S-type star

An S-type star (or just S 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.

The carbon stars have more carbon than oxygen in their atmospheres. In most stars, such as class M giants, the atmosphere is richer in oxygen than carbon and they are referred to as oxygen-rich stars. S-type stars are intermediate between carbon stars and normal giants. They can be grouped into two classes: intrinsic S stars, which owe their spectra to convection of fusion products and s-process elements to the surface; and extrinsic S stars, which are formed through mass transfer in a binary system.

The intrinsic S-type stars are on the most luminous portion of the asymptotic giant branch, a stage of their lives lasting less than a million years. Many are long period variable stars. The extrinsic S stars are less luminous and longer-lived, often smaller-amplitude semiregular or irregular variables. S stars are relatively rare, with intrinsic S stars forming less than 10% of asymptotic giant branch stars of comparable luminosity, while extrinsic S stars form an even smaller proportion of all red giants.

W Aquilae binary
W Aquilae is an S-type star and Mira variable with a close companion resolved by the Hubble Space Telescope.

Spectral features

Cool stars, particularly class M, show molecular bands, with titanium(II) oxide (TiO) especially strong. A small proportion of these cool stars also show correspondingly strong bands of zirconium oxide (ZrO). The existence of clearly detectable ZrO bands in visual spectra is the definition of an S-type star.[1]

The main ZrO series are:[1]

  • α series, in the blue at 464.06 nm, 462.61 nm, and 461.98 nm
  • β series, in the yellow at 555.17 nm and 571.81 nm
  • γ series, in the red at 647.4 nm, 634.5 nm, and 622.9 nm[2]

The original definition of an S star was that the ZrO bands should be easily detectable on low dispersion photographic spectral plates, but more modern spectra allow identification of many stars with much weaker ZrO. MS stars, intermediate with normal class M stars, have barely detectable ZrO but otherwise normal class M spectra. SC stars, intermediate with carbon stars, have weak or undetectable ZrO, but strong sodium D lines and detectable but weak C2 bands.[3]

S star spectra also show other differences to those of normal M class giants. The characteristic TiO bands of cool giants are weakened in most S stars, compared to M stars of similar temperature, and completely absent in some. Features related to s-process isotopes such as YO bands, SrI lines, BaII lines, and LaO bands, and also sodium D lines are all much stronger. However, VO bands are absent or very weak.[4] The existence of spectral lines from the period 5 element Technetium (Tc) is also expected as a result of the s-process neutron capture, but a substantial fraction of S stars show no sign of Tc. Stars with strong Tc lines are sometimes referred to as Technetium stars, and they can be of class M, S, C, or the intermediate MS and SC.[5]

Some S stars, especially Mira variables, show strong hydrogen emission lines. The Hβ emission is often unusually strong compared to other lines of the Balmer series in a normal M star, but this is due to the weakness of the TiO band that would otherwise dilute the Hβ emission.[1]

Classification schemes

The spectral class S was first defined in 1922 to represent a number of long-period variables (meaning Mira variables) and stars with similar peculiar spectra. Many of the absorption lines in the spectra were recognised as unusual, but their associated elements were not known. The absorption bands now recognised as due to ZrO are clearly listed as major features of the S-type spectra. At that time, class M was not divided into numeric sub-classes, but into Ma, Mb, Mc, and Md. The new class S was simply left as either S or Se depending on the existence of emission lines. It was considered that the Se stars were all LPVs and the S stars were non-variable,[6] but exceptions have since been found. For example, π1 Gruis is now known to be a semiregular variable.[7]

The classification of S stars has been revised several times since its first introduction, to reflect advances in the resolution of available spectra, the discovery of greater numbers of S-type stars, and better understanding of the relationships between the various cool luminous giant spectral types.

