnitak. For prominent Galactic O stars see → HD 93129.

O, letter of alphabet used in the Harvard spectral classification; → star.

Same as → O star.

O, letter of alphabet used in the → Harvard classification; → type; → star.

A collective designation for massive O and B stars.

→ O star; → B star.

A → carbon (C)-rich → O-type or → B-type star.

→ O star; → B star; → carbon; → star.

A → nitrogen (N)-rich → O-type or → B-type star.

→ O star; → B star; → nitrogen; → star.

An → O star whose spectrum displays strong N III 4634-4640-4642 emission and strong He II 4686 emission. The N III lines are always much stronger than C III 4647-4650-4651 when the latter are present. Historically, Of stars were considered to belong to the peculiar category, hence the f notation (see below). In his thesis work, Walborn (1971, ApJS 23, 257) removed them from that category and established them as the normal O-type → supergiants. He also used the notation ((f)), (f), and f to describe the progression from strong He II 4686 absorption, through weakened/absent, to emission, respectively, correlated with increasing N III emission strength, subsequently showing that it is a luminosity sequence -- the first such for stars earlier than O9.

The reason for the Of designation is that the letters Oa-Oe were used in the original Harvard classification to denote various types of → Wolf-Rayet and OB spectra. Therefore Of was the next available when Plaskett and Pearce (1931, Pub. Dominion Ap. Obs 5, 99) wished to distinguish O-type spectra with selective emission in N III 4634-4640-4642 and He II 4686 ("selective" because other lines from the same ions appear in absorption); → star.

A → massive star spectrum whose principal defining characteristics is the presence of C III 4647, 4650, 4651 emission lines with strength comparable to that of N III 4634, 4640, 4642. This category was introduced by Walborn (1972) to describe two well-known peculiar stars, HD 108 and HD 148937. → Of star

→ Of star; the question mark was intended to denote doubt that these stars are normal Of supergiants; p for "peculiar."

A small class of evolved → massive stars showing spectral properties intermediate between those of → Of star and → WN Wolf-Rayet stars. Several of them have been found to possess non-spherical nitrogen-rich circumstellar nebulae. Ofpe/WN9 stars are considered to be transition objects between Of and W-R stars. This type of stars was first identified by Walborn (1982), who introduced the classification Ofpe/WN9, indicating that the stars could not be classified solely as Of stars, nor as WNL stars. Ofpe/WN9 stars have been found in the → Milky Way, the → Large Magellanic Cloud, → M31, and M33. Currently 10 Ofpe/WN9 stars are known in the LMC. Observational evidence suggests a close relationship between the class of → LBVs and the Ofpe/WN9 stars. A notable example is the prototype Ofpe/WN9 star R127 in the LMC that became an LBV on a time-scale of the order of a year. The possibility of such a relationship has been explored by Smith et al. (1994), who proposed that some LBVs show spectral morphologies that make them appear as an extension of the WN sequence toward later spectral types. Hence, they reclassified Ofpe/WN9 stars as WN10-11.

Ofpe, from → Of star; p for "peculiar;" e refers to the presence of other emission lines in addition to the Of ones, mainly H and He I, although also Si III. This peculiar class in the LMC was first described by Walborn (1977, ApJ 215, 53), where he called them "O Iafpe extr" based on the most similar Galactic objects known at that time. Subsequently Walborn (1983, ApJ 256, 452) and Bohannan & Walborn (1989, PASP 101, 520) suggested an extension of the WN sequence and/or transition between Of and WN. This nomenclature Ofpe/WN9 has been rather widely adopted. Later on, Smith et al. (1994) broke the Ofpe/WN9 and related Galactic types into WN10-11 subtypes to include this group of emission line stars.

An evolved Mira-type star which is associated with strong OH maser and strong infrared (IR) emission from the surrounding shell of warm gas and dust.

→ OH; → infrared; → star.

A member of a population of stars that, according to stellar evolution theories, are almost as aged as the galaxy in which it resides.

→ old; → star.

A pair of stars that lie close to each other in the sky by chance, but are not physically associated, in contrast to a true → binary star.

→ optical; → double star.

