Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
---|---|
Constellation | Sagitta |
Right ascension | 20h 11m 56.05947s [1] |
Declination | +20° 20′ 04.3672″ [1] |
Apparent magnitude (V) | 8.7 - 23.0 [2] |
Characteristics | |
Spectral type | (O3 [3] -) B4Ieq - K2Ib [4] |
U−B color index | +0.75 [5] |
B−V color index | +1.21 [5] |
Astrometry | |
Parallax (π) | 0.7630 ± 0.2302 mas [6] |
Distance | approx. 4,000 ly (approx. 1,300 pc) |
Details | |
Mass | 0.8 [3] M☉ |
Radius | 0.9 - 184 [3] R☉ |
Luminosity | 2,692- 12,000 [3] L☉ |
Surface gravity (log g) | 0.2 - 2.2 [3] cgs |
Temperature | 4,467 - 45,000 [3] K |
Other designations | |
FG Sge, HIP 99527, PN ARO 169, ALS 10924, IRAS 20097+2010, PN G060.3-07.3, AN 377.1943, Hen 3-1844, JP11 5474, CSI+20-20097, Hen 2-457, LS II +20 19, TYC 1626-619-1, CSV 5066, Hen 1-5, 2MASS J20115606+2020044, UBV M 50884, PK 060-07 1, AAVSO 2007+20 | |
Database references | |
SIMBAD | data |
FG Sagittae is a supergiant star in the constellation Sagitta at a distance of 4000 light-years. When first noted in 1943, it was identified to be a variable star, and it was found to be a hot, blue star of stellar spectral type B in 1955. Since then it has expanded and cooled, becoming a yellow G-type star by 1991, [7] and then further cooling to become an orange K-type star. It started to pulsate when becoming an A-type star with a period of 15 days. This period later increased to over 100 days.[ citation needed ]
Since 1992 the star has exhibited fadings and recoveries similar to that of a R Coronae Borealis variable star; this behavior is emphasized by a hydrogen deficiency typical for this class of stars. [8] 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. [8]
FG Sagittae is the central star of the planetary nebula Henize 1-5.
In 1943, a star designated AN 377.1943 was discovered to be a previously-unknown variable star. [9] It was designated CSV 5066 as a suspected variable, [10] and then FG Sagittae as a confirmed variable star. At the time, its variations were described as being irregular, [9] but it was soon noted that the average brightness was steadily increasing. It brightened by about two magnitudes between 1943 and 1970, and then began to fade. Examination of old photographic observations found that the star had been brightening since at least 1900, with extrapolations suggesting that the minimum had occurred around 1880. [3] As it faded, FG Sagittae began to show periodic variations, at first a period of 80 days, but increasing to 130 days. In 1992, the periodic variations ceased and the brightness decreased by five magnitudes in only two months. [11] Since then, it has continued to show occasional deep fading events, appearing much like an R Coronae Borealis star. [8]
The spectrum of FG Sagittae when it was first noted as a variable star was that of a blue supergiant. The first reliable spectral class is B0 in 1930. Extrapolation of the brightness and colour indices suggest it may have been an O3 star in 1890. [3] It then steadily cooled, with the spectral class becoming as late as K2 in the 1980s. [12] The spectral class has since stayed as a G or K type supergiant, but there have been dramatic changes. The abundances of various elements have either increased or decreased: s-process elements became at least 25 times more abundant between 1967 and 1974; iron peak elements became less visible; and carbon-rich dust became strongly visible after 1992. Observations of the spectrum after 1992 are hindered by the dust formation, but the s-process and rare earth elements appear to have continued becoming more abundant. [13]
There is a very faint visible planetary nebula, Henize 1-5, around FG Sagittae, around visual magnitude 23. This formed when FG Sagittae first left the asymptotic giant branch. [14] FG Sagittae is now losing mass at about one M☉ every million years and a dust shell has formed around the star. This may form a second planetary nebula. [15]
The effective temperature of FG Sagittae in 1930 would have been about 25,000 K , possibly as hot as 45,000 K in 1890, then cooling to about 5,500 K by 1975. [12] Detailed analysis of the spectral energy distribution during the 1980s show a slow decrease in temperature to as low as 5,280 K. During the deep fades since 1992, even lower temperatures have been calculated, but these may represent observations of obscuring dust rather than the surface of the star itself. [11]
The bolometric luminosity of FG Sagittae increased steadily from around 2,700 L☉ at the end of the 19th century to over 10,000 L☉ by 1965. The luminosity then became more or less stable until 1992. As the star cooled and became more luminous, its radius increased from around one R☉ in 1900 to about 184 R☉ by 1992. [3]
When the star faded in 1992, it was obscured by dust formation and comparisons of temperature and luminosity became more difficult. The visual luminosity dropped by about five magnitudes, but the infrared brightness increased by a comparable amount. Models of the dust around the star suggest that the luminosity dropped sharply for a few hundred days as dust formed and was heated, but the underlying stellar luminosity was essentially constant and remained constant until at least 2001. [11]
The underlying properties of FG Sagittae changed on a timescale almost unheard-of for a star, from a small very hot post-asymptotic giant branch star becoming a white dwarf, to a hot supergiant and then a cool supergiant. This is believed to have been due to a helium flash in a shell that had previously been inactive since the star left the asymptotic giant branch. This is known as a late thermal pulse or very late thermal pulse, depending on the exact timing. Models approximate the behaviour of FG Sagittae although there are still detailed discrepancies. [13]
Sagitta is a dim but distinctive constellation in the northern sky. Its name is Latin for 'arrow', not to be confused with the significantly larger constellation Sagittarius 'the archer'. It was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations defined by the International Astronomical Union. Although it dates to antiquity, Sagitta has no star brighter than 3rd magnitude and has the third-smallest area of any constellation.
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.
Red supergiants (RSGs) are stars with a supergiant luminosity class of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.
A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) late in their lives.
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RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima.
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A technetium star, or more properly a Tc-rich star, is a star whose stellar spectrum contains absorption lines of the light radioactive metal technetium. The most stable isotope of technetium is 97Tc with a half-life of 4.21 million years, which is too short a time to allow the metal to be material from before the star's formation. Therefore, the detection in 1952 of technetium in stellar spectra provided unambiguous proof of nucleosynthesis in stars, one of the more extreme cases being R Geminorum.
A yellow supergiant (YSG) is a star, generally of spectral type F or G, having a supergiant luminosity class. They are stars that have evolved away from the main sequence, expanding and becoming more luminous.
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An S-type star is a cool giant with approximately equal quantities of carbon and oxygen in its atmosphere. The class was originally defined in 1922 by Paul Merrill for stars with unusual absorption lines and molecular bands now known to be due to s-process elements. The bands of zirconium monoxide (ZrO) are a defining feature of the S stars.
A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants are classed as Ia-0 or Ia+, but red supergiants are rarely assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution, especially star formation, stability, and their expected demise as supernovae.
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89 Herculis is a binary star system located about 4,700 light years away from the Sun in the northern constellation of Hercules. It is visible to the naked eye as a faint, fifth magnitude star. The system is moving closer to the Earth with a heliocentric radial velocity of −28.5 km/s.
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Y Lyncis is a semiregular variable star in the constellation Lynx. It is an asymptotic giant branch star of spectral type M6S, with a luminosity class of Ib, indicating a supergiant luminosity. It is around 1,160 light years away.
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