Given good viewing conditions, Thuban is relatively easy to spot in the night sky, due to its location in relation to the Big Dipper asterism of Ursa Major. While it is well known that the two outer stars of the 'dipper' point to the modern-day pole starPolaris, it is less well known that the two inner stars, Phecda and Megrez, point to Thuban, just 15 degrees of arc from Megrez. Thuban is not bright enough to be viewed from badly light-polluted areas.
Pole star
Due to the precession of Earth's rotational axis, Thuban was the naked-eye star closest to the north pole from 3942 BCE, when it superseded Iota Draconis as the Pole Star, until 1793 BCE, when it was superseded by Kappa Draconis. It was closest to the pole in 2830 BCE, when it was less than ten arc-minutes away from the pole. It remained within one degree of celestial north for nearly 200 years afterwards, and even 900 years after its closest approach, was just five degrees off the pole. Thuban was considered the pole star until about 1800 BCE, when the much brighter Beta Ursae Minoris began to approach the pole as well. Having gradually drifted away from the pole over the last 4,800 years, Thuban is now seen in the night sky at a declination of 64° 20' 45.6", RA 14h 04m 33.58s. After moving nearly 47 degrees off the pole by 10000 CE, Thuban will gradually move back toward the north celestial pole. In 20346 CE, it will again be the pole star, that year reaching a maximum declination of, at right ascension.
Preceded by
Pole Star
Succeeded by
Iota Draconis
c. 3900–1800 BCE
Kochab
Binary system
Thuban is a single-lined spectroscopic binary. Only the primary star can be detected in the spectrum. The radial velocity variations of the primary can be measured and the pair have a somewhat eccentric orbit of 51.4 days. Making some assumptions based on the faintness of the secondary, the stars are likely to be about 0.46 astronomical units apart and the secondary is a little less massive than the primary. The secondary is likely to be a main sequence star slightly cooler than the primary, possibly an A2 spectral class. The secondary star was detected in high spatial resolution observations using the Navy Precision Optical Interferometer. The secondary star is 1.8 magnitudes fainter than the primary star and was detected at separations ranging from 6.2 to 2.6 milli-arcseconds. Eclipses were detected using data obtained with the Transiting Exoplanet Survey Satellite. The presence of eclipses places Thuban into the class of binaries known as eclipsing binaries.
Properties
Thuban has a spectral class of A0III, indicating its similarity to Vega in temperature and spectrum, but more luminous and more massive. It has been used as an MK spectral standard for the A0III type. Thuban is not a main sequence star; it has now ceased hydrogen fusion in its core. That makes it a white giant star, being 120 times more luminous than the Sun. It is 300 light years away and its brightness is only decreased by 0.003 magnitudes by intervening gas and dust.
