Nu Octantis

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Brightest star in the constellation Octans

title: "Nu Octantis" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["octans", "k-type-giants", "spectroscopic-binaries", "bayer-objects", "durchmusterung-objects", "hipparcos-objects", "bright-star-catalogue-objects", "henry-draper-catalogue-objects", "gliese-and-gj-objects", "planetary-systems-with-one-confirmed-planet", "population-i-stars", "lucidae"] description: "Brightest star in the constellation Octans" topic_path: "science/astronomy" source: "https://en.wikipedia.org/wiki/Nu_Octantis" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Brightest star in the constellation Octans ::

| name = ν Octantis |image= |image=Octans constellation map.svg|alt=|float=center|width=280 |label=|position=right |mark=Red circle.svg|mark_width=10|mark_link=Nu Octantis |x=536|y=334 |caption=Location of ν Octantis in Octans (circled) | epoch = J2000 | constell = Octans | ra = | dec = | appmag_v = 3.73 | component1= A | type= Subgiant | class= K1IV | b-v = +1.00 | u-b = +0.89 | component2=B | type2 = white dwarf | radial_v = +34.40 | prop_mo_ra = +66.41 | prop_mo_dec = −239.10 | pm_footnote = | parallax = 45.25 | p_error = 0.25 | parallax_footnote =
| dist_pc = | dist_ly = | dist_footnote = | absmag_v = |reference= | period_unitless = days | eccentricity = | axis_unitless = AU | inclination = | node = | k2 = | periarg = | source = | component1 = Nu Octantis A | mass = | radius = | temperature = | luminosity = | rotational_velocity = 2.0 | gravity = | metal_fe= | age_gyr = | component2 = Nu Octantis B | mass2 = | names = | Simbad = HD+205478 | NSTED = nu Oct

ν Octantis, Latinised as Nu Octantis, is a binary star in the constellation of Octans. Unusually for having such a late greek letter in its name, it is the brightest star in this faint constellation at apparent magnitude +3.7. It is located at 22.54 pc from Earth, and is moving away at a radial velocity of +34.4 km/s. The primary star has an exoplanet whose orbit lies halfway between both stars.

Characteristics

This is a spectroscopic binary system,{{cite journal | last1=Eggleton | first1=P. P. | last2=Tokovinin | first2=A. A. | title=A catalogue of multiplicity among bright stellar systems | journal=Monthly Notices of the Royal Astronomical Society | volume=389 | issue=2 | pages=869–879 | date=September 2008 | doi=10.1111/j.1365-2966.2008.13596.x | doi-access=free | bibcode=2008MNRAS.389..869E | arxiv=0806.2878 | s2cid=14878976

The primary has a spectral type of K1IV, with the luminosity class IV indicating that it is a subgiant star that has fused up all the hydrogen at its core and has expanded. Nu Octantis A has 1.57 times the mass of the Sun, but has expanded to 5.04 times the radius of the Sun. and now is radiating 13.2 times as much luminosity as the Sun.

The secondary star is a white dwarf with 0.57 times the mass of the Sun. When it was on the main sequence, it had a mass of and was closer to its primary, at . When it evolved to a red giant, and then to a white dwarf, it lost most of its mass, thus increasing the orbital separation. The primary star accreted about from the secondary during this period.

Nu Octantis is unusual on that its Bayer designation would suggest it is one of the faintest stars in the constellation, but is actually the brightest, over one magnitude brighter than Alpha Octantis. It seems that Lacaille (who lettered the Bayer stars in Octans) believed that Nu Octantis was a double star (like Mu Octantis) of small angular separation, rather than a single bright star.

Planetary system

::figure[src="https://upload.wikimedia.org/wikipedia/commons/5/5a/Nu_Octantis_orbit.png" caption="Orbital configuration of the Nu Octantis system, centered on the subgiant primary."] ::

In 2009, the system was hypothesised to contain a superjovian exoplanet based on variations in the radial velocity. A prograde solution was quickly ruled out but a retrograde solution remains a possibility, although a study posited that it may instead be due to the secondary star being itself a close binary, since the formation of a planet in such a system would be difficult due to gravitational perturbations. Further evidence ruling out a stellar variability and favouring the existence of the planet was gathered by 2021. With new radial velocity measurements, a study in 2025 confirmed the planet's existence.

The binary components in the Nu Octantis system were initially separated by 1.3 AU, which overlap with the current orbital separation of the planet. Therefore, the planet did not form in its current orbit, but either migrated from a longer, circumbinary orbit, or originated from a protoplanetary disc that formed after the death of the white dwarf's progenitor.

| name = Nu Octantis A | table_ref= | exoplanet = b | mass = | semimajor = | period = | eccentricity = | inclination = 108.2

References

References

  1. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics.
  2. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters.
  3. (2014). "Sloan Magnitudes for the Brightest Stars". The Journal of the American Association of Variable Star Observers.
  4. Wilson, R. E.. (1953). "General Catalogue of Stellar Radial Velocities". [[Carnegie Institution for Science]].
  5. (2016). "The conjectured S-type retrograde planet in ν Octantis: more evidence including four years of iodine-cell radial velocities". Monthly Notices of the Royal Astronomical Society.
  6. "* nu. Oct".
  7. (1958). "Spectroscopic Binaries". Springer.
  8. Ramm, D. J.. (2015-06-01). "Line-depth-ratio temperatures for the close binary ν Octantis: new evidence supporting the conjectured circumstellar retrograde planet". Monthly Notices of the Royal Astronomical Society.
  9. Lynn, W. T.. (April 1911). "The constellation Octans". The Observatory.
  10. (April 2009). "Spectroscopic orbits for K giants β Reticuli and ν Octantis: what is causing a low-amplitude radial velocity resonant perturbation in ν Oct?". Monthly Notices of the Royal Astronomical Society.
  11. (October 2010). "On the reality of the suggested planet in the ν Octantis system". The Astrophysical Journal.
  12. (February 2012). "Precession due to a close binary system: an alternative explanation for ν-Octantis?". Monthly Notices of the Royal Astronomical Society.
  13. (March 2013). "Testing a hypothesis of the ν Octantis planetary system". Monthly Notices of the Royal Astronomical Society.
  14. (2021). "A photospheric and chromospheric activity analysis of the quiescent retrograde-planet host ν Octantis A". Monthly Notices of the Royal Astronomical Society.
  15. (May 2025). "A retrograde planet in a tight binary star system with a white dwarf". Nature.

::callout[type=info title="Wikipedia Source"] This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page. ::

octansk-type-giantsspectroscopic-binariesbayer-objectsdurchmusterung-objectshipparcos-objectsbright-star-catalogue-objectshenry-draper-catalogue-objectsgliese-and-gj-objectsplanetary-systems-with-one-confirmed-planetpopulation-i-starslucidae