40 Eridani B

40 Eridani
Robert Fealey: Tripple star system 40 Eridani, October 21st 2022

Robert Fealey published on October 21st on forum post including beautiful picture above:

The December Sky and Telescope mag motivated me to get out early this morning and see if I could observe and photograph “The First White Dwarf” star in 40 Eridani; the latter is a triple star with the B component a white dwarf.
In the attached photo I used the 10 in SCT and Orion 6.3 M pixel camera with a 8 second exposure. The bright component A is quite overexposed (but did have a nice yellow color) and component C was quite faint (11.2 mag). Visually I could barely see the white dwarf B (9.5 mag) and seeing conditions were poor unfortunately.

Sky and Telescope 2022-DecemberStory of 40 Eridani
West of Orion’s Rigel in the northern reaches of the meandering constellation Eridanus shines a triple star system 40 Eridanus. It lies 16.3 light years from Earth, about twice as far as Sirius. The brightest member of this stellar trio is orange dwarf, a K-type main sequence star, that like Sun converts hydrogen into helium in its core.
In 1783, on January 31st William Hershel saw that this naked eye star has two faint companions. The tight  dim pair was 1 1/3 arcminutes east of the orange dwarf, 40 Eridani A.
Two decades later Giuseppe Piazzi observed large proper motion of the 40 Eridani and published it in 1804, a sign it could be a close star. In the 1880s astronomers finally measured parallax and found distance value close to the current value 16.3 light years.
In early 20th century was known that 40 Eri B is type-A star with hot surface, but dim. It was lonely in lower left corner of the diagram spectral-type and luminosity, now known as a Hertzsprung-Russel diagram. It meant that it has to be small like Earth. The term white dwarf coined in 1922 Willem Luyten, student of Hertzsprung.
More details in the Sky & Telescope article The First White Dwarf Discovery.


Observe Write Dwarf!

Triple star 40 Eridani can be easily observed next to the famous constellation Orion. It’s visible by naked eye and it has name Keid, officially approved by IAU since 2016. Name Keid,in indigenous Arabic al-qaid, means “the Egg Shells” in unspecified ostrich’s nest. Below is one row from IAU table.

IAU Naming Stars
IAU Name Designation ID Const. # WDS_J Vmag RA(J2000) Dec(J2000) Approval Date
Keid HR 1325 ο2 Eri A 04153-0739 4.43 63.817999° -7.652872° 2016-09-12
40 Eridani – Keid Coordinates
Epoch RA Dec
J2000 04h 15m 16.31963s −07° 39′ 10.3404″
2023 04h 16m 20.4s −07° 37′ 05.7″
2025 04h 16m 26.6s −07° 36′ 51.5″

If Keid is in your eyepiece, you should look for 9.5mag white dwarf, about two Jupiter diameters apart.
More information can be found e.g. at SIMBAD: 40 Eri A, 40 Eri B. Latest measurement from GAIA astrometry observatory gives parallax for Keid 199.6080124181338 mili-arc-seconds then distance is 16.34 ly.


Constellation Eridanus including red circle location of the 40 Eridani. Source: en.wikipedia.org/wiki/40_Eridani

40 Eridani – Keid at stellarium-web.org. Adjust time of observing in the right bottom corner as needed.

40 Eri Keid
40 Eridani A,Keid, or Flare star. It’s companion 40 Eridani B is 9.5m white dwarf. Source: Stellarium-web.org

Another picture 40 Eridani by Robert Fealey, better seeing, on forum.

40 Eridani, 2022-10-22
Robert Fealey: 40 Eridani by ASI 186MC camera, October 22, 2022

The First White Dwarf The road to discovering a new type of star was long and winding. By Ken Croswell
IAU Naming Stars
A Dictionary of Modern Star Names: A Short Guide to 254 Star Names and Their Derivations