Alpha Doradus

At magnitude 3.27, Alpha Doradus is the brightest star in Dorado, the Swordfish. It is fairly prominent, outshining most of its deep southern surroundings.

Orbit and Components

α Dor is actually a double star with components so close together that not even a fine optical telescope can separate them visually. Close as they are, their orbit is rather well known, and a bit odd. Averaging 9.7 AU apart, they orbit each other in 12.1 years. The orbital eccentricity is remarkably high, however; the stars separate by as much as 17.5 AU, and then come as close as 1.9 AU. Their last close approach took place in 1998.

The dominating component is a magnitude 3.64 class A0 III giant; its magnitude 4.55 companion is classified as a B9 IV subgiant. Their separation, only three tenths of an arcsecond, renders the individuals difficult to study. α Dor A has a measured temperature of 12,200 K, anomalously high for class A0 star. It is more in line with that expected for a B9 star; quite possibly, the light from the primary A0 star has been contaminated by its companion.

Properties and Evolution

The pair lies 175 light years away. That gives respective luminosities of 157 and 68 suns, radii of 2.8 and 1.9 solar, and masses of 3.0 and 2.7 suns. The orbital parameters give a combined mass of 6.2 suns, a bit greater than the 5.7 solar derived above, but given the uncertainties the agreement is rather good. The primary (α Dor A) has begun to evolve off the main sequence, and has probably finished its core hydrogen fusion; hence its giant classification. The secondary (α Dor B) is really a hydrogen-fusing main-sequence star rather than a true subgiant.

The measured rotation speed of the primary, at least 70 km/sec, gives it a rotation period under two days. α Dor A's spectrum is peculiarly rich in silicon; this is the result of separation of chemical elements in a relatively quiet stellar atmosphere, some settle downward because of gravity, while others are lofted upward by radiation pressure. The silicon seems to be concentrated into a magnetic spot, which indicates a rotation period of 2.95 days. This shows that the above rotation speed cannot be correct, probably as a result of light interference from α Dor B.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]