Betelgeuse - Properties

Properties

Betelgeuse is a very large, luminous and cool star classified as a red supergiant of M2Iab class. The letter "M" in this designation means that it is a red star belonging to the M spectral class and therefore has a relatively low photospheric temperature; the "Iab" suffix luminosity class indicates that it is an intermediate luminous supergiant. Uncertainties regarding the star's surface temperature, angular diameter and distance, make it difficult to achieve a precise measurement of Betelgeuse's luminosity. Research from 2012 gives Betelgeuse an average luminosity of 120,000 ± 30,000 L_☉, assuming a median temperature of 3,300 K and a radius of 1,200 R_☉. However, because most of the star's radiation is in the near infrared, the human eye cannot perceive the star's intrinsic brightness. Since 1943, the spectrum of Betelgeuse has served as one of the stable anchor points by which other stars are classified.

The mass of Betelgeuse has never been measured because it has no known companion. A mass estimate is only possible using theoretical modeling, a situation which has produced mass estimates ranging from 5 to 30 M_☉ in the 2000s. Smith and colleagues calculated that Betelgeuse began its life as a star of 15 to 20 M_⊙, based on a photospheric measurement of 5.6 AU or 1,200 R_⊙. However, a novel method of determining the supergiant's mass was proposed in 2011 by Hilding Neilson and colleagues, arguing for a stellar mass of 11.6 M_⊙ with an upper limit of 16.6 and lower of 7.7 M_⊙, based on observations of the star’s intensity profile from narrow H-band interferometry and using a photospheric measurement of roughly 4.3 AU or 955 R_⊙. How the debate will be resolved is still open—at least until a companion is identified allowing for a direct calculation of stellar mass.

Due to its variability and the presence of hotspots, the photospheric temperature of Betelgeuse is uncertain. Studies since 2001 report temperatures ranging from 3,140 to 3,641 K with a median of about 3,300K. The star is also a slow rotator and the most recent velocity recorded was 5 km/s. Depending on its photospheric radius, it could take the star from 25 to 32 years to turn on its axis—much slower than Antares, which has a rotational velocity of 20 km/s.

In 2002, astronomers using computer simulations speculated that Betelgeuse might exhibit magnetic activity in its extended atmosphere, a factor where even moderately strong fields could have a meaningful influence over the star's dust, wind and mass-loss properties. A series of spectropolarimetric observations obtained in 2010 with the Bernard Lyot Telescope at Pic du Midi Observatory revealed the presence of a weak magnetic field at the surface of Betelgeuse, suggesting that the giant convective motions of supergiant stars are able to trigger the onset of a small-scale dynamo effect.