March 31, 2020

Astro-Challenge:Spy a White Dwarf!

This week, I’m going to introduce you to a unique but fascinating multiple star system, and one that’s definitely worth seeking out as it’s a good study in comparative stellar evolution; Omicron Eridani. This one will require a telescope of about 4′ aperture or greater, a go-to scope or a good finder chart, and patience. But the quarry is worth it; for Omicron Eridanus B is a white dwarf, the most easily observable in the sky, paired with C, a red dwarf star! Omicron Eridani is a triple star system, about 16.5 light years distant. The primary star, a K type main sequence star, is visible to the naked eye at a magnitude of about +4.5. Known to the Arabs as Al Keid (“the Egg”),

the star system may also be familiar to science fiction fans as 40 Eridani, home of the planet Vulcan in the Star Trek series. In fact, NASA has listed 40 Eridani as one of the top 100 targets for its Terrestrial Planet Finder mission. Omicron Eridani is orbited by a pair, one each white dwarf and red dwarf at 83” arc seconds of separation or an actual distance of 400 Astronomical units (AU). “B” has a visual magnitude of about +9, while “C” is a bit tougher, at about magnitude +11 and a very low stellar mass of 0.2 suns. The B-C pair orbit at a distance of about 34 AU and are closing the gap since a maximum separation of 9” in 1990. in 2080, a minimum separation of about 4” seconds will be attained. The binary nature of Omicron Eridani has been known since the discovery of the A-B pair by William Herschel in 1783, and the separation of the B-C pair was discovered by Otto Struve in 1851. The white dwarf nature of the B component was first deduced in 1910. Although not the nearest to our own solar system (Sirius B is closer at 7 light years), Omicron Eridani B is the easiest to observe, because Sirius B tends to be swamped in the primaries glare. It was also the first white dwarf discovered…and for those with setting circles, the coordinates for Omicron Eridani are;

R.A. 04H 15 min and 16 secs & Declination -07 degrees 39′ 10”.

As the Full Moon comes and goes on December 2nd, give this unique star system a shot…Live long, and prosper!

The astro-term of the week is white dwarf. A white dwarf such as Omicron Eridani B is a degenerate star at the end of its life. A main sequence star like our Sun will eventually wind up as a white dwarf once it has exhausted or shed its supply of hydrogen, helium and carbon. The remaining core is about the diameter of the Earth and will no longer be able to sustain itself via fusion and thus become incredibly dense; about 1 ton per cubic centimeter. Electron degeneracy is not a commentary on the stars’ moral state; rather, its the outward force that prevents further collapse and the maximum upward limit (also known as the Chandrasekhar limit) for a white dwarf is about 1.4 solar masses. White dwarfs can thus be thought of as slowly cooling cinders, radiating away energy until becoming cold black dwarfs. This happens over an extremely long time; its estimated that a white dwarf will last for about 1015 years, which is well past the current estimated 13.7 billion year age of our universe! White dwarfs will be with us for some time!


  1. [...] readers of this column will recall that Omicron Eridani & Sirius B are closer, but Omicron Eridani is in a trinary star system and Sirius B is [...]

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