A descriptional summary of Orion's left shoulder star
Dr. Achim Tegeler, March 08, 2024
Betelgeuse, Orion's left shoulder star, is a Red Supergiant that has become the focus of science and, of course, amateur astronomy at the latest after its significant brightness dimming at the end of 2019. The purpose of this summary is to present the current state of the facts about Betelgeuse.
Betelgeuse, Orion's left shoulder star, is a giant compared to our Sun. Although this star is only the second brightest in the constellation Orion after Rigel, it is referred to as Alpha Orionis.
The star was already mentioned in the medieval Arabic writings of the Persian astronomer Abd ar-Rahman as-Sufi [1] in the 10th century as "yad al-jauzā". Apparently, in the course of time, the Arabic initial letter "Yā'" (يـ = ي – with two dots) was adopted as "Bā'" (ب = بـ with one dot) and thus incorrectly transcribed into Latin. Thus, the origin of today's name Betelgeuse is based on this incorrect transcription.
Data and Stellar Physics
Recent investigations of the star, which is about 550 light-years away, assume a mass of about 16.5 to 19 solar masses, which is distributed over a spherical volume of about 700 times the Sun's radius (R☉) – this results in a huge surface over which the energy of the fusion reaction can be emitted in the interior. Therefore, Betelgeuse, with an effective temperature of about 3600 K, also shows a much cooler surface than our Sun (about 5778 K), and accordingly the star shows a spectrum clearly shifted into red (spectral class M1 – M2). The absolute luminosity is described as beeing about 126.000 times the luminosity of our Sun (∼105.1 L☉).
The star is currently thought to be at an early stage on the Red Supergiant Branch (RSB) in the Hertzsprung-Russel diagram. [2]
Red supergiants are among the variable stars that can fluctuate greatly in brightness due to the convection cells that occur on the surface. The fusion reaction in the interior of red supergiants is no longer limited to the nucleus in which hydrogen burning (fusion of hydrogen to helium) first started, but the fusion zone of hydrogen burning slowly shifts in the shape of a bowl towards the stellar surface, while helium burning begins in the core and further stages of nucleosynthesis take place depending on the mass. [3]
With a young age of about 10 million years (compared to about 4.6 billion years ago of our Sun), Betelgeuse is currently in the phase of helium burning, the hydrogen supply is almost completely fused to helium. The star thus produces larger elements such as carbon, oxygen and iron in its interior through the fusion of smaller atoms. In the process, the star loses large amounts of matter due to a strong stellar wind, which repeatedly carries the outer shells away into space.
It is debated whether Betelgeuse may have had a companion in the distant past whose mass (about 1 M☉) was accreted, which would explain both the high stellar mass of Betelgeuse and the conspicuously high rotational speed. [4]
Due to the repulsion of outer shell material in star winds and bowl-shaped expanding fusion zone lead to pulsation movements of the star due to gravitational effects and radiation pressure again and again. These cause the typical brightness fluctuations of such red supergiants. At some point, the radiation pressure will no longer be able to counter the ever-increasing gravity of the collapsing core, and Betelgeuse will collapse in a Type II(P) Supernova. According to the data known so far, Betelgeuse will then end up as a neutron star and release the heavy elements produced into interstellar space. However, it cannot be ruled out that stars like Betelgeuse with a mass of about 20 solar masses may also collapse into black holes. [5]
How long it will take for Betelgeuse to explode in a supernova is the subject of much study and just as much speculation. Currently, it is assumed that a supernova is not to be expected in the next 100,000 years. [6]
When Betelgeuse explodes in a Supernova, this Supernova would for about a week probably shine 10,000 to 100,000 times as brightly as the star currently does and afterwards shine as brightly as a crescent moon for months [7– Betelgeuse would also be visible in the sky during the day – it would be a great astronomical experience, to be able to observe this Supernova.
Acoording to Goldberg [7], it is also quite possible that a Pulsar (i.e. a rapidly rotating neutron star with a correspondingly large magnetic field and formation of jets) forms from the collapsed core of Betelgeuze after the Supernova, which causes the ejected remnants of the shell to glow for centuries, as in the Crab Nebula (M1/NGC1952). Neither the Supernova itself nor the resulting electromagnetic radiation (including gamma radiation) would be dangerous for the Earth and life on Earth, the distance is too great for that – fortunately for our home planet!