Comma notation

The formalisation of S star classification in 1954 introduced a two-dimensional scheme of the form SX,Y. For example, R Andromedae is listed as S6,6e.[1]

X is the temperature class. It is a digit between 1 (although the smallest type actually listed is S1.5) and 9, intended to represent a temperature scale corresponding approximately to the sequence of M1 to M9. The temperature class is actually calculated by estimating intensities for the ZrO and TiO bands, then summing the larger intensity with half the smaller intensity.[1]

Y is the abundance class. It is also a digit between 1 and 9, assigned by multiplying the ratio of ZrO and TiO bands by the temperature class. This calculation generally yields a number which can be rounded down to give the abundance class digit, but this is modified for higher values:[1]

  • 6.0 – 7.5 maps to 6
  • 7.6 – 9.9 maps to 7
  • 10.0 – 50 maps to 8
  • > 50 maps to 9

In practice, spectral types for new stars would be assigned by referencing to the standard stars, since the intensity values are subjective and would be impossible to reproduce from spectra taken under different conditions.[1]

A number of drawbacks came to light as S stars were studied more closely and the mechanisms behind the spectra came to be understood. The strengths of the ZrO and TiO are influenced both by temperature and by actual abundances. The S stars represent a continuum from having oxygen slightly more abundant than carbon to carbon being slightly more abundant than oxygen. When carbon becomes more abundant than oxygen, the free oxygen is rapidly bound into CO and abundances of ZrO and TiO drop dramatically, making them a poor indicator in some stars. The abundance class also becomes unusable for stars with more carbon than oxygen in their atmospheres.[8]

This form of spectral type is a common type seen for S stars, possibly still the most common form.[9]

Elemental intensities

The first major revision of the classification for S stars completely abandons the single-digit abundance class in favour of explicit abundance intensities for Zr and Ti.[10] So R And is listed, at a normal maximum, with a spectral type of S5e Zr5 Ti2.[9]

In 1979 Ake defined an abundance index based on the ZrO, TiO, and YO band intensities. This single digit between 1 and 7 was intended to represent the transition from MS stars through increasing C/O ratios to SC stars. Spectral types were still listed with explicit Zr and Ti intensity values, and the abundance index was included separately in the list of standard stars.[8]

Abundance index criteria and estimated C/O ratio[8]
Abundance index Criteria C/O ratio
1 TiO ≫ ZrO and YO
< 0 .90
2 TiO ≥ ZrO ≥ 2×YO
0 .90
3 2×YO ≥ ZrO ≥ TiO
0 .93
4 ZrO ≥ 2×YO > TiO
0 .95
5 ZrO ≥ 2×YO, TiO = 0
> 0 .95
6 ZrO weak, YO and TiO = 0
~ 1
7 CS and carbon stars
> 1

Slash notation

The abundance index was immediately adopted and extended to run from 1 to 10, differentiating abundances in SC stars. It was now quoted as part of the spectral type in preference to separate Zr and Ti abundances. To distinguish it from the earlier abandoned abundance class it was used with a slash character after the temperature class, so that the spectral class for R And became S5/4.5e.[3]

The new abundance index is not calculated directly, but is assigned from the relative strengths of a number of spectral features. It is designed to closely indicate the sequence of C/O ratios from below 0.95 to about 1.1. Primarily the relative strength of ZrO and TiO bands forms a sequence from MS stars to abundance index 1 through 6. Abundance indices 7 to 10 are the SC stars and ZrO is weak or absent so the relative strength of the sodium D lines and Cs bands is used. Abundance index 0 is not used, and abundance index 10 is equivalent to a carbon star Cx,2 so it is also never seen.[4]

Abundance index criteria and estimated C/O ratio[4]
Abundance index Criteria C/O ratio
MS Strongest YO and ZrO bands just visible
1 TiO ≫ ZrO and YO
< 0 .95
2 TiO > ZrO
0 .95:
3 ZrO = TiO, YO strong
0 .96
4 ZrO > TiO
0 .97
5 ZrO ≫ TiO
0 .97
6 ZrO strong, TiO = 0
0 .98
7 (SC) ZrO weaker, D lines strong
0 .99
8 (SC) No ZrO or C2, D lines very strong
1 .00
9 (SC) C2 very weak, D lines very strong
1 .02
10 (SC) C2 weak, D lines strong
1 .1:

