beta Hydri

{{Starbox begin

| name = β Hydri

}}

{{Starbox image

| image=

{{Location mark

|image=Hydrus constellation map.svg|alt=|float=center|width=240

|label=|position=right

|mark=Red circle.svg|mark_width=10|mark_link=β Hydri

|x=731|y=891

}}

|caption=Location of β Hydri (circled)

}}

{{Starbox observe

| epoch = J2000

| constell = Hydrus

| ra = {{RA|00|25|45.07036}}

| dec = {{DEC|−77|15|15.2860}}

| appmag_v = 2.80

}}

{{Starbox character

| class = G2 IV

| b-v = +0.62

| u-b = +0.11

| variable =

}}

{{Starbox astrometry

| radial_v = +22.4

| prop_mo_ra = +2,219.54

| prop_mo_dec = +324.09

| parallax = 134.07

| p_error = 0.11

| parallax_footnote =

| absmag_v = {{nowrap|3.45 ± 0.01}}

}}

{{Starbox detail

| source =

| mass = {{val|1.107|0.009}}

| radius = {{val|1.831|0.009}}

| luminosity = {{val|3.45|0.10}}

| gravity = {{val|3.97|0.04}}

| temperature = {{val|5917|25|fmt=commas}}

| rotation = {{val|23.0|0.8}} days

| metal_fe = {{val|-0.12|0.02}}

| rotational_velocity = 6.0

| age_gyr = 6.1{{emdash}}7.5

}}

{{Starbox catalog

| names={{odlist | B=β Hyi | CD=-77 15 | CPD=-77 16 | FK5=11 | GJ=19 | HD=2151 | HIP=2021 | HR=98 | LFT=43 | LHS=6 | LPM=22 | LTT=226 | PLX=69 | SAO=255670}}

}}

{{Starbox reference

|Simbad=bet+Hyi

}}

Beta Hydri (β Hyi, β Hydri) is a star in the southern circumpolar constellation of Hydrus. (Note that Hydrus is not the same as Hydra.) With an apparent visual magnitude of 2.8, this is the brightest star in the constellation. Based upon parallax measurements the distance to this star is about {{Convert|24.33|ly|pc|abbr=off|lk=on}}.

At a distance of 13°, it is the closest easily visible star to the south celestial pole, and around 150 BC it was within two degrees of it, which made it the southern pole star.{{Cite web|url=http://stars.astro.illinois.edu/Sow/betahyi.html|title=Beta Hydri}}

Characteristics

The spectrum of this star matches a stellar classification of G2 IV, with the luminosity class of 'IV' indicating this is a subgiant star. As such, it is a slightly more evolved star than the Sun, with the supply of hydrogen at its core becoming exhausted. Around 113% of the mass of the Sun, it has expanded to 184% of the Sun's radius, with more than three times the Sun's luminosity. It is one of the oldest stars in the solar neighborhood, with an age between 6.1 and 7.5 billion years. This star bears some resemblance to what the Sun might look like in the far distant future, making it an object of interest to astronomers.

Beta Hydri has a strong magnetic field, which is not common for evolved stars that lost most of it during their main sequence phase. It is believed that during the subgiant phase, the star temporarily re-established a large magnetic field.

Search for planets

In 2002, Endl et al. inferred the possible presence of an unseen companion orbiting Beta Hydri as hinted by a radial velocity linear trend with a periodicity exceeding 20 years. A substellar object such as a planet with a minimum mass of 4 Jupiter masses and orbital separation of roughly 8 AU could explain the observed trend.{{cite journal | arxiv=astro-ph/0207512| author=Endl | title=The planet search program at the ESO Coudé Echelle spectrometer. III. The complete Long Camera survey results | journal=Astronomy and Astrophysics | volume=392| pages=671–690 | year=2002 | issue=2 | bibcode=2002A&A...392..671E | doi=10.1051/0004-6361:20020937| s2cid=17393347 |display-authors=etal}} If confirmed, it would be a true Jupiter-analogue, though 4 times more massive. So far no planetary/substellar object has been certainly detected.