According to data published by Davies et al. at the end of 2022, a typical sign of an imminent type II P supernova, such as the one that will occur at Betelgeuse, is a drop in brightness of about 99%. This massive dimming results from a dense cloud of matter that forms around the star from several outbursts of matter over the course of a few months before the supernova, thus almost completely obscuring it. [8]
Such massive darkening has not yet been observed in Betelgeuse.
The Great Dimming event in 2019
The observed brightness dip (Great Dimming) of up to 60% at the end of 2019 was interpreted and discussed, especially in the press and the non-specialist media, as a possible omen of an imminent supernova. ESO observed Betelgeuse both before and during the eclipse using the Very Large Telescope (VLT) instrument SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch instrument) before and during the dimming effect. The data from this observation is remarkable and shows how much the star's appearance had changed.
In fact, however, the dimming was a huge cloud of dust and gas that was ejected randomly from the star's surface in the direction of our observation axis and apparently part of the known but still not fully understood mass loss of supergiants in the form of constantly peeling layers of the outer stellar surface as stellar wind and in the form of large bursts of matter that occur from time to time. Apparently, about 400 billion times as much mass was ejected as our Sun loses in a coronal mass outburst.6]
The astrophysicist Miguel Montargès, who worked at the LESIA (Laboratoire d'Études Spatiales et d'Instrumentation en Astrophysique) in Paris in 2022 (see also the film above) has written a summary article with the LESIA team, which was published in the renowned scientific journal Nature. [10]
The reason for the spectacular outburst of matter has also been investigated by other research groups and is classified as a short-term sequence of pulsations superimposed on long-term pulsations. Thus, in the fluctuation of its brightness, Betelgeuse shows several overlapping short- and long-term changes on the surface, that are difficult to detect. [11]
Betelgeuse is therefore a star that shows very interesting astronomical processes. which make the reddish-looking giant all the more fascinating when you observe it in the telescope. Betelgeuse, along with only a few other stars, can also be imaged as a flat surface instead of a single point from Earth with large telescopes and shows a parallax of about 50 milliarcseconds (mas).
With all this information in mind, it's all the more worthwhile to take a closer look at Orion's left shoulder as often as possible.
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Sources:
[1] https://de.wikipedia.org/wiki/Abd_ar-Rahman_as-Sufi
[2] Joyce et al., The Astrophysical Journal, 902:63 (25pp), 2020 October 10, https://iopscience.iop.org/article/10.3847/1538-4357/abb8db/pdf
[3] https://de.wikipedia.org/wiki/Roter_%C3%9Cberriese
[4] Wheeler et al., „The Betelgeuse Project: constraints from rotation“ in Monthly Notices of the Royal Astronomical Society, Volume 465, Issue 3, March 2017, Pages 2654–2661, https://doi.org/10.1093/mnras/stw2893
[5] Heger et al., „How Massive Single Stars End Their Life“ in The Astrophysical Journal, 591:288–300, 2003 July 1 https://iopscience.iop.org/article/10.1086/375341/pdf
[6] Chelsea Gohd, NASA https://science.nasa.gov/missions/hubble/what-is-betelgeuse-inside-the-strange-volatile-star
[7] Goldberg et al. 2020, „Apparent Magnitude of Betelgeuse as a Type IIP Supernova“ in Res. Notes AAS 4 35
[8] Davies et al. 2022, „Explosion imminent: the appearance of red supergiants at the point of core-collapse“ in Monthly Notices of the Royal Astronomical Society, Volume 517, Issue 1, November 2022, Pages 1483–1490 https://doi.org/10.1093/mnras/stac2427
[9] https://www.eso.org/public/germany/videos/eso2003c
[10] Montargès, M., Cannon, E., Lagadec, E. et al. „A dusty veil shading Betelgeuse during its Great Dimming“ in Nature 594, 365–368 (2021). https://arxiv.org/pdf/2201.10551.pdf
[11] Dupree et al. 2022, „The Great Dimming of Betelgeuse: a Surface Mass Ejection (SME) and its Consequences“ in The Astrophysical Journal, 936:18 (9pp), 2022 September 1
