Theta Hydri, Latinized from θ Hydri, is 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. ...

for a blue-white hued

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 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 ...

of Hydrus. It is faintly visible to the naked eye with an apparent visual magnitude of +5.53. Based upon an annual

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...

shift of as seen from Earth, is located approximately from the Sun. At that distance, the visual magnitude of the star is diminished by an

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

of 0.10 due to

interstellar dust Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are c ...

. It is moving away from the Sun with a

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 ...

of . 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 B8 III/IV suggests it is an evolving

B-type star A B-type main-sequence star (B V) is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type stars ...

showing mixed traits of a subgiant or giant star. It is a PGa star – a sub-class of the higher temperature chemically peculiar stars known as

mercury-manganese star A mercury-manganese star (also HgMn star) is a type of chemically peculiar star with a prominent spectral line at 398.4 nm, due to absorption from ionized mercury. These stars are of spectral type B8, B9, or A0, corresponding to surface tempera ...

s (HgMn stars). That is, it displays a rich spectra of singly-ionized phosphorus and gallium, in addition to ionized mercury and manganese. As such, Theta Hydri forms a typical example of this type. The

absorption 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 for these ionized elements are found to vary, most likely as the result of uneven surface distribution combined with the star's rotation. It is a

helium-weak star Helium-weak stars are chemically peculiar stars which have a weak helium lines for their spectral type. Their helium lines place them in a later (i.e. cooler) spectral type then their hydrogen lines. List of helium-weak stars This is a non-exte ...

, having helium

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 ...

that are anomalously weak for its spectral type. A weak and variable longitudinal

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

has been detected. There is a nearby companion star of class A0 IV located at an

angular separation Angular distance \theta (also known as angular separation, apparent distance, or apparent separation) is the angle between the two sightlines, or between two point objects as viewed from an observer. Angular distance appears in mathematics (in pa ...

of along a position angle of 179°, as of 2002. Schöller et al. (2010) consider this to be a visual companion, although Eggleton and Tokovinin (2008) listed the pair as a probable

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

system.

References

{{DEFAULTSORT:Theta Hydri B-type giants B-type subgiants Mercury-manganese stars Helium-weak stars Hydrus Hydri, Theta 0939 Durchmusterung objects 019400 014131