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Betelgeuse (α Orionis/SDSS J05552+0724): A Type II Core-Collapse Candidate

05/04/2012

By any stretch of the imagination Betelgeuse (also known as α Orionis, HR 2061 and sometimes colloquially “beetlejuice“) is a remarkable object. As a cool supergiant star some 650 times larger than the Sun (Michelson & Pease 1921), Betelgeuse is in its last death throes; the terminal throes of shedding vast winds of gas into space. To find Betelgeuse, you need to look towards the Orion constellation. Specifically looking towards the northeastern region (5:55:10.3+7:24:25.4, ICRS 2000) of the constellation. The stars red colouration have been noted since antiquity; the classical astronomer Ptolemy described its color as “ὑπόκιρρος” (hypókirros), a term which was later described by a translator of Ulugh Beg’s Zij-i Sultani as “rubedo”, which is Latin for “ruddiness”. The rusty Betelgeuse is also extremely bright, as the eigth brightest star in the sky and one of the brightest spectral M type stars in the sky, and large in the constellation, lying at a distance of approximately 643±147 ly (Harper et al. 2008) from Earth. As such, the fate of Betelgeuse is extreme: with many believing the star will be one of the brightest supernova events ever seen in human history.

Fig. 1: Visual representation of the Orion constellation with Betelgeuse to the top left of the asterism of Orion. (Credit: Virtual Astronomical Observatory Data Discovery Tool: [http://vao.stsci.edu/…/datadiscovery.html]).

Fig. 1: An artist’s impression of the Orion constellation if Betelgeuse was to undergo a core-collapse supernova event. The three closely aligned stars centred in the picture are that of the Orion belt asterism: Alnitak, Alnilam and Mintaka. The bright star to the bottom right of the image is Rigel.

The future fate of Betelgeuse is entirely dependent its mass, a critical factor which is not well understood for such an important star in the night sky. Betelgeuse is believed to have a mass of 16.0±2.0 (Posson-Brown et al. 2006), but mass estimates range from 5-30. Eventually the star will fuse higher elements all the way to iron and collapse and explode as a supernova. The most likely star-formation scenario for Betelgeuse suggests that it is “a runaway star” from the Orion OB1 Association and it was originally a member of a high-mass multiple star system within Orion OB1a, which includes the late type-O Alnitak (Zeta Orionis), triple star system (Harper et al 2008).

The association of Betelgeuse with Alnitak, in the sense that the are both believed to have both been born from within the same molecular cloud, could provide clues to the mass estimation of Betelgeuse. Studies by Remie & Lamers (1982) suggest that the mass of Alnitak is 28. This is quite extreme: meaning Alnitak will almost certainly end it’s stellar evolutionary sequence as a singularity. At a conservative estimation, and using similar applicable mass-radius relationships, it would seem that the minimum mass of Betelgeuse is highly probably to be greater than ~8.

The fate is well known for stars with initial masses above ~8. Stars above this threshold are thought to end their lives with the irreversible collapse of their core, when a compact remnant is formed (a neutron star, or a black hole for the more massive stars) and a powerful explosion expels the outer layers of the star. This explosion is observed as a supernova (SN). As most astronomers would expect Betelgeuse to be more than this limit, a consensus amongst popular astronomy is that the core-collapse of Betelgeuse is something of a formality.

Betelgeuse is already old for its size class and is expected by many to explode relatively soon compared to its age, some have even theorised within our lifetime. I’m always conservative on these issues, and I find it foolish to make such sensationalist and dramatic predictions. Betelgeuse may explode tomorrow, it may explode in a million years.  However, one certainty, if it did explode within our lifetime: it would be the greatest astronomical event in human history. Greater even than that of SN1987A. At its current distance from Earth, such a supernova explosion would be the brightest recorded, outshining the Moon in the night sky and becoming easily visible in broad daylight.

Further Reading:

  • Hummel C.A.; White N.M.; Elias N.M. II; Hajian A.R.; Nordgren T.E. (2000). ζ Orionis A Is a Double StarThe Astrophysical Journal, 540 (2): L91–L93.
  • North, G. (2004) Observing Variable Stars: Novae and SupernovaeCambridge University Press, Cambridge.
  • Smith, N. et al. (2009) Red Supergiants As Potential Type IIn Supernova Progenitors: Spatially Resolved 4.6 μm CO Emission Around VY CMa & Betelgeuse. The Astrophysical Journal, 137 (3): pp.3558-3573.
  • Dessart, L. et al. (2010) Determining The Main Sequence Mass Of Type II Supernova ProgenitorsMonthly Notices Royal Astronomical Society, 408 (2): pp. 827-840.
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