Eridanus, the River, is a large constellation that begins just west of Orion’s brightest star, Rigel. From Rigel, it stretches about 60° southwest before ending at a bright star called Achernar. Achernar lies far enough south that it is only visible from the southernmost parts of the continental United States. However, the northern part of Eridanus is easily observable from areas much farther north.
Eridanus contains a surprisingly low number of good telescopic targets in relation to its size, but there are a few objects here that are worth finding. One such object is a triple star called Keid. To find Keid, start from the stars that mark the feet of Orion. Extend an imaginary line from Saiph through Rigel; then go the same distance again past Rigel. A few degrees west of the end of your line, you should see a little pair of stars that are similar in brightness. These stars are called Beid and Keid (Arabic for “eggs” and “eggshells,” because the surrounding stars represented an ostrich nest to the Arabs). Keid is the southeastern star of the pair.
Keid is one of the closer stars to Earth, at only sixteen light-years away. The A (brightest) component star of Keid is a Sun-like yellow dwarf star that shines at 4th magnitude. The B component star is a 9th magnitude white dwarf star that lies 83 arcseconds away from the A star—far enough for any telescope to easily split.
White dwarf stars such as Keid B contain an enormous amount of matter packed into a tiny volume. Keid B is only 50% larger in diameter than Earth, yet it contains half the mass of the Sun. One cubic inch of material from this star would weigh about four tons. All white dwarf stars are relatively dim; Keid B is the easiest white dwarf to observe.
While Keid is not particularly difficult to view, it is most beautiful in larger telescopes. Even a 6” telescope will show all of Keid’s component stars if the atmosphere is stable, but more aperture is necessary to really see the contrasting colors of the fainter stars.
Keid C, an 11th magnitude red dwarf, lies a modest 8 arcseconds from Keid B. The C star is just faint enough that you will probably need moderate to high magnification and a stable atmosphere to split it from Keid B.
NGC 1535, a fine planetary nebula, lies just south of Keid. To find the nebula, start at Keid and go 2.5° south to the star 39 Eridani (which is, incidentally, a tight double star). NGC 1535 is another 2.5° south of 39 Eri. Use low to moderate magnification to find the nebula; it will look like a fuzzy, defocused star. Once you’ve found it, boost the magnification as high as the viewing conditions allow. When the atmosphere is stable, high magnification may reveal the nebula’s central star surrounded by a bright ring of nebulosity.
Keid and Beid make a good starting point for finding our third target, the double star 32 Eridani. Starting at Keid, go northwest through Beid; then go another 6°, or 5 times the Keid–Beid distance. This will bring you right to 32 Eri, which is rather dim, but visible from rural and suburban skies. With a separation of 7 arcseconds, 32 Eri is not a wide double, but moderate magnification will split it easily. This is a classic color-contrast double, with yellow and blue component stars.
