BH Crucis, also known as Welch's Red Variable, is a

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Crux Crux () is a constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross. It lies on the southern end of the Milky Way's visible band. The name ''Crux'' is Latin for c ...

. A long period ( Mira-type) variable, its

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 ...

ranges from 6.6 to 9.8 over 530 days. Hence at its brightest it is barely visible with the unaided eye in a rural sky. A

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

, it had been classified ranging between spectral types SC4.5/8-e and SC7/8-e, but appears to have evolved into a C-type (

carbon star A carbon star (C-type 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 mos ...

) spectrum by 2011. Ronald G. Welch discovered the star while looking for new variables in October 1969. In the first thirty years since discovery, it has become redder and brighter (mean magnitude changing from 8.047 to 7.762) and its period lengthened by 25% from 421 to 530 days. Retrospective examination of

photographic plate Photographic plates preceded photographic film as a capture medium in photography, and were still used in some communities up until the late 20th century. The light-sensitive emulsion of silver salts was coated on a glass plate, typically thinn ...

s taken in South West Africa in 1937 and 1951 and stored at

Sonneberg Observatory Sonneberg Observatory (german: Sternwarte Sonneberg) is an astronomical observatory and was formerly an institute of the Academy of Science in the German Democratic Republic. It was founded in 1925 by Cuno Hoffmeister and is located in Sonneberg, ...

suggest the amplitude of variation might have been smaller in the earliest records. A study of the star published in 2011 found that the increase in period appeared to have stopped or even begun reversing (estimated at 524 days in 2011), and that the spectral class had changed from SC to C, with

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

emission becoming more prominent.

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 ...

was also recorded in the emission spectrum and its surface temperature deemed to have cooled to 3000 K. Unusually for a Mira variable, BH Crucis had a double maximum, with two peaks in brightness, reminiscent of an

RV Tauri variable RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima. History and discovery German astronomer Friedrich Wilhelm Argelander monitored the distinctive variations in brightne ...

. However, with the lengthening of its period, this feature disappeared. Guandalini and Cristallo calculated the luminosity of Mira variables based on their periods. Using a period of 421 days, they calculated the

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...

of BH Crucis to be -4.80. Uttenthaler and colleagues calculated a

bolometric magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it w ...

of -5.59.

Gaia Data Release 2 The ''Gaia'' catalogues are star catalogues created using the results obtained by ''Gaia'' space telescope. The catalogues are released in stages that will contain increasing amounts of information; the early releases also miss some stars, especia ...

gives a parallax of and a corresponding distance of around .

References

{{DEFAULTSORT:BH Crucis Crux (constellation) Crucis, BH Mira variables Carbon stars S-type stars 059844 Welch's Red Variable 196910?? Emission-line stars