How Big Can Stars Get? Exploring the Universe's Giants from Bellatrix to UY Scuti

Overview

Join host Alex McColgan as he guides viewers from our Sun to the universe’s most colossal stars. Using the Hertzsprung Russell diagram, the video explains how a star’s temperature, brightness and mass determine its life path, from stable hydrogen fusion on the main sequence to dramatic expansions as red giants and red supergiants. Along the way we meet famous giants such as Bellatrix, R136A1, Betelgeuse, UY Scuti, and WOH G64, and learn why heftier stars burn hotter and live shorter lives. The episode emphasizes that some stars become enormous through mergers, discusses the Hayashi limit that caps size, and notes the challenges of measuring distant giants. It’s a reminder of our small place in a vast cosmos and the value of curiosity.

Introduction to Cosmic Giants

The video opens with a reminder that the universe is vast beyond human experience and that within it exist stellar monsters that push the limits of scale. It grounds the discussion in the Hertzsprung-Russell diagram, explaining how it plots stellar temperatures against luminosities and how most stars reside on the main sequence while still differing in mass, brightness and lifetime.

The Main Sequence and Mass

We learn that the Sun sits in the middle of its class, a G-type main sequence star, and that stellar mass correlates with brightness. A bright, hot star like Bellatrix is introduced as an example of a high mass, high temperature star, whose larger mass drives stronger core fusion and reveals why such giants burn through fuel quickly and have shorter lifetimes than the Sun.

Behemoths Born of Mass and Merger

The narrative then shifts to the idea that the heaviest stars are limited by instability, but that binary mergers can create objects far larger than solitary accretion would allow. The candidate R136A1 is highlighted as potentially the most massive and luminous star known, with estimates placing its mass around 265 solar masses and a radius far larger than the Sun. The star group to which it belongs, Wolf-Rayet stars, is described as highly luminous and extremely unstable, shedding mass through powerful winds that shape the surrounding nebulae and contribute to chemical enrichment of the cosmos.

Red Giants to Red Supergiants

As hydrogen runs out in a star’s core, it contracts and heats the interior enough to ignite helium fusion. This drives the outer layers to expand dramatically, producing red giants or, for stars above roughly eight solar masses, red supergiants. Betelgeuse is cited as a quintessential red supergiant, enormous enough to engulf inner planets if placed at the center of our solar system, while Mira A exemplifies a pulsating red giant with a radius hundreds of times that of the Sun.

Distance, Size Limits and the Hayashi Line

The video discusses how distance and temperature uncertainties complicate size measurements for distant giants like WOH G64, a red supergiant in the Large Magellanic Cloud. It also introduces the Hayashi limit, a theoretical boundary in the HR diagram that constrains a star’s maximum radius for a given mass, implying that truly gigantic stars may be rarer than their core physics might suggest.

The Ubiquity of Stellar Diversity

Beyond size, the episode emphasizes diverse stellar life cycles, from main sequence hydrogen burning to explosive ends such as supernovae. It notes that hypergiants can shed mass in dramatic bursts and contribute to the formation of stellar nurseries through their ejected material. The audience is reminded of the scale: galaxies, cosmic filaments, and the vast distances that separate these wonders from Earth.

Closing Thoughts

The host wraps by reaffirming the sense of wonder that underpins astronomy. While monsters like UY Scuti and R136A1 capture our imagination, the video stresses that even these giants are tiny in the context of the universe. The ending underscores human curiosity and invites viewers to stay engaged with space science through ongoing content and community updates.