Aldebaran is classified as a type K5III star, which indicates it is an orange giant star that has moved off the main sequence line of the Hertzsprung–Russell diagram after exhausting the hydrogen at its core. Compression from gravity increased the temperature and pressure at the core until it passed through a helium flash stage, when it began the generation of energy through the triple-alpha fusion process of helium.[9] Additional energy is being produced through hydrogen fusion along a shell surrounding the helium core. As a result of the increase in the core temperature, the star has expanded to a diameter of 44.2 times the diameter of the Sun,[4][10] approximately 61 million kilometres (see 10 gigametres for similar sizes).
The Hipparcos satellite has measured it as 65.1 light years (20.0 pc) away, and it shines with 425 times the Sun's luminosity.[3] Aldebaran is a slightly variable star, of the slow irregular variable type LB. It varies by about 0.2 in apparent magnitude from 0.75 to 0.95.[2] With a near-infrared J band magnitude of −2.1,[1] only Betelgeuse (−2.9), R Doradus (−2.6), and Arcturus (−2.2) are brighter.
Visibility
Aldebaran is one of the easiest stars to find in the night sky, partly due to its brightness and partly due to its spatial relation to one of the more noticeable asterisms in the sky. If one follows the three stars of Orion's belt from left to right (in the Northern Hemisphere) or right to left (in the Southern), the first bright star found by continuing that line is Aldebaran.
Since the star is located (by chance) in the line of sight between the Earth and the Hyades, it has the appearance of being the brightest member of the more scattered Hyades open star cluster that makes up the bull's-head-shaped asterism; however, the star cluster is actually more than twice as far away, at about 150 light years.
In this predawn occultation, Aldebaran has just reappeared on the dark limb of the waning crescent Moon (July 1997 still frame captured from video).
Aldebaran is close enough to the ecliptic to be occulted by the Moon. Such occultations occur when the Moon's ascending node is near the autumnal equinox. A number of such events occur every revolution of the Moon starting at 29 Jan 2015 and ending at 3 Sep 2018. Each event is visible from a different location on Earth, but always in the northern hemisphere (or southern close to the equator). That means that people from Australia or South Africa never can observe an Aldebaran occultation. This is due to the fact that Aldebaran is slightly too far south of the ecliptic. A reasonably accurate estimate for the diameter of Aldebaran was obtained during the September 22, 1978 occultation.[11] Aldebaran is in conjunction with the Sun around June 1 of each year.[12]
Double star
Five faint stars appear close enough to Aldebaran in its visual field for astronomers to consider it[citation needed] a double star association. These stars were given alphabetic secondary star designations more or less in the order of their discovery, with the letter A reserved for the primary star. The better known characteristics of these optical double stars are listed in the table below with the primary star, Aldebaran (Alpha Tauri A), shown for reference.[13]
what can explain the rare partial solar eclipses I captured for 5 days in a row?