The derivation of the temperature class is also refined, to use line ratios in addition to the total ZrO and TiO strength. For MS stars and those with abundance index 1 or 2, the same TiO band strength criteria as for M stars can be applied. Ratios of different ZrO bands at 530.5 nm and 555.1 nm are useful with abundance indices 3 and 4, and the sudden appearance of LaO bands at cooler temperatures. The ratio of BaII and SrI lines is also useful at the same indices and for carbon-rich stars with abundance index 7 to 9. Where ZrO and TiO are weak or absent the ratio of the blended features at 645.6 nm and 645.0 nm can be used to assign the temperature class.[4]

Asterisk notation

With the different classification schemes and the difficulties of assigning a consistent class across the whole range of MS, S, and SC stars, other schemes are sometimes used. For example, one survey of new S/MS, carbon, and SC stars uses a two-dimensional scheme indicated by an asterisk, for example S5*3. The first digit is based on TiO strength to approximate the class M sequence, and the second is based solely on ZrO strength.[2]

Standard stars

This table shows the spectral types of a number of well-known S stars as they were classified at various times. Most of the stars are variable, usually of the Mira type. Where possible the table shows the type at maximum brightness, but several of the Ake types in particular are not at maximum brightness and so have a later type. ZrO and TiO band intensities are also shown if they are published (an x indicates that no bands were found). If the abundances are part of the formal spectral type then the abundance index is shown.

Comparison of spectral types under different classification schemes
Star Keenan
(1954)[1]
Keenan et al.
(1974)[11]
Ake
(1979)[8]
Keenan-Boeshaar
(1980)[4]
R Andromedae S6,6e: Zr4 Ti3 S4,6e S8e Zr6 4 S5/4.5e Zr5 Ti2
X Andromedae S3,9e Zr3 Ti0 S2,9e: S5.5e Zr4 5 S5/4.5e Zr2.5 Tix
RR Andromedae S7,2e: Zr2 Ti6.5 S6,2e: S6.5e Zr3 Ti6 2 S6/3.5e Zr4+ Ti4
W Aquilae S4,9: Zr4 Ti0 S3,9e: S6/6e Zr6 Ti0
BD Camelopardalis S5,3 Zr2.5 Ti4 S3.5 Zr2.5 Ti3 2 S3.5/2 Zr2+ Ti3
BH Crucis SC8,6:[12] SC4.5/8-e Zr0 Tix Na10:
Chi Cygni S7,1e: Zr0-2 Ti7 S7,2e S9.5 Zr3 Ti9 1 S6+/1e = Ms6+ Zr2 Ti6
R Cygni S3.5,9e: Zr3.5 Ti0 S3,9e S8e Zr7 Ti3: 4 S5/6e Zr4 Tix
R Geminorum S3,9e: Zr3 Ti0 S3,9e S8e Zr5 5 S4/6e Zr3.5 Tix

Formation

There are two distinct classes of S-type stars: intrinsic S stars; and extrinsic S stars. The presence of Technetium is used to distinguish the two classes, only being found in the intrinsic S-type stars.

Intrinsic S stars

Evolution on the TP-AGB
Stellar properties as a 2 M solar-metallicity red giant evolves along the TP-AGB to become an S star and then a carbon star[13]

Intrinsic S-type stars are thermal pulsing asymptotic giant branch (TP-AGB) stars. AGB stars have inert carbon-oxygen cores and undergo fusion both in an inner helium shell and an outer hydrogen shell. They are large cool M class giants. The thermal pulses, created by flashes from the helium shell, cause strong convection within the upper layers of the star. These pulses become stronger as the star evolves and in sufficiently massive stars the convection becomes deep enough to dredge up fusion products from the region between the two shells to the surface. These fusion products include carbon and s-process elements.[14] The s-process elements include zirconium (Zr), yttrium (Y), lanthanum (La), technetium (Tc), barium (Ba), and strontium (Sr), which form the characteristic S class spectrum with ZrO, YO, and LaO bands, as well as Tc, Sr, and Ba lines. The atmosphere of S stars has a carbon to oxygen ratio in the range 0.5 to < 1.[15] Carbon enrichment continues with subsequent thermal pulses until the carbon abundance exceeds the oxygen abundance, at which point the oxygen in the atmosphere is rapidly locked into CO and formation of the oxides diminishes. These stars show intermediate SC spectra and further carbon enrichment leads to a carbon star.[16]

