π¹ Gruis (Pi¹ Gruis) is a

semiregular variable star In astronomy, a semiregular variable star, a type of variable star, is a giant or supergiant of intermediate and late (cooler) spectral type showing considerable periodicity in its light changes, accompanied or sometimes interrupted by various irre ...

in the constellation Grus around 530

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 101 ...

s from Earth. It forms a close naked-eye double with π² Gru four arc-minutes away. Pi1GruLightCurve

π¹ Gruis is an

asymptotic giant branch 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) lat ...

(AGB) star of spectral type S5. It is one of the brightest members of a class of stars known as S stars. It is also a semi-regular variable star ranging from

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...

5.3 to 7.0 over a period of 198.8 days. It is an ageing star, thought to be well on its way transitioning from a red giant to 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 ...

. A shell of material has been detected at a distance of 0.91

s (0.28

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...

s), which is estimated to have been ejected 21,000 years ago. Closer to the star, there appears to be a cavity within , suggesting a drop off in the ejection of material in the past 90 years. The presence of one companion makes the shape of the shell irregular (rather than spherical), and there may as yet be another undetected companion contributing to this. π¹ Gruis has a companion star of apparent magnitude 10.9 that is sunlike in properties—a yellow

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

star of spectral type G0V. Separated by , the pair make up a likely binary system. The primary star has a measured diameter of 18.37 milliarcseconds, corresponding to a size 350 times that of the Sun. The star was catalogued by French explorer and astronomer

Nicolas Louis de Lacaille Abbé Nicolas-Louis de Lacaille (; 15 March 171321 March 1762), formerly sometimes spelled de la Caille, was a French astronomer and geodesist who named 14 out of the 88 constellations. From 1750 to 1754, he studied the sky at the Cape of Good ...

in 1756 but not given a name. Instead, he gave the

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. T ...

of "π Gruis" to π². It was

Thomas Brisbane Major General Sir Thomas Makdougall Brisbane, 1st Baronet, (23 July 1773 – 27 January 1860), was a British Army officer, administrator, and astronomer. Upon the recommendation of the Duke of Wellington, with whom he had served, he was appoint ...

who designated this star as π¹.

Annie Jump Cannon Annie Jump Cannon (; December 11, 1863 – April 13, 1941) was an American astronomer whose cataloging work was instrumental in the development of contemporary stellar classification. With Edward C. Pickering, she is credited with the creation of ...

was the first to report its unusual

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of color ...

, sending a plate of its spectrograph made in 1895 to

Paul W. Merrill Paul Willard Merrill (August 15, 1887 – July 19, 1961) was an American astronomer whose specialty was spectroscopy. He was the first to define S-type stars in 1922. Career He received his Ph.D. at the University of California (now UC Berkeley) ...

and noting its similarity to

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 t ...

. Merrill selected these two stars along with

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 ...

to be the three prototypes of the S star class. π¹ Gruis was one of the first 17 stars defined as S-stars by Merrill in 1922; the only star not observed from Mount Wilson due to its southerly location in the sky. Analysis of its spectrum showed bands indicating the presence of

technetium Technetium is a chemical element with the symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. All available technetium is produced as a synthetic element. Naturally occurring technetium is a spontaneous ...

, as well as oxides of

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...

lanthanum Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lantha ...

cerium Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 o ...

and

yttrium Yttrium is a chemical element with the symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a " rare-earth element". Yttrium is almost always found in c ...

but not

titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...

nor

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. ...

which have been recorded in other S stars.

References

{{DEFAULTSORT:Pi1 Gruis Grus (constellation) Gruis, Pi1 S-type stars G-type main-sequence stars 110478 8521 212087 Durchmusterung objects Semiregular variable stars Asymptotic-giant-branch stars Binary stars