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

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

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 first constellati ...

Crux CRUX is a lightweight x86-64 Linux distribution targeted at experienced Linux users and delivered by a tar.gz-based package system with BSD-style initscripts. It is not based on any other Linux distribution. It also utilizes a ports system ...

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

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

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 stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...

, 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 Stellar atmosphere, atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monox ...

) 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 film as the primary medium for capturing images in photography. These plates, made of metal or glass and coated with a light-sensitive emulsion, were integral to early photographic processes such as heliography, d ...

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

Sonneberg Observatory Sonneberg Observatory () is an astronomical observatory located at 638 m altitude on Erbisbühl in the Neufang district of Sonneberg, Germany. The observatory was founded in 1925 on the initiative of Cuno Hoffmeister by the town of Sonneber ...

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; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

emission becoming more prominent.

Technetium Technetium is a chemical element; it has Symbol (chemistry), symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. Technetium and promethium are the only radioactive elements whose neighbours in the sense ...

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. 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 of BH Crucis to be -4.80. Uttenthaler and colleagues calculated a bolometric magnitude of -5.59.

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

gives a parallax of and a corresponding distance of around .

References

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