Extrinsic S stars

The Technetium isotope produced by neutron capture in the s-process is 99Tc and it has a half life of around 200,000 years in a stellar atmosphere. Any of the isotope present when a star formed would have completely decayed by the time it became a giant, and any newly formed 99Tc dredged up in an AGB star would survive until the end of the AGB phase, making it difficult for a red giant to have other s-process elements in its atmosphere without technetium. S-type stars without technetium form by the transfer of technetium-rich matter, as well as other dredged-up elements, from an intrinsic S star in a binary system onto a smaller less-evolved companion. After a few hundred thousand years, the 99Tc will have decayed and a technetium-free star enriched with carbon and other s-process elements will remain. When this star is, or becomes, a G or K type red giant, it will be classified as a Barium star. When it evolves to temperatures cool enough for ZrO absorption bands to show in the spectrum, approximately M class, it will be classified as an S-type star. These stars are called extrinsic S stars.[16][17]

Distribution and numbers

Stars with a spectral class of S only form under a narrow range of conditions and they are uncommon. The distributions and properties of intrinsic and extrinsic S stars are different, reflecting their different modes of formation.

TP-AGB stars are difficult to identify reliably in large surveys, but counts of normal M-class luminous AGB stars and similar S-type and carbon stars have shown different distributions in the galaxy. S stars are distributed in a similar way to carbon stars, but there are only around a third as many as the carbon stars. Both types of carbon-rich star are very rare near to the galactic centre, but make up 10% – 20% of all the luminous AGB stars in the solar neighbourhood, so that S stars are around 5% of the AGB stars. The carbon-rich stars are also concentrated more closely in the galactic plane. S-type stars make up a disproportionate number of Mira variables, 7% in one survey compared to 3% of all AGB stars.[18]

Extrinsic S stars are not on the TP-AGB, but are red giant branch stars or early AGB stars. Their numbers and distribution are uncertain. They have been estimated to make up between 30% and 70% of all S-type stars, although only a tiny fraction of all red giant branch stars. They are less strongly concentrated in the galactic disc, indicating that they are from an older population of stars than the intrinsic group.[16]

Properties

Very few intrinsic S stars have had their mass directly measured using a binary orbit, although their masses have been estimated using Mira period-mass relations or pulsations properties. The observed masses were found to be around 1.5 – 5 M[16] until very recently when Gaia parallaxes helped discover intrinsic S stars with solar-like masses and metallicities.[15] Models of TP-AGB evolution show that the third dredge-up becomes larger as the shells move towards the surface, and that less massive stars experience fewer dredge-ups before leaving the AGB. Stars with masses of 1.5 – 2.0 M will experience enough dredge-ups to become carbon stars, but they will be large events and the star will usually skip straight past the crucial C/O ratio near 1 without becoming an S-type star. More massive stars reach equal levels of carbon and oxygen gradually during several small dredge-ups. Stars more than about 4 M experience hot bottom burning (the burning of carbon at the base of the convective envelope) which prevents them becoming carbon stars, but they may still become S-type stars before reverting to an oxygen-rich state.[19] Extrinsic S stars are always in binary systems and their calculated masses are around 1.6 – 2.0 M. This is consistent with RGB stars or early AGB stars.[17]

Intrinsic S stars have luminosities around 5,000 – 10,000 L,[20][21] although they are usually variable.[16] Their temperatures average about 2,300 K for the Mira S stars and 3,100 K for the non-Mira S stars, a few hundred K warmer than oxygen-rich AGB stars and a few hundred K cooler than carbon stars. Their radii average about 526 R for the Miras and 270 R for the non-miras, larger than oxygen-rich stars and smaller than carbon stars.[22] Extrinsic S stars have luminosities typically around 2,000 L, temperatures between 3,150 and 4,000 K, and radii less than 150 R. This means they lie below the red giant tip and will typically be RGB stars rather than AGB stars.[23]

Mass loss and dust

Extrinsic S stars lose considerable mass through their stellar winds, similar to oxygen-rich TP-AGB stars and carbon stars. Typically the rates are around 1/10,000,000th those of the sun, although in extreme cases such as W Aquilae they can be more than ten times higher.[20]