A → protostellar object which has been dynamically ejected from a newborn → multiple star system, either into a tenuously bound orbit or into an escape, thus depriving it from gaining much additional mass. Recent observations have shown that → Class I protostellar sources have a population of distant companions at separations ~ 1000 to 5000 → astronomical unit (AU)s. Moreover, the companion fraction diminishes as the sources evolve. According to N-body simulations of unstable → triple systems embedded in dense cloud cores, many companions are ejected into unbound orbits and quickly escape, but others are ejected with insufficient momentum to climb out of the potential well of the cloud core and associated binary. These loosely bound companions reach distances of many thousands of AU before falling back and eventually being ejected into escapes as the cloud cores gradually disappear (B. Reipurth et al. 2010, arXiv:1010.3307).

→ orphan; → protostar.

1) A variable star in the constellation → Cygnus, and one of the most luminous stars in the Milky Way Galaxy. It is a → Luminous Blue Variable (LBV) star of → spectral type B2 Ia located about 7000 light-years away. It was unknown until the 17th century, when it suddenly brightened to third magnitude. It briefly disappeared and returned, and today has a magnitude of about 4.8.
2) A member of a class of high-luminosity, early-type stars, in which spectral lines have a characteristic → P Cygni profile.

P, letter of alphabet; Cygni, pertaining to → Cygnus; → star.

A system for wide-field astronomical imaging developed and operated by the Institute for Astronomy at the University of Hawaii. Its goal is to survey the sky for moving or variable objects on a continual basis, and also produce accurate astrometry and photometry of already detected objects. It is situated at Haleakala Observatories near the summit of Haleakala in Hawaii. Pan-STARRS1 (PS1) was the first part of Pan-STARRS. The survey used a 1.8 meter telescope and a 1.4 Gigapixel camera to image the sky in five broadband filters (g, r, i, z, y). The PS1 consortium is made up of astronomers and engineers from 14 institutions and six countries. The survey was completed in April 2014. The Pan-STARRS Project is now focusing on building PS2.

Short for Panoramic Survey Telescope and Rapid Response System.

A star strongly compressed due to the → tidal force of a → massive black hole. The intense → gravity of the → black hole pulls harder on the nearest part of the star, creating an imbalance. When the star penetrates the → tidal radius, first it becomes cigar-shaped, then the squeezing of the tidal forces flattens the star in its orbital plane to the shape of a → pancake. Next the star rebounds, and as it leaves the tidal radius, it starts to expand. A little further on its orbit the star finally breaks up into gas fragments. This flattening would increase the → density and → temperature inside the star enough to trigger intense nuclear reactions that would tear it apart (Brassart & J.-P. Luminet, 2008, Astron. Astrophys. 481, 259).

→ pancake; → star.

A star with a spectrum that cannot be conveniently fitted into any of the standard → spectral classifications.

→ peculiar; → star.

A peony-shaped nebula around the → Wolf-Rayet star WR 102ka as shown by the → Spitzer Space Telescope image at 24 μm. The formation of this infrared bright nebula is attributed to the recent evolutionary history of WR 102ka during → LBV-type eruptions and/or its strong → stellar wind (see, e.g., A. Barniske et al. 2008, A&A 486, 971).

→ peony; → star.

A member of the class of stars in transition between → post-AGB and → white dwarf stars, with temperatures as high as 200,000 K, mean mass about 0.6 M_sun, and log g = 5.5-8. PG 1159 stars have no hydrogen or He I lines in their spectra, but do show weak He II lines and stronger lines of ionized carbon and oxygen. These stars are thought to be the exposed inner core of a star that has exploded as a → planetary nebula and is on its way to become a white dwarf. Also called → pre-degenerate star

Named after their prototype PG 1159-035, from the Palomar-Green Catalog of Ultraviolet Excess Stellar Objects (Green et al. 1986, ApJS 61, 305); → star.

Same as → 61 Cygni and → Bessel's star.

Giuseppe Piazzi (1746-1826) was the first to notice the large → proper motion of the star, in 1804. His observations over a period of 10 years revealed the largest proper motion ever detected for any star at the time, leading him to baptize it the "Flying Star;" → fly; → star.