PG 1159 star
Encyclopedia
A PG 1159 star, often also called a pre-degenerate, is a star
with a hydrogen
-deficient atmosphere which is in transition between being the central star of a planetary nebula
and being a hot white dwarf
. These stars are hot, with surface temperature
s between 75,000 K and 200,000 K, and are characterized by atmospheres with little hydrogen
and absorption lines for helium
, carbon
and oxygen
. Their surface gravity
is typically between 104 and 106 meters per second squared
. Some PG 1159 stars are still fusing helium., § 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
-excess stellar objects
, 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
. 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 simultaneously, with typical periods between 300 and 3,000 second
s., 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. These stars are called GW Vir stars, after their prototype, or the class may be split into DOV and PNNV stars., § 1.1;
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
with a hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
-deficient atmosphere which is in transition between being the central star of a planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
and being a hot white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
. These stars are hot, with surface temperature
Effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...
s between 75,000 K and 200,000 K, and are characterized by atmospheres with little hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and absorption lines for helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. Their surface gravity
Surface gravity
The surface gravity, g, of an astronomical or other object is the gravitational acceleration experienced at its surface. The surface gravity may be thought of as the acceleration due to gravity experienced by a hypothetical test particle which is very close to the object's surface and which, in...
is typically between 104 and 106 meters per second squared
Metre per second squared
The metre per second squared is the unit of acceleration in the International System of Units . As a derived unit it is composed from the SI base units of length, the metre, and the standard unit of time, the second...
. Some PG 1159 stars are still fusing helium., § 2.1.1, 2.1.2, Table 2. The PG 1159 stars are named after their prototype, PG 1159-035
PG 1159-035
PG 1159-035 is the prototypical PG 1159 star after which the class of PG 1159 stars was named. It was discovered in the Palomar-Green survey of ultraviolet-excess stellar objects and, like the other PG 1159 stars, is in transition between being the central star of a planetary nebula and being a...
. This star, found in the Palomar-Green survey of ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
-excess stellar objects
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
, 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
Asymptotic Giant Branch
The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....
, they have reignited helium fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
. 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
Gravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
pulsations within themselves. They vibrate in a number of modes simultaneously, with typical periods between 300 and 3,000 second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
s., 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. These stars are called GW Vir stars, after their prototype, or the class may be split into DOV and PNNV stars., § 1.1;