It is expected that the existence of dust drives the mass loss in cool stars, but it is unclear what type of dust can form in the atmosphere of an S star with most carbon and oxygen locked into CO gas. The stellar winds of S stars are comparable to oxygen-rich and carbon-rich stars with similar physical properties. There is about 300 times more gas than dust observed in the circumstellar material around S stars. It is believed to be made up of metallic iron, FeSi, silicon carbide, and forsterite. Without silicates and carbon, it is believed that nucleation is triggered by TiC, ZrC, and TiO2.[21]

Detached dust shells are seen around a number of carbon stars, but not S-type stars. Infrared excesses indicate that there is dust around most intrinsic S stars, but the outflow has not been sufficient and longlasting enough to form a visible detached shell. The shells are thought to form during a superwind phase very late in the AGB evolution.[20]

Examples

BD Camelopardalis is a naked-eye example of an extrinsic S star. It is slow irregular variable in a symbiotic binary system with a hotter companion which may also be variable.[24]

The mira variable Chi Cygni is an intrinsic S star. When near maximum light, it is the sky's brightest S-type star.[25] It has a variable late type spectrum about S6 to S10, with features of zirconium, titanium and vanadium oxides, sometimes bordering on the intermediate MS type.[4] A number of other prominent Mira variables such as R Andromedae and R Cygni are also S-type stars, as well as the peculiar semiregular variable π1 Gruis.[25]

The naked-eye star ο1 Ori is an intermediate MS star and small amplitude semiregular variable[7] with a DA3 white dwarf companion.[26] The spectral type has been given as S3.5/1-,[4] M3III(BaII),[27] or M3.2IIIaS.[7]

References

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  13. ^ Weiss, A.; Ferguson, J. W. (2009). "New asymptotic giant branch models for a range of metallicities". Astronomy and Astrophysics. 508 (3): 1343. arXiv:0903.2155. Bibcode:2009A&A...508.1343W. doi:10.1051/0004-6361/200912043.
  14. ^ Gallino, Roberto; Arlandini, Claudio; Busso, Maurizio; Lugaro, Maria; Travaglio, Claudia; Straniero, Oscar; Chieffi, Alessandro; Limongi, Marco (1998). "Evolution and Nucleosynthesis in Low-Mass Asymptotic Giant Branch Stars. II. Neutron Capture and the S-Process". The Astrophysical Journal. 497 (1): 388. Bibcode:1998ApJ...497..388G. doi:10.1086/305437.
  15. ^ a b Shetye, S.; Goriely, S.; Siess, L.; Van Eck, S.; Jorissen, A.; Van Winckel, H. (2019). "Observational evidence of third dredge-up occurrence in S-type stars with initial masses around 1 M". Astronomy and Astrophysics. 625: L1. arXiv:1904.04039. Bibcode:2019A&A...625L...1S. doi:10.1051/0004-6361/201935296.
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  17. ^ a b Jorissen, A.; Van Eck, S.; Mayor, M.; Udry, S. (1998). "Insights into the formation of barium and Tc-poor S stars from an extended sample of orbital elements". Astronomy and Astrophysics. 332: 877. arXiv:astro-ph/9801272. Bibcode:1998A&A...332..877J.
  18. ^ Hollis R. Johnson; Ben Zuckerman (22 June 1989). Evolution of Peculiar Red Giant Stars. IAU Colloquium. 106. Cambridge University Press. pp. 342–. ISBN 978-0-521-36617-5.
  19. ^ Groenewegen, M. A. T.; Van Den Hoek, L. B.; De Jong, T. (1995). "The evolution of galactic carbon stars". Astronomy and Astrophysics. 293: 381. Bibcode:1995A&A...293..381G.
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  21. ^ a b Ferrarotti, A. S.; Gail, H.-P. (2002). "Mineral formation in stellar winds". Astronomy and Astrophysics. 382: 256–281. Bibcode:2002A&A...382..256F. doi:10.1051/0004-6361:20011580.
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  23. ^ Van Eck, S.; Jorissen, A.; Udry, S.; Mayor, M.; Pernier, B. (1998). "The HIPPARCOS Hertzsprung-Russell diagram of S stars: Probing nucleosynthesis and dredge-up". Astronomy and Astrophysics. 329: 971. arXiv:astro-ph/9708006. Bibcode:1998A&A...329..971V.
  24. ^ Ake, Thomas B.; Johnson, Hollis R.; Perry, Benjamin F. (1988). "Companions to peculiar red giants: HR 363 and HR 1105". In ESA. 281: 245. Bibcode:1988ESASP.281a.245A.
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  27. ^ Sato, K.; Kuji, S. (1990). "MK classification and photometry of stars used for time and latitude observations at Mizusawa and Washington". Astronomy and Astrophysics Supplement Series. 85: 1069. Bibcode:1990A&AS...85.1069S.
98 Herculis

