40 Eridani is a multiple-star system in the constellation Eridanus. Its bright primary, 40 Eridani A, is also known as Keid. The white dwarf 40 Eridani B is a different physical star, not another name for the primary. The system also contains the much fainter red dwarf 40 Eridani C. This distinction matters whenever a chart or catalogue gives a position for the system as a whole: the white dwarf has its own identity, and measurements belonging to A or C must not be silently transferred to B. In the Star Almanack, this component has permanent fixed-object ID 1261. Its component-specific catalogue reference is Gaia DR3 3195919254111315712.
The discovery story begins well before astronomers knew what a white dwarf was. William Herschel identified the close pair now called B and C in 1783. The companions orbit one another, while the pair and the brighter A form a wider gravitational association. Their relative motion provides more than a picturesque double-star view. Repeated measurements of a binary orbit can reveal the total mass of its components through gravity, turning points of light into objects that can be weighed. The Washington Double Star Catalog is a useful reference for the system's measured components and changing separations: https://crf.usno.navy.mil/wds.
The extraordinary nature of B became clear through spectroscopy. In 1914 Walter Sydney Adams reported that this faint star has a spectrum associated with a hot, relatively white star. Its low luminosity was therefore puzzling: if it was hot but faint, its radiating surface had to be very small. The paradox was not that the star emitted no light, but that its temperature and brightness implied an object far smaller than ordinary stars. White dwarfs were among the clues that stellar physics could not be explained by familiar, diffuse gas alone. For historical context, see the American Astronomical Society's historical literature and the Smithsonian Astrophysical Observatory's ADS bibliography: https://ui.adsabs.harvard.edu/.
Today, a white dwarf is understood as the compact remnant of a star that has exhausted its central nuclear fuel and shed its outer layers. The remnant is supported largely by electron degeneracy pressure, a quantum-mechanical effect that does not depend on ordinary thermal gas pressure in the same way as a main-sequence star. This does not make the star immune to gravity: there is an upper mass scale beyond which that support cannot maintain a stable white dwarf. 40 Eridani B is especially valuable because its binary orbit allows astronomers to test white-dwarf mass estimates against orbital dynamics. A general introduction to white dwarfs and stellar remnants is available from NASA: https://science.nasa.gov/universe/stars/.
One caution is important when quoting a mass for 40 Eridani B. Published values have changed as orbital solutions and distance measurements improved. Rather than treating a familiar textbook number as timeless, researchers compare the orbit, parallax, atmosphere models and theoretical mass–radius relation. A frequently discussed modern analysis is Bond and collaborators' study of the dynamical mass and gravitational redshift of 40 Eridani B; its literature record can be located through ADS: https://ui.adsabs.harvard.edu/search/q=40%20Eridani%20B%20Bond%20mass. The scientific interest lies as much in the agreement or tension among methods as in any single quoted value.
For an observer, 40 Eridani B is not a naked-eye target. The primary is comparatively easy to locate in Eridanus, but separating the much fainter white dwarf from the primary requires a telescope, a suitable sky, and enough angular resolution and contrast. The red dwarf C lies near B and is harder still. Their positions relative to one another evolve, so a printed finder should be checked against an observation date rather than regarded as a permanent scale drawing. A catalogue position marks where to aim; it does not guarantee that every component will be visible through the instrument. The WDS measurements and SIMBAD's component-specific records help keep those questions separate: https://simbad.cds.unistra.fr/simbad/.
The remarkable lesson is that 40 Eridani B's importance does not come from spectacular brightness. Its light, spectrum and orbital motion helped establish that stars can end as extraordinarily dense remnants. The faint point beside Keid is therefore a practical observing challenge and a landmark in the history of stellar physics. When using the Star Almanack finder, look for the primary first, then consult the component labels and the observing notes before attempting B; never mistake the easier primary for the white dwarf.