These results were not confirmed in CES and HARPS measurements published in 2012. Instead, the long-term radial velocity variations may be caused by the star's magnetic cycle.{{cite journal|author1=M. Zechmeister |author2=M. Kürster |author3=M. Endl |author4=G. Lo Curto |author5=H. Hartman |author6=H. Nilsson |author7=T. Henning |author8=A. P. Hatzes |author9=W. D. Cochran |title=The planet search programme at the ESO CES and HARPS. IV. The search for Jupiter analogues around solar-like stars|arxiv=1211.7263|year=2012|doi=10.1051/0004-6361/201116551 |volume=552 |journal=Astronomy & Astrophysics |page=A78 |bibcode=2013A&A...552A..78Z|s2cid=53694238 }} Similarly, a 2023 study detected long-period radial velocity variations that were attributed to stellar activity.

References

{{Reflist|30em|refs=

{{citation|last1=Metcalfe |first1=Travis S. |title=TESS Asteroseismology of β Hydri: A Subgiant with a Born-again Dynamo |date=2024-08-10 |arxiv=2408.05482 |last2=van Saders |first2=Jennifer L. |last3=Huber |first3=Daniel |last4=Buzasi |first4=Derek |last5=Garcia |first5=Rafael A. |last6=Stassun |first6=Keivan G. |last7=Basu |first7=Sarbani |last8=Breton |first8=Sylvain N. |last9=Claytor |first9=Zachary R.|journal=The Astrophysical Journal |volume=974 |issue=1 |page=31 |doi=10.3847/1538-4357/ad6dd6 |doi-access=free |bibcode=2024ApJ...974...31M }}

{{citation |last=Soubiran |first=C. |last2=Creevey |first2=O. L. |last3=Lagarde |first3=N. |last4=Brouillet |first4=N. |last5=Jofré |first5=P. |last6=Casamiquela |first6=L. |last7=Heiter |first7=U. |last8=Aguilera-Gómez |first8=C. |last9=Vitali |first9=S. |last10=Worley |first10=C. |last11=de Brito Silva |first11=D. |date=2024-02-01 |title=Gaia FGK benchmark stars: Fundamental Teff and log g of the third version |bibcode=2024A&A...682A.145S |journal=Astronomy and Astrophysics |volume=682 |pages=A145 |doi=10.1051/0004-6361/202347136 |issn=0004-6361}} [https://vizier.cds.unistra.fr/viz-bin/VizieR?-source=J/A%2BA/682/A145&HIP=HIP2021 Beta Hydri's database entry] at VizieR.

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{{citation | last1=Johnson | display-authors=1 | first1=H. L. | last2=Iriarte | first2=B. | last3=Mitchell | first3=R. I. | last4=Wisniewskj | first4=W. Z. | title=UBVRIJKL photometry of the bright stars | journal=Communications of the Lunar and Planetary Laboratory | volume=4 | issue=99 | year=1966 | page=99 | bibcode=1966CoLPL...4...99J }}

{{Citation |last=Santos |first=A. R. G. |last2=Metcalfe |first2=T. S. |last3=Kochukhov |first3=O. |last4=Ayres |first4=T. R. |last5=Gafeira |first5=R. |last6=Campante |first6=T. L. |title=Magnetic braking and dynamo evolution of β Hydri |date=2025-06-20 |arxiv=2506.17049}}

{{citation | title=The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ˜14 000 F and G dwarfs | display-authors=1 | last1=Nordström | first1=B. | last2=Mayor | first2=M. | last3=Andersen | first3=J. | last4=Holmberg | first4=J. | last5=Pont | first5=F. | last6=Jørgensen | first6=B. R. | last7=Olsen | first7=E. H. | last8=Udry | first8=S. | last9=Mowlavi | first9=N. | journal=Astronomy and Astrophysics | volume=418 | pages=989–1019 |date=May 2004 | doi=10.1051/0004-6361:20035959 | bibcode=2004A&A...418..989N |arxiv = astro-ph/0405198 | s2cid=11027621 }}

{{cite journal |last1=Laliotis |first1=Katherine |last2=Burt |first2=Jennifer A. |display-authors=etal |date=April 2023 |title=Doppler Constraints on Planetary Companions to Nearby Sun-like Stars: An Archival Radial Velocity Survey of Southern Targets for Proposed NASA Direct Imaging Missions |journal=The Astronomical Journal |volume=165 |issue=4 |page=176 |doi=10.3847/1538-3881/acc067 |doi-access=free |arxiv=2302.10310 |bibcode=2023AJ....165..176L}}

}}