98 Herculis is a single star located approximately 590 light years from the Sun in the northern constellation Hercules. It is visible to the naked eye as a dim, red-hued point of light with an apparent visual magnitude of 4.96. The brightness of the star is diminished by an extinction of 0.19 due to interstellar dust. The star is moving closer to the Earth with a heliocentric radial velocity of −19 km/s.This is an aging red giant star on the asymptotic giant branch with a stellar classification of M3-SIII, where the suffix notation indicating this is an S-type star. It is a mild barium star with an intensity class of 0.2, and is a suspected variable star, although Percy and Shepherd (1992) were unable to confirm this. With the hydrogen at its core exhausted, the star has expanded to around 85 times the Sun's radius. It is radiating 1,330 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 3,772 K.

Canis Minor

Canis Minor is a small constellation in the northern celestial hemisphere. In the second century, it was included as an asterism, or pattern, of two stars in Ptolemy's 48 constellations, and it is counted among the 88 modern constellations. Its name is Latin for "lesser dog", in contrast to Canis Major, the "greater dog"; both figures are commonly represented as following the constellation of Orion the hunter.

Canis Minor contains only two stars brighter than the fourth magnitude, Procyon (Alpha Canis Minoris), with a magnitude of 0.34, and Gomeisa (Beta Canis Minoris), with a magnitude of 2.9. The constellation's dimmer stars were noted by Johann Bayer, who named eight stars including Alpha and Beta, and John Flamsteed, who numbered fourteen. Procyon is the seventh-brightest star in the night sky, as well as one of the closest. A yellow-white main sequence star, it has a white dwarf companion. Gomeisa is a blue-white main sequence star. Luyten's Star is a ninth-magnitude red dwarf and the Solar System's next closest stellar neighbour in the constellation after Procyon. The fourth-magnitude HD 66141, which has evolved into an orange giant towards the end of its life cycle, was discovered to have a planet in 2012. There are two faint deep-sky objects within the constellation's borders. The 11 Canis-Minorids are a meteor shower that can be seen in early December.

Carbon star

A carbon star is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance. There are also some dwarf and supergiant carbon stars, with the more common giant stars sometimes being called classical carbon stars to distinguish them.

In most stars (such as the Sun), the atmosphere is richer in oxygen than carbon. Ordinary stars not exhibiting the characteristics of carbon stars but cool enough to form carbon monoxide are therefore called oxygen-rich stars.

Carbon stars have quite distinctive spectral characteristics, and they were first recognized by their spectra by Angelo Secchi in the 1860s, a pioneering time in astronomical spectroscopy.

Chi Cygni

Chi Cygni (Latinised from χ Cygni) is a Mira variable star in the constellation Cygnus, and also an S-type star. It is around 500 light years away.

χ Cygni is an asymptotic giant branch star, a very cool and luminous red giant nearing the end of its life. It was discovered to be a variable star in 1686 and its apparent visual magnitude varies from as bright as 3.3 to as faint as 14.2.

