highly complex, each cell-surface molecule 94143, USA, and in the Chan Zuckerberg rather than billions of . is a that is involved in guiding axons to their appro- Biohub, San Francisco. supergiant — a stage in the evolution of priate targets probably serves multiple such e-mail: [email protected] massive that begins when these stars functions in different circuits, depending on transition from fusing hydrogen in their cores the cellular and developmental context. It will to fusing ; this leads to the cooling and

be crucial to account for each molecule’s con- 1. Sperry, R. W. Proc. Natl Acad. Sci. USA 50, 703–710 (1963). expansion of their outer layers. The cores then text-dependent roles during the assembly of 2. Sanes, J. R. & Zipursky, S. L. Cell 181, 536–556 (2020). spend several million years fusing progres- diverse neuronal circuits. 3. Pederick, D. T. et al. 372, 1068–1073 (2021). sively heavier elements before collapsing. 4. Igarashi, K. M., Ito, H. T., Moser, E. I. & Moser, M.-B. FEBS Lett. 588, 2470–2476 (2014). These dying stars produce the spectacular Yajun Xie and Corey Harwell are in the 5. Berns, D. S., DeNardo, L. A., Pederick, D. T. & Luo, L. Nature show of a , leaving behind Department of Neurobiology, Harvard Medical 554, 328–333 (2018). neutron stars or black holes, and enriching 6. Del Toro, D. et al. Cell 180, 323–339e.19 (2020). School, Boston, Massuchusetts 02115, USA. 7. Sando, R., Jiang, X. & Südhof, T. C. Science 363, eaav7969 their surroundings as they hurl the elements C.H. is also in the Department of Neurology, (2019). made in their interiors into interstellar space. Eli and Edythe Broad Center of Regeneration Red supergiants represent an extreme stage C.H. declares potential competing interests. See go.nature. Medicine, University of California, com/3ib1kwz for details. of . They are the largest stars San Francisco, San Francisco, California This article was published online on 4 June 2021. in the Universe — for instance, Betelgeuse has a radius 900 times that of the Sun4, and if it were placed at the centre of the Solar Sys- tem, it would swallow all 4 inner and nearly reach the of . The huge cold outer layers of red supergiants pulsate, and Great Dimming of host a handful of enormous convective cells (volumes of material that move as a result of Betelgeuse explained ). Furthermore, these outer layers shed mass that can eventually form dust in the Emily M. Levesque ’s surrounding environment. Modelling the outer layers of red super- suggest that an unexpected dimming of the giants is extremely challenging, because the massive star Betelgeuse resulted from dust forming over a mechanisms driving mass loss and dust pro- cold patch in the star’s southern hemisphere. This finding duction are complex, and the effects of these various quirks on the star’s brightness, evo- improves our understanding of such massive stars. See p.365 lution and eventual death are far from clear. Nevertheless, efforts to better understand red supergiants are worth the trouble because In December 2019, reported1 and amateur stargazers alike because such these stars are key players in the cycle of stellar a surprising change in the appearance of rapid and visible changes in the are birth and death and in the chemical evolution Betelgeuse. The bright red star in the shoulder rare. Now, a after Betelgeuse’s recovery of the cosmos. of the had begun dimming from what has become known as its Great Betelgeuse’s Great Dimming was evident dramatically during the preceding two Dimming, Montargès et al.3 (page 365) pres- with the , but the observations months. In the following weeks, Betelgeuse’s ent a detailed picture of and compelling presented by Montargès et al. reveal the full rapid and unprecedented dimming contin- explan­ation for this strange behaviour. details of the star’s sudden change in appear- ued. By mid-February 2020, the star had Betelgeuse was born with about 20 times ance. Betelgeuse’s large size and close prox- plummeted to about 35% of its typical bright- the mass of the Sun4. Such massive stars imity to (about 220 , or 724 ness2 before swiftly recovering over the next evolve much faster than their lower-mass years5) make it one of only a few stars that can few months. The event captivated professional counterparts, with lifetimes of mere millions be seen as a spatially resolved disk rather

a b January 2019 January 2020





Figure 1 | Location and observations of Betelgeuse. a, Normally, Betelgeuse Dimming, in which the star was comparable in brightness to Bellatrix. The is the brightest star in the Orion constellation; is the second brightest observations show that the light loss was concentrated in Betelgeuse’s southern and Bellatrix the third brightest. b, Montargès et al.3 observed Betelgeuse hemisphere. A detailed analysis by the authors suggests that a southern dust before (January 2019) and during (January 2020) a period known as the Great cloud temporarily blocked much of the star’s light.

