Editing Aldebaran (section)

==Physical characteristics==
[[File:Aldebaran-Sun comparison-en.svg|thumb|left|upright=1.0|Size comparison between Aldebaran and the [[Sun]]]]

Aldebaran is listed as the [[stellar classification|spectral standard]] for [[K-type giant|type K5+ III stars]].<ref name=perkins/> Its spectrum shows that it is a [[giant star]] that has evolved off the [[main sequence]] band of the [[Hertzsprung–Russell diagram|HR diagram]] after exhausting the [[hydrogen]] at its core. The collapse of the center of the star into a degenerate [[helium]] core has ignited a shell of hydrogen outside the core and Aldebaran is now on the [[red giant branch]] (RGB).<ref name=stock/>

The [[effective temperature]] of Aldebaran's [[photosphere]] is {{val|3,900|ul=K|fmt=commas}}. It has a surface gravity of {{val|1.45|ul=cgs}}, typical for a giant star, but around 35 times lower than the Earth's and nearly a thousand times lower than the Sun's. Its [[metallicity]] is about half the [[Sun]]'s.

Measurements by the [[Hipparcos]] satellite and other sources put Aldebaran around {{convert|65.3|ly|pc|abbr=off}} away.<ref name="Gatewood2008"/> Asteroseismology has determined that it is about 16% more massive than the Sun, yet it shines with 439 times the Sun's luminosity due to the expanded radius.  It is 45.1 times the [[Solar radius|diameter of the Sun]], approximately 63 million kilometres.  The angular diameter of Aldebaran has been measured many times. The value adopted as part of the Gaia benchmark calibration is {{val|20.580|0.030|ul=mas}}.<ref name=heiter/>

Aldebaran is a slightly [[variable star]], assigned to the [[slow irregular variable|slow irregular]] type ''LB''. The [[General Catalogue of Variable Stars]] indicates variation between apparent magnitude 0.75 and 0.95 from historical reports.<ref name=gcvs/> Modern studies show a smaller amplitude, with some showing almost no variation.<ref name=wasatonic/> Hipparcos photometry shows an amplitude of only about 0.02 magnitudes and a possible period around 18 days.<ref name=koen/> Intensive ground-based photometry showed variations of up to 0.03 magnitudes and a possible period around 91 days.<ref name=wasatonic/> Analysis of observations over a much longer period still find a total amplitude likely to be less than 0.1 magnitudes, and the variation is considered to be irregular.<ref name=percy/>

The [[photosphere]] shows abundances of [[carbon]], [[oxygen]], and [[nitrogen]] that suggest the giant has gone through its first [[dredge-up]] stage—a normal step in the evolution of a star into a red giant during which material from deep within the star is brought up to the surface by [[Convection zone|convection]].<ref name="Ohnaka2013"/> With its slow rotation, Aldebaran lacks a [[Dynamo theory|dynamo]] needed to generate a [[stellar corona|corona]] and hence is not a source of [[X-ray astronomy|hard X-ray emission]]. However, small scale [[Stellar magnetic field|magnetic fields]] may still be present in the lower atmosphere, resulting from convection turbulence near the surface. The measured strength of the magnetic field on Aldebaran is {{val|0.22|ul=G}}.<ref name=aa574_A90_30/> Any resulting soft X-ray emissions from this region may be attenuated by the [[chromosphere]], although ultraviolet emission has been detected in the [[stellar spectrum|spectrum]].<ref name=apj598_1_610/> The star is currently losing mass at a rate of {{val|1|–|1.6|e=-11|ul= M_Solar|up=yr}} (about one [[Earth mass]] in 300,000 years) with a velocity of {{val|30|u=km/s}}.<ref name="Ohnaka2013"/> This [[stellar wind]] may be generated by the weak magnetic fields in the lower atmosphere.<ref name=apj598_1_610/>

Beyond the chromosphere of Aldebaran is an extended molecular outer atmosphere (''MOLsphere'') where the temperature is cool enough for molecules of gas to form. This region lies at about 2.5 times the radius of the star and has a temperature of about {{val|1,500|fmt=commas|ul=K}}. The spectrum reveals lines of [[carbon monoxide]], [[water]], and [[titanium oxide]].<ref name="Ohnaka2013"/> Outside the MOLSphere, the stellar wind continues to expand until it reaches the [[termination shock]] boundary with the hot, ionized [[interstellar medium]] that dominates the [[Local Bubble]], forming a roughly spherical [[astrosphere]] with a radius of around {{val|1,000|u=au}}, centered on Aldebaran.<ref name=Wood2007/>