Author Credit: Alex Masegian, University of Chicago '23
Sitting high in the left shoulder of the Orion constellation, the massive star known as Betelgeuse is nearing the end of its life. As one of the largest and most luminous stars in the sky, Betelgeuse has been identified as an M1-2 type red supergiant star [1]. When it dies, it will undergo a catastrophic process known as a supernova, flinging its outer layers into space in a massive explosion and leaving behind a core so dense that it will likely become a black hole. The only question is when that supernova will occur — a question that, over the course of the past year, astrophysicists thought was on the verge of being answered.
The changing surface of fading Betelgeuse. Source: NASA, 2020
Betelgeuse is known to be a semi-regular variable star, meaning that its brightness fluctuates periodically on multiple timescales. Because it is so bright, these fluctuations are often visible to the naked eye and were noticed even by the earliest of astronomers. In fact, the first recorded instance of Betelgeuse’s variability dates back to 1836. The event was described in Sir John Herschel’s 1849 Outlines of Astronomy, who wrote, “The variations of Alpha Orionis, which were most striking and unequivocal in the years 1836-1840, within the years since elapsed became much less conspicuous” [2]. By piecing together observations like Herschel’s and more modern data, astrophysicists have deduced that Betelgeuse primarily pulsates on a timescale of almost 425 days, with secondary periods of 100-180 days and 2,153 days (5.9 years) [3]. This means that slight fluctuations in the star’s brightness are both commonplace and expected — within the confines of the expected pattern, of course.
Over the course of the past year, however, the massive star’s brightness has been fluctuating in ways that defy this usual pattern. In October of 2019, Betelgeuse began to dim at a point in its cycle where it normally would have been bright. Though the change was not noticeable at first, by December of 2019 the supergiant had lost over two-thirds of its usual brilliance, enough to reduce it from one of the top ten brightest stars in the sky to the twenty-first. [3] The star’s dimming was the most severe since precise monitoring of its atmosphere began nearly a century ago, and it lasted much longer than would be normal for the star’s typical cycle. (If the dimming was just a product of two of the star’s cycles overlapping at their minimum points, for instance, it would have only lasted a few weeks rather than several months.) Astronomers around the world began to take notice. Was Betelgeuse on the verge of supernova?
Little is known about how massive stars behave in the years leading up to their explosive deaths. Though historical evidence indicates that humanity has witnessed supernovae before, the last such event to occur in our galaxy was in 1604, long before the advent of modern telescopes and observing technology. [4] Even though we have never been able to observe a star during its final moments before the supernova, astrophysicists have made predictions as to what some of the early warning signs could be. One of those possible signs is what astrophysicist Sarafina Nance calls “insane and violent mass loss.” [5] In theory, a dying star will shoot a large portion of its mass into space right before its death, which could cause the star to appear dimmer as clouds of ejected dust block its light from reaching the Earth. Betelgeuse’s sudden and significant dimming, therefore, seemed to be a sign that the red supergiant was in the throes of death.
As astronomers scrambled to develop theories for what could be causing the star’s abrupt change, Harvard-Smithsonian astrophysicist Andrea Dupree and her team turned to the Hubble Space Telescope, which they started using to monitor Betelgeuse in early January of 2019. They were able to isolate several months of ultraviolet-light spectroscopic observations of the star in the time leading up to its dimming, and analysis of the data revealed signs of dense, hot material moving through the star’s atmosphere in the months of September, October, and November 2019. [6] Traveling at nearly 200,000 miles per hour, the material continued beyond Betelgeuse’s visible surface and out into space at around the same time that the star underwent its most significant dimming. Dupree theorizes that once the material had separated from the incredibly hot stellar surface, it was able to cool enough to form a large gas cloud, which would have blocked a large portion of Betelgeuse’s light and made it appear much dimmer to us here on Earth. [6] Meanwhile, the star’s normal pulsational cycle continued as usual, the behavior of the star’s outer atmosphere returning to normal even as the dust lingered.
It is not yet understood what caused the stellar outburst, though Dupree and her colleague Klaus Strassmeier of the Leibniz Institute for Astrophysics in Potsdam think that it may have been a result of the star expanding in its pulsation cycle at the same time that a pocket of material underneath the star’s surface was experiencing an upwelling. The surge of force accompanying Betelgeuse’s expansion could have propelled the hot plasma in the convection cell outward, providing it with enough momentum to escape the star’s atmosphere. Though the star has since stabilized and seems to have returned to its normal pulsational cycle and brightness, the mass ejection that Dupree observed has been found to contain nearly twice the amount of material that is typical of a Betelgeuse outburst. [6] The question of whether or not it is on the brink of supernova remains an open one.
When Betelgeuse eventually does reach the end of its life, the resulting explosion will be bright enough to be visible during the day and cast shadows at night here on Earth. Though our planet is far enough away from the massive star to avoid the majority of the radiation that the supernova will produce, the afterimage of the supergiant’s death will linger in the sky for months, serving as a stark reminder of the vastness and beauty of our universe. The star’s odd behavior this past year might not be the signal we’re looking for to indicate Betelgeuse’s imminent death, but it certainly is a sign that the star is growing more unstable — and, whenever it does finally explode, its death will undoubtedly be one of the most exciting astrophysical events of the millennium.
References
[1] Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71:245. DOI: 10.1086/191373.
[2] Davis, Kate. Alpha Orionis (Betelgeuse). American Association of Variable Star Observers. <https://www.aavso.org/vsots_alphaori>
[3] King, Bob. (2019 December 21). Betelgeuse is Dimming… Why? Sky & Telescope. <https://skyandtelescope.org/observing/fainting-betelgeuse/>
[4] Vink J. (2016) “Supernova 1604, Kepler’s Supernova, and Its Remnant.” In: Alsabti A., Murdin P. (eds) Handbook of Supernovae. Springer, Cham. DOI:10.1007/978-3-319-20794-0_49-1
[5] Drake, Nadia. (2019 December 26). A giant star is acting strange, and astronomers are buzzing. National Geographic. <https://www.nationalgeographic.com/science/2019/12/betelgeuse-is-acting-strange-astronomers-are-buzzing-about-supernova/>
[6] Dupree, Andrea et al. (2020). “Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse.” The Astrophysical Journal. 899:1. DOI: 10.3847/1538-4357/aba516