Nature | Vol 594 | 17 June 2021 | 343 ©2021 Spri nger Nature Li mited. All ri ghts reserved. ©2021 Spri nger Nature Li mited. All ri ghts reserved.

News & views than a single point. Using instruments on the late 2019, a combination of normal pulsa- supergiants show signs of their own Great European Southern ’s Very Large tion behaviour and a convection-driven cold Dimmings? Next-generation facilities focused in Chile, Montargès and colleagues patch in the star’s southern hemisphere began on monitoring stellar brightness over time, captured pictures of Betelgeuse both before to cool the local environment. The drop in or on studying the signatures of dust in the and during the Great Dimming. A comparison temperature allowed the recently shed gas in spectra of stars, could prove inval- of these images shows that the star hadn’t sim- that region to rapidly form dust. This process uable for expanding the lessons learnt here. ply shrunk or dimmed uniformly. Instead, the generated a dense southern dust cloud that This exquisitely detailed study of Betelgeuse’s light loss was concentrated in the star’s south- temporarily blocked much of Betelgeuse’s unexpected behaviour lays the groundwork ern hemisphere (Fig. 1). light, giving us what we saw as the Great for unravelling the properties of an entire Could the dimming be explained by a short- Dimming. population of stars. lived cold patch on Betelgeuse’s southern What does this mean for Betelgeuse? Its surface, produced by the churn of enormous behaviour in late 2019 made headlines, in part Emily M. Levesque is in the Department of convective cells? Or might a clump of dust be because of the tantalizing prospect that the Astronomy, , Seattle, blocking our view of the star, making it tempo- dimming might be a sign of the star’s imminent Washington 98195, USA. rarily seem dimmer? Some early observations demise. Betelgeuse is an irresistible target on e-mail: [email protected] concluded that the dimming was probably which to pin hopes of seeing a supernova caused by dust, and that this dust had been happen in our own Galaxy. Such events are made by Betelgeuse itself 6,7. However, other incredibly rare, with the recent obser- work presented evidence of temperature vation of a supernova pre-dating changes in the star’s atmosphere, and sug- the invention of the telescope. However, 1. Guinan, E. F., Wasatonic, R. J. & Calderwood, T. J. Astron. Telegr. 13341 (2019). gested that cooling might also have played Montargès et al. make clear that the Great 2. Guinan, E., Wasatonic, R., Calderwood, T. & Carona, D. a part in the dimming8,9. Montargès et al. Dimming does not indicate any sign of an Astron. Telegr. 13512 (2020). addressed this uncertainty by modelling both impending supernova. Importantly, they also 3. Montargès, M. et al. Nature 594, 365–368 (2021). 4. Dolan, M. M. et al. Astrophys. J. 819, 7 (2016). scenarios and comparing the models with their note that Betelgeuse might not give us much 5. Harper, G. M. et al. Astronom. J. 154, 11 (2017). images. They conclude that, although dust was warning — astronomers currently cannot pre- 6. Levesque, E. M. & Massey, P. Astrophys. J. 891, L37 (2020). the ultimate culprit, the two scenarios are not dict, at least on any realistic human timescale, 7. Dupree, A. K. et al. Astrophys. J. 899, 68 (2020). 8. Harper, G. M., Guinan, E. F., Wasatonic, R. & Ryde, N. at odds but are linked. when a star will die. Astrophys. J. 905, 34 (2020). Observations show evidence of a mass-loss The lack of an explosive conclusion might 9. Dharmawardena, T. E. et al. Astrophys. J. 897, L9 (2020). episode that began nearly a year before the seem disappointing, but Montargès and col- 10. Kravchenko, K. et al. Preprint at https://arxiv.org/ abs/2104.08105 (2021). Great Dimming7,10, releasing a clump of gas. leagues’ results go beyond explaining one Montargès and colleagues suggest that, in brief wink of a nearby star. Could other red The author declares no competing interests.

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