ASELLUS AUSTRALIS (Delta Cancri). After the Pleiades and Hyades of Taurus, the Beehive cluster (Messier 44) of Cancer takes its place as one of the more prominent open clusters of the northern sky. The cluster is equally well-known as the Praesepe, or "manger," and as such it is flanked by a pair of asses, donkeys, or "Aselli." The northern one, Gamma Cancri, to the northeast of the Beehive, then takes on the name Asellus Borealis, the brighter fourth magnitude (3.94) southeastern one of the unrelated pair the name Asellus Australis. It may be fairly dim, but even though Bayer assigned it the fourth Greek letter, it's still the third brightest star in the constellation, which is led by Al Tarf (Beta Cancri), the letters rather out of order (Acubens, Alpha Cnc, is fourth). Clearly, the Aselli (Delta and Gamma) got their letters more from position. Delta Cnc also comes to prominence as one of the brighter "ecliptic stars," residing just five minutes of arc north of the apparent solar path. The Sun crosses it about August 1. While a seemingly ordinary class K (K0) giant, Delta Cnc still has a bit of mystery surrounding it. From its distance of 131 light years (give or take just 1) and a well-determined temperature of 4585 Kelvin, the star is seen to shine with the light of 53 Suns, which gives a radius 11.0 times solar. From interferometry, an angular radius of 0.00245 seconds of arc coupled with distance gives an exact match, a radius of 11.0 solar, implying that all the data and results are correct. An uncertain rotation velocity gives a rotation period under 240 days, which is not much of a constraint. From theory, the mass then comes in at about double that of the Sun, the star most likely a common "clump" core helium-burner (so called because on a graph of luminosity and temperature one finds a lot of stars). A speed relative to the Sun of about three times normal suggests that our star is a bit of a visitor from the thicker part of the Galaxy's disk. Off in the distance, 40 seconds of arc away, is a 12th magnitude "companion," Delta Cnc B. Over the past two centuries, however, it has moved far too much and is clearly just a line-of-sight coincidence. The mystery involves another purported, but fifth magnitude, companion, Delta Aa, suspected only a tenth of a second of arc away. If real, it reduces the luminosity of Delta A proper by 60 percent and the radius to 8.4 solar, which is not at all consistent with the directly-determined radius of 11 solar. Neither is there any confirmation of duplicity from the Hipparcos parallax satellite. Most likely the neighbor does not exist.
Written by Jim Kaler 5/14/10. Return to STARS.