Grus (constellation)

Grus (, or colloquially ) is a constellation in the southern sky. Its name is Latin for the crane, a type of bird. It is one of twelve constellations conceived by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman. Grus first appeared on a 35-centimetre-diameter (14-inch) celestial globe published in 1598 in Amsterdam by Plancius and Jodocus Hondius and was depicted in Johann Bayer's star atlas Uranometria of 1603. French explorer and astronomer Nicolas-Louis de Lacaille gave Bayer designations to its stars in 1756, some of which had been previously considered part of the neighbouring constellation Piscis Austrinus. The constellations Grus, Pavo, Phoenix and Tucana are collectively known as the "Southern Birds".

The constellation's brightest star, Alpha Gruis, is also known as Alnair and appears as a 1.7-magnitude blue-white star. Beta Gruis is a red giant variable star with a minimum magnitude of 2.3 and a maximum magnitude of 2.0. Six star systems have been found to have planets: the red dwarf Gliese 832 is one of the closest stars to Earth to have a planetary system. Another—WASP-95—has a planet that orbits every two days. Deep-sky objects found in Grus include the planetary nebula IC 5148, also known as the Spare Tyre Nebula, and a group of four interacting galaxies known as the Grus Quartet.

II Lupi

II Lupi (IRAS 15194-5115) is a Mira variable and carbon star located in the constellation Lupus. It is the brightest carbon star in the Southern Hemisphere at 12 μm.

In 1987, the infrared source IRAS 15194-5115 was identified as an extreme carbon star. It was seen to be strongly variable at optical and infrared wavelengths. It is very faint visually, 15th or 16th magnitude in a red filter and below 21st magnitude in a blue filter, but at mid-infrared wavelengths (N band) it is the third-brightest carbon star in the sky. A star at the location had earlier been catalogued as WOS 48, a possible S-type star, on the basis of strong LaO bands in its spectrum.On the basis of infrared photometry, IRAS 15194-5115 was given the variable star designation II Lupi in 1995, although the variability type was still unknown. More detailed infrared photometry confirmed that II Lupi was a Mira variable and showed regular variations with a period of 675 days over 18 years. The mean magnitude also dimmed and brightened during that time and has been characterised as a 6,900-day secondary period although less than a full cycle was observed. The secondary period could be interpreted as an isolated or irregular obscuration event in a dust shell surrounding the star.

Omicron1 Orionis

Omicron1 Orionis (ο1 Ori) is a binary star in the northeastern corner of the constellation Orion. It is visible to the naked eye with an apparent visual magnitude of 4.7. Based upon an annual parallax shift of 5.01±0.71 mas, it is located approximately 650 light years from the Sun. At that distance, the visual magnitude of the star is diminished by an interstellar absorption factor of 0.27 due to intervening dust.The two components of this system have an orbital period of greater than 1,900 days (5.2 years). The primary component is an evolved red giant with the stellar classification of M3S III. This is an S-type star on the asymptotic giant branch. It is a semiregular variable that is pulsating with periods of 30.8 and 70.7 days, each with nearly identical amplitudes of 0.05 in magnitude. The star has an estimated 90% of the mass of the Sun but has expanded to 214 times the Sun's radius. It shines with 4,046 times the solar luminosity from its outer atmosphere at an effective temperature of 3,465 K.

Pi Gruis

π Gruis, Latinised as Pi Gruis, is an optical double comprising two unrelated stars in the constellation Grus appearing close by line of sight:

π1 Gruis (HR 8521), a semiregular S-type star

π2 Gruis (HR 8524), an F-type star

R Andromedae

R Andromedae (R And) is a Mira-type variable star in the constellation Andromeda. Its spectral class is type S because it shows absorption bands of zirconium monoxide (ZrO) in its spectrum. It was among the stars found by Paul Merrill to show absorption lines of the unstable element technetium, establishing that nucleosynthesis must be occurring in stars. The SH molecule was found for the first time outside earth in the atmosphere of this star. The star is losing mass due to stellar winds at a rate of 1.09×10−6 M☉/yr.

