page_title: Betelgeuse
section_title: Variability
hierarchical_section_title: Betelgeuse / Observation / Variability
caption_attribution_description: English: Betelgeuse (Alpha Orionis), the reddish star at the shoulder of Orion, is a semiregular variable star, which usually shines at about magnitude +0.5, only slightly fainter than Rigel (Beta Orionis), the blueish star at the foot of Orion at maigitude +0.1. In late 2019 and early 2020, however, Betelgeuse has undergone a unusually deep minima, when it's magnitude dropped to about +1.6. That is, it shone at less than 40% of it's usual luminosity, so it's brightness was compareable to Bellatrix (Gamma Orionis), the other shoulder star (+1.6 mag). In this view, the usual appearance of Orion (left, in an image taken on Feburary 22, 2012), is compared with a image taken taken on February 21, 2020 (right), clearly showing the unusual dimming of the red giant star. Both images were taken with identical setup and exposure data: Each is a 30 second exposure taken with a Canon EOS 550D camera at ISO 3200, 17mm f/2.8 lens, Cokin P830 diffusor, tracked with the Vixen Polarie star tracker).
context_page_description: Betelgeuse is usually the tenth-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of Orion. It is a distinctly reddish semiregular variable star whose apparent magnitude, varying between +0.0 and +1.6, has the widest range displayed by any first-magnitude star. At near-infrared wavelengths, Betelgeuse is the brightest star in the night sky. Its Bayer designation is α Orionis, Latinised to Alpha Orionis and abbreviated Alpha Ori or α Ori.
  Classified as a red supergiant of spectral type M1-2, Betelgeuse is one of the largest stars visible to the naked eye. If it was at the center of our Solar System, its surface would lie beyond the asteroid belt and it would engulf the orbits of Mercury, Venus, Earth, Mars, and possibly Jupiter. Nevertheless, there are several larger red supergiants in the Milky Way, including VY Canis Majoris and possibly Mu Cephei. Calculations of Betelgeuse’s mass range from slightly under ten to a little over twenty times that of the Sun. It is calculated to be about 700 light-years from the Sun, indicating an absolute magnitude of about −6.
context_section_description: Betelgeuse is classified as a semiregular variable star, indicating that some periodicity is noticeable in the brightness changes, but amplitudes may vary, cycles may have different lengths, and there may be standstills or periods of irregularity. It is placed in subgroup SRc; these are pulsating red supergiants with amplitudes around one magnitude and periods from tens to hundreds of days.
  Betelgeuse typically shows only small brightness changes near to magnitude +0.5, although at its extremes it can become as bright as magnitude 0.0 or as faint as magnitude +1.6. Betelgeuse is listed in the General Catalogue of Variable Stars with a possible period of 2,335 days. More detailed analyses have shown a main period near 400 days and a longer secondary period around 2,100 days.  The lowest reliably-recorded V-band magnitude of +1.614 was reported recently, as above.
  Radial pulsations of red supergiants are well-modelled and show that periods of a few hundred days are typically due to fundamental and first overtone pulsation. Lines in the spectrum of Betelgeuse show doppler shifts indicating radial velocity changes corresponding, very roughly, to the brightness changes. This demonstrates the nature of the pulsations in size, although corresponding temperature and spectral variations are not clearly seen. Variations in the diameter of Betelgeuse have also been measured directly.
  The source of the long secondary periods is unknown, but they cannot be explained by radial pulsations. Interferometric observations of Betelgeuse have shown hotspots that are thought to be created by massive convection cells, a significant fraction of the diameter of the star and each emitting 5–10% of the total light of the star. One theory to explain long secondary periods is that they are caused by the evolution of such cells combined with the rotation of the star. Other theories include close binary interactions, chromospheric magnetic activity influencing mass loss, or non-radial pulsations such as g-modes.
  In addition to the discrete dominant periods, small-amplitude stochastic variations are seen. It is proposed that this is due to granulation, similar to the same effect on the sun but on a much larger scale.
caption: Orion, with Betelgeuse at its usual magnitude (left) and during the unusually deep minimum in early 2020 (right)
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