Chapter 1: Betelgeuse's Final Stage

The supergiant Betelgeuse is nearing the end of its life cycle. While we cannot pinpoint the exact moment, it is destined to undergo a supernova explosion, which will obliterate it, leaving just a small, dense remnant—a neutron star. Until that magnificent event unfolds, several intriguing questions remain, particularly regarding its unusual rotational speed. Researchers have proposed a captivating theory involving a stellar cannibalism event that may have occurred around 100,000 years ago.

Section 1.1: Understanding Betelgeuse's Characteristics

Currently in the red supergiant phase, Betelgeuse exemplifies a typical evolutionary stage for stars with masses ranging from 10 to 40 times that of our Sun. This stage is marked by an expansive stellar envelope resulting from shifts in energy generation processes that signal the star's departure from the main sequence. Many astronomers agree that Betelgeuse is ascending the supergiant branch for the first time. Its energy production now relies on the fusion of helium into carbon and oxygen within its core, alongside hydrogen fusion into helium occurring in an outer shell.

The classification of Betelgeuse has placed it within the spectral types M1-M2Ia-Iab since the adoption of the Morgan-Keenan system. Variations in its spectral type reflect the star's intrinsic variability. Overall, Betelgeuse is characterized as a luminous red supergiant.

Subsection 1.1.1: The Challenge of Mass Estimation

Estimating Betelgeuse's mass presents a significant challenge. Despite numerous investigations, no binary companion has been confirmed, complicating mass calculations based on orbital parameters. Thus, astronomers must rely on stellar evolution models to deduce its mass from observable characteristics.

In 2008, Michelle M. Dolan and colleagues estimated Betelgeuse's mass at 16 ± 2 solar masses using these models. A subsequent study published in 2016 in The Astrophysical Journal provided a comprehensive identity card for Betelgeuse, integrating observational data and model-derived parameters. They calculated an initial mass of around 20 solar masses, with a margin of uncertainty.

Key parameters from this study include:

• Distance: 197 ± 45 parsecs
• Radial velocity: 21.91 ± 0.51 km/s
• Diameter: 887 ± 203 solar diameters (approximately 1.23 ± 0.28 × 10⁹ km)
• Luminosity: 125,900 solar luminosities (4.845 × 10³¹ watts)
• Effective temperature: 3,500 ± 200 K
• Rotational velocity: 5 km/s (inclination angle: 20 degrees)
• Rotational period: 8.4 years
• Composition: 70% hydrogen, 28% helium, 2.4% other elements
• Current mass: 19.4 solar masses (3.86 × 10³¹ kg)
• Mass loss: (2 ± 1)×10⁻⁶ solar masses/year
• Surface gravity: 0.3 cm/s²
• Age: 8−8.5 million years
• Expected demise: Type II-P supernova

These figures illustrate Betelgeuse's extraordinary nature. Its diameter is between 1 and 1.5 billion kilometers; if it occupied the Sun's position, its photosphere would extend nearly to Jupiter's orbit. With brightness exceeding 125,000 times that of the Sun, Earth would need to be about 55 billion kilometers away to receive the same radiation it currently gets from the Sun.

Additionally, Betelgeuse experiences a remarkable mass loss, approximately eight orders of magnitude greater than that of the Sun. The red supergiant loses around two-millionths of a solar mass annually through stellar wind, equating to nearly 4 × 10²⁴ kg—about 66% of Earth's mass—meaning it sheds enough material equivalent to two Earth-sized planets every three years!

In summary, Betelgeuse's attributes are vastly disproportionate compared to typical stellar measures, calibrated against the Sun's characteristics. This immense power comes at a cost: time. Betelgeuse consumes its nuclear fuel at an astonishing rate, far faster than the Sun. Consequently, with an age of merely 8.5 million years—537 times younger than the Sun (4.57 billion years)—it is rapidly approaching its end, expected to culminate in a catastrophic supernova explosion.

Chapter 2: The Imminent Demise of Betelgeuse

The core of Betelgeuse is accumulating substantial amounts of carbon and oxygen, the byproducts of helium fusion, making up 40-50% of its core mass. This area contains approximately three solar masses, with a density reaching 1,000 g/cm³, compared to the Sun's core density of 150 g/cm³. The core temperature soars to around 100 million K, starkly higher than the Sun's 15.6 million K.

The outermost core region consists of residual helium, also weighing around three solar masses. Once this helium is exhausted and transformed into carbon and oxygen, Betelgeuse will enter a critical phase lasting a few thousand years, where core pressure and temperature will ignite carbon fusion, followed by neon, oxygen, and eventually silicon.