R Cygni

R Cygni is a variable star of the Mira type in the constellation Cygnus, less than 4' from θ Cygni. This is a red giant star on the asymptotic giant branch located around 2,200 light years away. It is an S-type star ranging between spectral types S2.5,9e to S6,9e(Tc).Stars at this mass range and evolutionary stage are pulsationally unstable, displaying a variation in their light output. R Cygni has a maximum magnitude of 6.1 and a minimum magnitude of 14.4, with a period of 426.45 days. The variation of this star was discovered by English astronomer N. R. Pogson in 1852, and it has a history of recorded brightness measurements stretching back more than a century. R Cygni shows distinct period-doubling, where alternate maxima are of different brightness, hence the real period of pulsation could be considered to be twice that from one maximum to the next.The Catalog of Components of Double and Multiple Stars lists 10th magnitude BD+49 3065 as a companion to R Cygni, at a separation of 91", and both stars lie at approximately the same distance. The Washington Double Star Catalog additionally lists a 15th magnitude star as a companion at a separation of about 14".

R Geminorum

R Geminorum (R Gem) is a Mira variable and technetium star in the constellation Gemini. When at maximum light its apparent visual magnitude usually is between 6 and 7, while at minimum light it is typically near magnitude 14. It is located approximately 575 parsecs (1,880 ly) away.It is one of the brightest known examples of an S-type star, a type that is similar to M-type star, but whose spectra shows zirconium oxide, yttrium oxide and technetium. These exotic elements are formed in the star's core. Technetium has a half-life of just 4.2 million years, so it must have been brought up from the core relatively recently. R Gem has an unusual amount of it, even for an S-type star.

S-Type

S-Type may refer to:

S-Type (Music), Scottish producer and DJ

S-type asteroid

S type carriage, a type of railroad car

S-type star

Jaguar S-Type, an automobile

Jensen S-type, an automobile

Soviet S-class submarine

S Cassiopeiae

S Cassiopeiae (S Cas, HD 7769) is a Mira variable and S-type star in the constellation Cassiopeia. It is an unusually cool star, rapidly losing mass and surrounded by dense gas and dust producing masers.

Semiregular variable star

Semiregular variable stars are giants or supergiants of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light curves may be rather different and variable with each cycle. The amplitudes may be from several hundredths to several magnitudes (usually 1-2 magnitudes in the V filter).

Technetium star

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 98Tc with a half-life of 4.2 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.Stars containing technetium belong to the class of asymptotic giant branch stars (AGB)—stars that are like red giants, but with a slightly higher luminosity, and which burn hydrogen in an inner shell. Members of this class of stars switch to helium shell burning with an interval of some 100,000 years, in "dredge-ups". Technetium stars belong to the classes M, MS, S, SC and C-N. They are most often variable stars of the long period variable types.

Current research indicate that the presence of technetium in AGB stars occurs after some evolution, and that a significant number of these stars do not exhibit the metal in their spectra. The presence of technetium seems to be related to the "third dredge-up" in the history of the stars.

W Andromedae

W Andromedae is a variable star in the constellation of Andromeda. It is classified as a Mira variable of S-type star, and varies from an apparent visual magnitude of 14.6 at minimum brightness to a magnitude of 6.7 at maximum brightness, with a period of approximately 397.3 days. The star is losing mass due to stellar winds at a rate of 2.79×10−7 M☉/yr.

W Aquilae

W Aquilae (W Aql / SAO 143184 / GC 2525) is a variable star in the constellation of Aquila. Its distance from the solar system is estimated between 1,100 and 7,502 light-years.

Y Lyncis

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 800 light years away.

Y Lyncis ranges in brightness from magnitude 6.8 to 8.9. Its changes in brightness are complex with at least two different periods showing. The General Catalogue of Variable Stars lists a period of 110 days. More recent studies show a primary pulsation period of 133 days, with and a long secondary period with an amplitude of 0.2 magnitudes and duration 1,300 days. The long secondary period variations are possibly caused by long-lived convection cells.Y Lyncis has a mass around 1.5-2.0 M☉ and a luminosity around 10,000 L☉. It is a thermally pulsing asymptotic giant branch star, an evolved star with a carbon-oxygen core that is fusing helium in a shell and hydrogen in a separate shell. It is also an S-type star, where third dredge-ups have brought some carbon to the surface, but not enough to create a carbon star.

Zirconium dioxide

Zirconium dioxide (ZrO2), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabilized cubic structured zirconia, cubic zirconia, is synthesized in various colours for use as a gemstone and a diamond simulant.

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