A PG 1159 star, often also called a pre-degenerate, is a star with a

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

-deficient atmosphere that is in transition between being the central star of a

planetary nebula A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelate ...

and being a hot

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

. These stars are hot, with surface temperatures between 75,000 K and 200,000 K,Observational constraints on the evolutionary connection between PG 1159 stars and DO white dwarfs
S. D. Huegelmeyer, S. Dreizler, K. Werner, J. Krzesinski, A. Nitta, and S. J. Kleinman. arXiv:astro-ph/0610746. and are characterized by atmospheres with little

and absorption lines for

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

. Their

surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experien ...

is typically between 10⁴ and 10⁶ meters per second squared. Some PG 1159 stars are still fusing helium.The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars
Klaus Werner and Falk Herwig, ''Publications of the Astronomical Society of the Pacific'' 118, #840 (February 2006), pp. 183–204^{, § 2.1.1, 2.1.2, Table 2.} The PG 1159 stars are named after their prototype, PG 1159-035. This star, found in the Palomar-Green survey of

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...

-excess stellar

objects Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...

,The Palomar-Green catalog of ultraviolet-excess stellar objects
R. F. Green, M. Schmidt, and J. Liebert, ''Astrophysical Journal Supplement'' 61 (June 1986), pp. 305–352.

CDS The compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then released in Octo ...

ID
II/207
. was the first PG 1159 star discovered. It is thought that the atmospheric composition of PG 1159 stars is odd because, after they have left the asymptotic giant branch, they have reignited helium

fusion Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole. Fusion may also refer to: Science and technology Physics *Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nucl ...

. As a result, a PG 1159 star's atmosphere is a mixture of material which was between the hydrogen- and helium-burning shells of its AGB star progenitor.^, §1. They are believed to eventually lose mass, cool, and become DO white dwarfs.^;^{, §4.} Some PG 1159 stars have varying luminosities. These stars vary slightly (5–10%) in brightness due to non-radial gravity wave pulsations within themselves. They vibrate in a number of

modes Mode ( la, modus meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * '' MO''D''E (magazine)'', a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is ...

simultaneously, with typical periods between 300 and 3,000 seconds.Mapping the Instability Domains of GW Vir Stars in the Effective Temperature-Surface Gravity Diagram
Quirion, P.-O., Fontaine, G., Brassard, P., ''Astrophysical Journal Supplement Series'' 171 (2007), pp. 219–248.^{, Table 1.} The first known star of this type is also PG 1159-035, which was found to be variable in 1979, and was given the variable star designation ''GW Vir'' in 1985.The 67th Name-List of Variable Stars
P. N. Kholopov, N. N. Samus, E. V. Kazarovets, and N. B. Perova, ''Information Bulletin on Variable Stars'', #2681, March 8, 1985. These stars are called ''

GW Vir star A pulsating white dwarf is a white dwarf star whose luminosity varies due to non-radial gravity wave pulsations within itself. Known types of pulsating white dwarfs include DAV, or ZZ Ceti, stars, with hydrogen-dominated atmospheres and the spec ...

s'', after their prototype, or the class may be split into '' DOV'' and '' PNNV'' stars.^{, § 1.1;}§1
Detection of non-radial g-mode pulsations in the newly discovered PG 1159 star HE 1429-1209
T. Nagel and K. Werner, ''Astronomy and Astrophysics'' 426 (2004), pp. L45–L48.

References

{{white dwarf Star types White dwarfs

See also

References