Betelgeuse: Over 900 Times the Sun, the Star That Will Explode as a Supernova

Betelgeuse: The Hunter's Shoulder

General characteristics

Betelgeuse, also called Alpha Orionis, is one of the most massive and luminous stars visible to the naked eye. Located about 640 light-years from Earth in the constellation Orion, it marks the left shoulder of the celestial hunter. This red supergiant has a radius of about 900 times that of the Sun, easily engulfing Mars' orbit if placed at the center of our solar system. Its mass, estimated between 10 and 20 solar masses, places it among the giants that will end their lives in a cataclysmic event: a supernova (core collapse of a massive star).

Betelgeuse: Internal structure and instabilities

As a red supergiant of spectral type M1-2 Ia, Betelgeuse has left the main sequence for several hundred thousand years. It has already exhausted hydrogen in its core, halting the central proton-proton fusion. Gravity then contracted the core, increasing its temperature until helium fusion into carbon and oxygen via the triple-alpha process began. Around this helium‑burning core, concentric shells host fusion reactions of increasingly lighter elements as one moves outward: carbon, neon, oxygen, then layers of residual hydrogen at the periphery. This onion‑skin structure is characteristic of massive stars in their late stages.

Convective phenomena and pulsations

The extreme thermal gradients between these layers generate powerful convective currents, transporting not only energy but also heavy elements to the surface. These convective cells, colossal in size (sometimes equivalent to a quarter of the star's radius), induce dynamic variations in the photosphere, visible from Earth as irregularities in brightness. Furthermore, Betelgeuse exhibits semi‑regular pulsations due to temporary hydrostatic imbalances between radiation pressure and gravity, causing variations in radius and effective temperature over periods of a few hundred days.

Betelgeuse: A Strange Stellar Dimming

The Great Dimming

The spectacular dimming observed between late 2019 and early 2020, called the Great Dimming, highlighted the complexity of atmospheric phenomena in these supergiants. Two main explanations have been advanced: on one hand, a local temperature drop linked to extreme convection or an internal shock wave that temporarily modified the effective photospheric temperature; on the other hand, the ejection of a dense, dusty gas plume that, condensing into silicates at low temperature, temporarily obscured part of the stellar disk by absorbing visible light. This latter hypothesis is supported by infrared and polarimetric observations that revealed a dust cloud forming near the line of sight.

Causes and interpretations

This erratic behavior is typical of stars close to gravitational collapse. With an initial mass estimated between 15 and 20 solar masses, Betelgeuse is destined to end its life as a type II supernova, perhaps within the next 100,000 years. Monitoring its brightness variations therefore offers a unique window into the final stages of massive star evolution, a phase where plasma physics, shell fusion dynamics, and radiative instabilities combine to produce spectacular and still imperfectly understood phenomena.

Betelgeuse: An Explosive Fate as a Supernova

Supernova scenario

Betelgeuse's fate is sealed: when iron accumulates in its core, preventing any further exothermic fusion, gravitational pressure will take over. In a fraction of a second, the core will collapse, generating a shock wave that will blast the outer layers into a type II supernova. This explosion will release as much energy as an entire galaxy over several days. If it occurs within the next millennia, the event would be visible even in broad daylight from Earth, without posing any danger, as Betelgeuse is too far for radiation to significantly affect our biosphere.

Betelgeuse: A Window into the Final Stellar Breaths

Observations and modelling

Studying Betelgeuse helps us better understand the final phases of massive stars. Thanks to interferometry, infrared observations, and hydrodynamic modelling, astrophysicists probe the interior of this red giant and its convective, pulsational, and mass‑loss phenomena. Betelgeuse teaches us that even the apparently most stable stars are subject to colossal internal dynamics, heralding a violent but also formative transformation, as supernovae enrich the interstellar medium with heavy elements – ingredients for life.

Calculating Betelgeuse's Radius by Interferometry

One of the most direct methods to estimate the physical radius of a nearby star like Betelgeuse relies on optical interferometry. This technique measures the star's angular diameter, i.e., the angle under which its diameter is seen from Earth. Combining this data with the star's distance yields an estimate of its true radius using purely trigonometric geometry.

Formula used

The radius R of the star is obtained by the relation:

R = (d × θ) / 2

• d is the distance to the star (in metres),
• θ is the apparent angular diameter of the star (in radians).

Observational data for Betelgeuse

• Distance d ≈ 197 pc = 6.07 × 10¹⁸ m (Gaia DR3 measurement)
• Angular diameter θ ≈ 42 mas = 2.04 × 10⁻⁷ rad (obtained by optical interferometry)

Calculation of Betelgeuse's radius

R = (6.07 × 10¹⁸ m × 2.04 × 10⁻⁷) / 2 ≈ 6.2 × 10¹¹ m

This radius corresponds to approximately:

R / R_☉ ≈ (6.2 × 10¹¹) / (6.96 × 10⁸) ≈ 891 R☉

Interferometric analysis places Betelgeuse's radius at about 890 times that of the Sun. This value, very consistent with other indirect methods, confirms the red supergiant status of this star, whose diameter is comparable to Jupiter's orbit in the Solar System.

Comparative sizes of some giant and supergiant stars

Stellar size comparison

Our Sun is really tiny compared to some stars. Planets are mere dust compared to the blue and red giants of our universe. In this YouTube video, the relative sizes of planets and stars are shown from smallest to largest. The video first shows our Moon, the planets of our solar system ordered by increasing size, then the Sun. Next, the largest stars of our galaxy parade. Their approximate sizes were calculated from their luminosities, temperatures (deduced from their colours), and distances.