SUPERMASSIVE stars: fact or fiction?

Have you ever looked up at the night sky and wondered how massive can stars be? Or what is the biggest star out there? Is there even a limit to how massive they can be? Well we can try and answer this question in two ways - first with theory, with our understanding of how stars fuel themselves with nuclear fusion fusing hydrogen into helium to produce energy which can counteract gravity and how that all contributes. And then secondly by surveying the stars we can see in the sky, counting how many of each mass we see to work out if there is an obvious cut off in mass that we see. And when we do that we get a distribution of that drops off towards 150 times heavier than the Sun, suggesting that is the limit. But is that just for stars forming now, what about stars in the early Universe when conditions were very different? Could they be much heavier? Could they be supermassive stars...? Abbott et al. (1981) - https://articles.adsabs.harvard.edu/pdf/1981ApJ...250..645A Bestenlehner et al. (2020) - https://arxiv.org/pdf/2009.05136 Crowther et al. (2016) - https://arxiv.org/pdf/1603.04994 De Becker et al. (2006) - https://arxiv.org/pdf/astro-ph/0606379 Figer (2005) - https://arxiv.org/pdf/astro-ph/0503193 Hosek et al. (2019) - https://arxiv.org/pdf/1808.02577 Wang et al. (2024) - https://arxiv.org/pdf/2212.04476 Zwick et al. (2025) - https://arxiv.org/pdf/2507.22014 My previous video on the history of figuring out the most massive star limit - https://www.youtube.com/watch?v=TSQS4OGCVD4 Alexander Hager lecture slides on "Stars and Stellar Evolution", Monash University (derivation of Eddington limit) - https://2sn.erc.monash.edu/Class/AST-4001-2008-S2/notes/Lecture_02.pdf 00:00 Introduction 01:34 The science behind the 150 MSun limit 05:03 The nearby stars that break that limit 10:07 The supermassive stars that might exist in the early Universe 12:17 Bloopers --- 📚 My new book, "A Brief History of Black Holes", out NOW in hardback, paperback, e-book and audiobook (which I narrated myself!): http://lnk.to/DrBecky --- 👕 My new merch, including JWST designs, are available here (with worldwide shipping!): https://dr-becky.teemill.com/ --- 🎧 Royal Astronomical Society Podcast that I co-host: podfollow.com/supermassive --- 🔔 Don't forget to subscribe and click the little bell icon to be notified when I post a new video! --- 👩🏽‍💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford (Christ Church). I love making videos about science with an unnatural level of enthusiasm. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars. http://drbecky.uk.com