FG Sagittae
Encyclopedia
FG Sagittae, in the constellation Sagitta
, is a supergiant
star
in the constellation
Sagitta at a distance of 8000 light-year
s. When first noted in 1943, it was noticed to be a variable star
, and its spectrum
was registered as of B4Ieq ("blue") in 1955. Since then it has expanded and reddened to G ("yellow") in 1991, and then further to about K ("orange"). FG started to pulsate when becoming an A-type star (soon after being registered as B4Ieq) with a period of 15 days, now the period has increased to over 100 days.
FG Sagittae is the central star of the planetary nebula
Henize 1-5.
Since 1992 the star has exhibited fadings and recoveries similar to that of a R Coronae Borealis variable
star, and this behavior is emphasized by a hydrogen deficiency typical for this class of stars.
It has been proposed that this star has undergone a late thermal pulse (LTP) of helium fusion after having left the asymptotic giant branch
(AGB) to move towards the hottest end of the white dwarf cooling track. This thermal pulse is believed to have revived this aged star to once again, for a short time, behave as an AGB star.
Sagitta
Sagitta is a constellation. Its name is Latin for "arrow", and it should not be confused with the larger constellation Sagittarius, the archer. Although ancient, it is insignificant, for it has no star brighter than the 4th magnitude and is the third smallest of all constellations...
, is a supergiant
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...
star
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...
in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Sagitta at a distance of 8000 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s. When first noted in 1943, it was noticed to be a variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
, and its spectrum
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
was registered as of B4Ieq ("blue") in 1955. Since then it has expanded and reddened to G ("yellow") in 1991, and then further to about K ("orange"). FG started to pulsate when becoming an A-type star (soon after being registered as B4Ieq) with a period of 15 days, now the period has increased to over 100 days.
FG Sagittae is the central star of the 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...
Henize 1-5.
Since 1992 the star has exhibited fadings and recoveries similar to that of a R Coronae Borealis variable
R Coronae Borealis variable
A R Coronae Borealis variable is an eruptive variable star that varies in luminosity in two modes, one low amplitude pulsation , and one irregular unpredictably sudden fading by 1 to 9 magnitudes...
star, and this behavior is emphasized by a hydrogen deficiency typical for this class of stars.
It has been proposed that this star has undergone a late thermal pulse (LTP) of helium fusion after having 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....
(AGB) to move towards the hottest end of the white dwarf cooling track. This thermal pulse is believed to have revived this aged star to once again, for a short time, behave as an AGB star.
See also
- Sakurai's ObjectSakurai's ObjectSakurai's Object in the constellation of Sagittarius, an object discovered to behave as a "slow nova" by Yukio Sakurai, a Japanese amateur astronomer, in 1996...
, also known as V4334 Sgr, another presumed late thermal pulsing object. - V605 AquilaeV605 AquilaeV605 Aquilae, in the constellation Aquila, is the variable central star of the planetary nebula Abell 58, exhibiting features similar to R Coronae Borealis variable stars , but also peculiar brightenings since 1919. Its spectrum exhibit the characteristic hydrogen deficiency and other features...
.