HR 4729 (HD 108250) is a

multiple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speaki ...

located about from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

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 ...

and part of the asterism known as the Southern Cross. It is a close companion of

α Crucis Acrux is the brightest star in the southern constellation of Crux. It has the Bayer designation α Crucis, which is Latinised to Alpha Crucis and abbreviated Alpha Cru or α Cru. With a combined visual magnitude of +0.76, it is the 13 ...

and sometimes called α Crucis C.

Nomenclature

HR 4729 is the star's designation in the

Bright Star Catalogue The Bright Star Catalogue, also known as the Yale Catalogue of Bright Stars, Yale Bright Star Catalogue, or just YBS, is a star catalogue that lists all stars of stellar magnitude 6.5 or brighter, which is roughly every star visible to the na ...

. It is also often referred to by its Henry Draper Catalogue listing of HD 108250. Because of its closeness to α Crucis it is included in many multiple star catalogues as α Crucis C. It is also listed as star 25 in Crux in the Uranometria Argentina, displayed as 25 G. Crucis.

Discovery

HR 4729 was first observed in 1829, as a companion to α Crucis, by

James Dunlop James Dunlop FRSE (31 October 1793 – 22 September 1848) was a Scottish astronomer, noted for his work in Australia. He was employed by Sir Thomas Brisbane to work as astronomer's assistant at his private observatory, once located at Paramat ...

from Paramatta in

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

. As early as 1916, HR 4729 was reported to have a variable radial velocity indicating a likely binary system, but the orbital elements were not calculated until 1979.

System

HR 4729 lies 90 arcseconds away from the triple star system of α Crucis and shares its motion through space, suggesting it may be gravitationally bound to it, and it is therefore generally assumed to be physically associated. In the context of being a companion to α Crucis it is usually referred to as α Crucis C. HR 4729 is itself a close spectroscopic binary system with a period of 1 day 5 hours. It also has a faint visual companion 2.1" away. A further seven faint stars are also listed as members of the α Crucis group out to a distance of about two arc-minutes. One particular companion very close to HR 4729 has been resolved using

adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

wavelengths. It has been named α Crucis P, or α Crucis CP because it is only 2" from HR 4729. Rizzuto and colleagues determined in 2011 that the α Crucis system, including HR 4729, was 66% likely to be a member of the Lower Centaurus–Crux sub-group of the

Scorpius–Centaurus association The Scorpius–Centaurus association (sometimes called Sco–Cen or Sco OB2) is the nearest OB association to the Sun. This stellar association is composed of three subgroups (Upper Scorpius, Upper Centaurus–Lupus, and Lower Centaurus–Crux) ...

. It was not previously seen to be a member of the group because of confusion over the true radial velocity of the spectroscopic pair. On 2008 October 2, the

Cassini–Huygens ''Cassini–Huygens'' ( ), commonly called ''Cassini'', was a space research, space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, i ...

spacecraft resolved three of the components (A, B and C) of the multiple star system as

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

's disk occulted it.

Stellar properties

HR 4729 is a hot class B

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 nearly ten times as massive as the sun. It is only about twelve million years old, but already shows signs of evolving away from the main sequence. Several studies have assigned a

subgiant A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of ...

luminosity class to the star. The spectroscopic companion cannot be seen in the spectrum, therefore little is known about its properties. Analysis of the orbit shows that it has a mass greater than the sun. The physically associated companion α Crucis D or α Crucis CP is a 15th magnitude star. Its relative faintness suggests an M0 V spectral type. Two other, even fainter stars, lie within of HR 4729.

References

External links

* http://www.daviddarling.info/encyclopedia/A/Acrux.html {{DEFAULTSORT:HR 4729 Double stars B-type main-sequence stars Crux (constellation) Lower Centaurus Crux Crucis, 25 Spectroscopic binaries 4729 108250 Durchmusterung objects