Visibility
Betelgeuse is easy to spot in the night sky, as it appears in proximity to the belt of Orion and has a distinctive orange-red color to the naked eye. In the Northern Hemisphere, beginning in January of each year, it can be seen rising in the east just after sunset. By mid-March, it is visible to virtually every inhabited region of the globe, with only a few research stations in Antarctica at latitudes south of 82° unable to see it. In May, the red giant can be seen briefly on the western horizon after sunset, reappearing again a few months later on the eastern horizon before sunrise.
The apparent magnitude of Betelgeuse is listed in the astronomical database SIMBAD at 0.42, making it on average the eighth brightest star in the celestial sphere excluding the Sun—ahead of Achernar. Because Betelgeuse is a variable star whose brightness ranges between 0.2 and 1.2, there are periods when it will surpass Procyon to become the seventh brightest star. Occasionally it can even outshine Rigel and become the sixth brightest star, as the latter star, with a nominal apparent magnitude of 0.12, has been reported to fluctuate slightly in brightness, by 0.03 to 0.3 magnitudes. At its faintest, Betelgeuse will fall behind Deneb as the 19th brightest star and compete with Mimosa for the 20th position.
Betelgeuse has a color index (B–V) of 1.85—a figure which points to the advanced "redness" of this celestial object. The photosphere has an extended atmosphere which displays strong lines of emission rather than absorption, a phenomenon which occurs when a star is surrounded by a thick gaseous envelope. This extended gaseous atmosphere has been observed moving away from and towards Betelgeuse, depending on radial velocity fluctuations in the photosphere. Betelgeuse is the brightest near-infrared source in the sky with a J band magnitude of −2.99. As a result, only about 13% of the star's radiant energy is emitted in the form of visible light. If human eyes were sensitive to radiation at all wavelengths, Betelgeuse would appear as the brightest star in the sky.
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