R Hydrae, abbreviated R Hya, is a single

star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...

in the equatorial constellation of

Hydra Hydra generally refers to: * Lernaean Hydra, a many-headed serpent in Greek mythology * ''Hydra'' (genus), a genus of simple freshwater animals belonging to the phylum Cnidaria Hydra or The Hydra may also refer to: Astronomy * Hydra (constel ...

, about 2.7° to the east of

Gamma Hydrae Gamma Hydrae (γ Hya, γ Hydrae) is a star in the equatorial constellation of Hydra. It has an apparent visual magnitude of 3.0, placing it second in brightness among the members of this generally faint constellation. Based upon parallax ...

. It is a Mira-type variable that ranges in apparent visual magnitude from 3.5 down to 10.9 over a period of 389 days. At maximum brightness the star can be seen with the naked eye, while at minimum a telescope of at least 5 cm is needed. This star is located at a distance of approximately 410

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 101 ...

s from the Sun based on

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 object ...

measurements, but is drifting closer 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 rate of change of the distance or range between the two points. It is equivalent to the vector projection ...

of −10 km/s. This is an aging red giant star with a

stellar classification In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting t ...

of M6e/M9e:, currently near the thermal-pulsing tip of the

asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

. The 1998 detection of weak technicium (Tc) lines in the spectrum suggests the star has recently gone through a third dredge-up. However, a 2010 study failed to detect technicium in the atmosphere, indicating this oxygen-rich AGB star is Tc-poor. O. J. Eggen found the star is a likely member of the Hyades supergroup, which suggests an age of 500 million to one billion years and a mass double that of the Sun. RHyaLightCurve

The first record of observations for this star was in 1662 by

Johannes Hevelius Johannes Hevelius Some sources refer to Hevelius as Polish: * * * * * * * Some sources refer to Hevelius as German: * * * * *of the Royal Society * (in German also known as ''Hevel''; pl, Jan Heweliusz; – 28 January 1687) was a councillo ...

. It was found to be variable by

Giacomo Filippo Maraldi Giacomo Filippo Maraldi (21 August 1665 – 1 December 1729) was a French- Italian astronomer and mathematician. His name is also given as Jacques Philippe Maraldi. Born in Perinaldo (modern Liguria) he was the nephew of Giovanni Cassini, and ...

in 1702; thereafter he tracked its various appearances until 1712. The evidence suggests the period of variability during this period was ~495 days. In between 1770 and 1950, the period declined linearly down to 385 days. It has remained fairly stable since that time. The decline may be due to a prior thermal pulse caused by a helium flash under a hydrogen- fusing shell. The period change would have been accompanied by a decrease in the stellar radius. An alternative explanation is a relaxation of the stellar envelope brought on by a change in the structure within the star. An extended emission component has been detected in the

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 aroun ...

that indicates a detached, expanding dust shell surrounds the star. The inner boundary of this shell is located at 60–100 times the stellar radius, and it extends outward to at least 1,000 times the radius with an expansion velocity of . The data indicate a mass loss rate of ·yr⁻¹, which declined by a factor of 20 around the year 1788. Observations using the Spitzer Space Telescope show an extended double shell with a spiral structure, which extends out to . It forms a

bow shock In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of t ...

where it interacts with the surrounding

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

. The orientation of this feature is consistent with the star's

proper motion Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dista ...

References

External links

AAVSO Variable Star of the Month. May 2002: R Hydraefinding chart
at AAVSO. {{DEFAULTSORT:R Hydrae M-type giants Mira variables Hydra (constellation) Durchmusterung objects 117287 065835 5080 Hydrae, R

References

Further reading

External links