Once silicon fusion ends, the core will experience rapid gravitational collapse, leading to a supernova explosion that will obliterate the star's outer layers, ejecting them into space. Betelgeuse will be left as a compact neutron star with a mass of roughly 1.5 solar masses.

This supernova will unleash immense energy: 2.0 × 10⁴⁶ joules in neutrinos and 2.0 × 10⁴⁴ joules of kinetic energy. Observers on Earth will witness a dazzling explosion reaching a visual magnitude of approximately −12.4, brighter than a full Moon. While a barrage of X-rays and gamma rays will reach Earth, they won't be potent enough to penetrate our atmosphere.

The shockwave, however, will travel more slowly, reaching our Solar System six million years post-explosion. Plasma particles from Betelgeuse will compress the heliosphere but won't breach Earth's atmosphere. They will meet the solar wind at around 2.5 astronomical units from the Sun, well beyond Earth’s orbit.

In essence, Betelgeuse's explosive demise poses no threat to life on Earth. Instead, it will offer a breathtaking celestial display and a significant opportunity for future astronomers to gain insight into stellar evolution. Despite recent fluctuations in brightness, predictions suggest Betelgeuse will explode in about 100,000 years, a mere blink in astronomical terms.

Chapter 3: The Mystery of Fast Rotation

The investigation into Betelgeuse's evolutionary stage isn't unique to Dolan's team. Another research group, led by J. Craig Wheeler from the University of Texas at Austin, has also delved into this matter. Their study, published in November 2016 in the Monthly Notices of the Royal Astronomical Society, examined evolutionary tracks for Betelgeuse with initial mass values between 15 and 25 solar masses using a computational program called MESA (Modules for Experiments in Stellar Astrophysics).

Unlike the previous study, Wheeler's team factored in rotational effects. They derived the rotational velocity from spectroscopic observations conducted in the 1990s with the Hubble Space Telescope. Their findings indicated that Betelgeuse rotates at the equator at a speed of 15 km/s—substantially faster than the Sun's equatorial speed of just over 2 km/s.

Size plays a crucial role in this context. Each star has an inherent rotational speed at birth based on the conservation of angular momentum of the material in its originating molecular cloud. As Betelgeuse expands into the red supergiant phase, its size increases dramatically, necessitating a decrease in rotational speed according to angular momentum conservation.

Despite this, observational data suggest Betelgeuse rotates 150 times faster than models predict. This discrepancy presents a genuine astrophysical puzzle.

One possible resolution is that the rotation measurements are fundamentally flawed. It could be that the observed Doppler effect pertains to circumstellar material rather than the star itself. Alternatively, existing models like MESA may inaccurately portray the physics of a star's inner layers, particularly regarding viscosity.

Another theory posits that Betelgeuse is undergoing a brief transitional phase—lasting up to a thousand years—where its core transfers angular momentum to its outer layers. However, this scenario seems highly improbable.

The hypothesis of stellar cannibalism offers another intriguing explanation for Betelgeuse's high rotation speed. If Betelgeuse had a binary companion that it "devoured," the angular momentum from the consumed star would enhance its rotation speed.

Evidence supporting this theory includes a far-infrared image of Betelgeuse captured by ESA's Herschel space telescope in 2013, displaying a broad arc of material ahead of the star. While initially interpreted as a shock wave from Betelgeuse's stellar wind compressing surrounding gas and dust, Wheeler's team suggests it might instead represent the remnants of a consumed companion star.

This arc, moving away from Betelgeuse, could indicate the star's "meal" has been expelled over time. Assuming an expulsion velocity of about 10 km/s, the arc's position could correlate with a merger event that occurred approximately 100,000 years ago.

Additionally, a ring of matter just four arc minutes from Betelgeuse may signify this celestial banquet, potentially dating back to 24,000 years ago.

Despite these hypotheses, some features in the Herschel image remain unexplained, indicating that more research is needed to determine whether Wheeler's theories align with reality. One thing is clear: Betelgeuse's rapid rotation requires a substantial explanation beyond the ordinary.

Notes

[1] One solar mass equals 1.989 × 10³⁰ kg.

[2] The luminosity figure of 125,900 solar luminosities is based on a distance of 197 parsecs. If Betelgeuse were further away, its brightness would increase accordingly.

[3] An astronomical unit represents the average distance from Earth to the Sun, roughly 150 million kilometers.

[4] A study by Pierre Kervella et al., published in 2018, suggests Betelgeuse's rotational speed is significantly lower than indicated in Wheeler's research, with a minimum equatorial speed of 5.47 ± 0.25 km/s.

The constellation Orion captured in an image from the DSS2 survey. Betelgeuse shines brightly as the orange star in the upper left corner.