Hubby and Wifey's Blog

Aldebaran is the brightest star in the constellation Taurus and one of the brightest stars in the nighttime sky. It has the Bayer designation Alpha Tauri. Because of its location in the head of Taurus, it has historically been called the Bull's Eye. Aldebaran has the appearance of being the brightest member of the more scattered Hyades cluster, which is the closest star cluster to Earth. However, it is merely located in the line of sight between the Earth and the Hyades, and is actually an independent star.

In 1997, a possible substellar companion was reported, with a mass at least 11 times that of Jupiter with an orbital period of around 2 years, however this has not been confirmed.

Aldebaran is a K5 III star, which means it is orangish, large, and has moved off of the main sequence after exhausting the hydrogen fuel in its core. It has a minor companion (a dim M2 dwarf orbiting at several hundred AU). Now primarily fusing helium, the main star has expanded to a diameter of approximately 5.3 × 10⁷ km, or about 38 times the diameter of the Sun. The Hipparcos satellite has measured it as 65.1 light years away, and it shines with 150 times the Sun's luminosity. With an apparent magnitude of 0.87, it is the 14th brightest star. It is slightly variable, of the irregular variable type, by about 0.2 magnitude.

Radial velocity variations

In 1993, radial velocity measurements of Aldebaran A, Arcturus and Pollux showed that Aldebaran A exhibited a long-period radial velocity oscillation, which could be interpreted as a substellar companion with a minimum mass 11.4 times that of Jupiter in a 643-day orbit at a separation of 2.0 AU in a mildly eccentric orbit.^[1] However, all three stars surveyed showed similar oscillations yielding similar companion masses, and the authors concluded that the variation was likely to be intrinsic to the star rather than due to the gravitational effect of a companion. On the other hand, it should be noted that the companion to the star Pollux, postulated in the 1993 paper has since been confirmed (see Pollux b).^[2]

Further analysis of the spectrum of Aldebaran A did not show any variation correlated to the 643-day radial velocity oscillation, which would be expected if the cause was intrinsic. The best orbital solution was found to be around 653.8 days with an eccentricity of 0.182, and a lower mass limit of 11 Jupiter masses. The high value for the minimum mass means that it is likely that the object, if it exists, exceeds 13 Jupiter masses and is therefore a brown dwarf. However it is possible that the oscillation is due to a pulsation or combination of pulsations, and as of 2006, this companion has not been confirmed.^[3]