HR 6819, also known as HD 167128 or QV Telescopii (abbreviated QV Tel), is a double

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 speak ...

in the southern

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 ...

Telescopium Telescopium is a minor constellation in the southern celestial hemisphere, one of twelve named in the 18th century by French astronomer Nicolas-Louis de Lacaille and one of several depicting scientific instruments. Its name is a Latinized for ...

. It is in the south-western corner of the constellation, near Pavo to the south and

Ara ARA may refer to: Media and the arts * American-Romanian Academy of Arts and Sciences * '' Artistička Radna Akcija'', compilation album released in former Yugoslavia * Associate of the Royal Academy, denoting membership in the British Royal Aca ...

to the west. The system appears as a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

that is dimly visible to the

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to normal ...

with an

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 ...

that ranges from 5.32 down to 5.39, which is comparable to the maximum brightness of the planet

Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...

. It is 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 ...

, and is drifting farther away at a rate of . Due to its location in the sky, it is visible only to observers south of 33°N latitude. A May 2020 study hypothesized that the system contained two stars and a

black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...

, which would have been the closest known black hole, and the first one in a system visible to the naked eye. A study in July of the same year concluded that rather than a triple system it was much more likely to be a

and a star orbiting at one distance from here and another star at a different distance. Three further papers in 2020 and one in 2022 concluded that HR 6819 is simply a binary system with two mainstream stars and no black hole at all.

Nomenclature

HR 6819 is 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 ...

designation for this star. It also has the '' Henry Draper Catalogue'' designation HD 167128 and the ''

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...

'' designation HIP 89605. The General Catalogue of Variable stars includes this system as a variable similar to but not properly of the

Gamma Cassiopeiae Gamma Cassiopeiae, Latinized from γ Cassiopeiae, is a bright star at the center of the distinctive "W" asterism in the northern circumpolar constellation of Cassiopeia. Although it is a fairly bright star with an apparent visual mag ...

type, with the

variable star designation In astronomy, a variable star designation is a unique identifier given to variable stars. It uses a variation on the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the constell ...

QV Telescopii, indicating that it is the 330th confirmed variable star (excluding stars with

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. ...

s) in the constellation Telescopium.

History of discoveries

HR 6819 was originally considered a single star but it was noted by astronomers Dachs and Slettebak that its absorption spectrum displays characteristics of both Be and B3III stars. In 2003, Monika Maintz concluded that the spectrum of HR 6819 contained the signatures of two stars, though there were not enough observations to deduce an orbital period. Further observations by Rivinius ''et al''. in 2009 were able to disentangle the spectra of the two stars, and the same team conducted thorough

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...

measurements in 2019 and proposed the presence of an unseen

stellar-mass black hole A stellar black hole (or stellar-mass black hole) is a black hole formed by the gravitational collapse of a star. They have masses ranging from about 5 to several tens of solar masses. The process is observed as a hypernova explosion or as a g ...

within the system. This triple star hypothesis was challenged in 2020 by Mohammadtaher Safarzadeh, Sivia Toonen, and

Abraham Loeb Abraham "Avi" Loeb ( he, אברהם (אבי) לייב; born February 26, 1962) is an Israeli-American theoretical physicist who works on astrophysics and cosmology. Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University. He had ...

System

The

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 colors i ...

of HR 6819 contains both narrow and broadened

lines Line most often refers to: * Line (geometry), object with zero thickness and curvature that stretches to infinity * Telephone line, a single-user circuit on a telephone communication system Line, lines, The Line, or LINE may also refer to: Arts ...

. The broad lines originate from a rapidly-rotating

Be star Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer e ...

(a B-class star with

emission line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

s in its spectrum), while the narrow lines are from a more slowly-rotating B-class star. The radial velocity variations of the lines indicate that the normal B-class star is in a 40-day orbit, but the Be star does not show detectable radial velocity oscillation. QVTelLightCurve

According to the interpretation of Rivinius ''et al''. in 2019, HR 6819 was a hierarchical triple containing a classical

in a wide orbit of unknown period around an inner 40.3 day binary consisting of a B3 III star and a non-emitting ( non-accreting)

(), designated Ab. Assuming that the two stars have similar masses would mean that there is a third, invisible, body in the system, as the other component in the 40-day orbit. Analysis of the orbital parameters then indicated that the third body was so massive that it could only be a black hole. However, later work found another explanation, namely that the moving star is actually much lighter, but emitting a similar amount of light. In that case there is no need to invoke the presence of a black hole. This explanation is supported by the detection of motion in the hydrogen disk surrounding the other star, with the 40-day period. The model with a Be star and a stripped giant star, but no black hole, has been confirmed in a further paper published in 2022. Although the HR 6819 system has been described as a member of the Sco OB2 association of co-moving stars, a more recent analysis indicates it is an older system and not part of the association.

B-class giant

One component in the system is a B3 III blue star. If it were a normal star of this type, it would have a mass of approximately , but if it is a stripped-down helium star it has a mass of around . The spectral type of this component is well-defined at about B3 from the distinct narrow lines in the composite spectrum. Its temperature is around .

Be star

The second stellar component is a Be star with a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...

of B3IIIpe. The 'e' suffix indicates

s in its spectrum. It is a rapidly rotating blue-white star with a hot decretion disk surrounding it. It is estimated to be 50 million years old, with a

projected rotational velocity Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bulge ...

of . The emission lines in the spectrum are strong, but absorption lines from the Be star are weak and so the exact spectral type is difficult to determine. Overall the spectral class is similar to the other blue giant, but the relative weakness of some luminosity-dependent lines suggest that it could be a

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. It appears to be slightly hotter and slightly less luminous than the other giant star, but the exact properties are difficult to determine due to its rapid rotation, the weak absorption lines, and the presence of strong emission lines from the disk.

References

External links

* (Friday, 2020 May 22) {{DEFAULTSORT:HR 6819 B-type giants Be stars Telescopium (constellation) Durchmusterung objects 167128 089605 6819 Telescopium, QV