Betelgeuse is the striking orange-red star visible in the shoulder of the constellation Orion, a celestial marker that has captured human attention for millennia. Its prominent color and shifting brightness make it one of the most recognizable objects in the night sky. The sheer size and unpredictable behavior of this star signal a stellar life cycle far different from that of our Sun.
The Definitive Classification: Red Supergiant
Betelgeuse is classified as a Red Supergiant, a designation that places it in a rare and short-lived category of massive stars nearing the end of their lives. It holds the spectral class M1-2 Ia-ab, where the “M” denotes its relatively cool surface temperature and reddish hue. The “Ia-ab” is a luminosity class, indicating that it is an intermediate-luminosity supergiant.
This classification means Betelgeuse has already consumed the hydrogen fuel in its core and expanded dramatically. On the Hertzsprung-Russell (H-R) diagram, which plots stars by temperature and luminosity, Betelgeuse occupies the extreme upper-right corner. This position highlights its combination of a low surface temperature—around 3,600 Kelvin—and an exceptionally high energy output.
The Red Supergiant stage is an unstable, temporary phase in the evolution of high-mass stars. Betelgeuse’s mass, estimated to be between 14 and 20 times that of our Sun, is the primary factor driving this classification. Only stars born with such immense mass evolve into a Red Supergiant.
Physical Scale and Specific Characteristics
The most remarkable feature of Betelgeuse is its colossal physical scale, a direct consequence of its supergiant classification. While its exact size varies as it pulsates, its radius is typically estimated to be between 640 and 764 times the radius of the Sun. If Betelgeuse were placed at the center of our solar system, its outer layers would extend past the orbit of Mars, and possibly even engulf the orbit of Jupiter.
Despite this enormous volume, Betelgeuse has a surprisingly low density, often described as a “red-hot vacuum.” The star’s vast expansion means its mass is spread across a tremendous volume, resulting in an average density far less than one-ten-thousandth that of ordinary air.
Betelgeuse is also a semi-regular variable star, meaning its brightness changes over time in a somewhat predictable pattern. It typically shows a main pulsation period of about 400 days, driven by massive, roiling convection cells on its surface. This variability is driven by the expansion and contraction of its outer atmosphere.
The star’s luminosity is staggering, shining with an energy output between 7,500 and 120,000 times greater than the Sun. Although its surface temperature is relatively cool, its sheer size allows it to radiate an immense amount of energy. Only about 13% of this energy is emitted as visible light, making Betelgeuse the brightest star in the night sky when observed at near-infrared wavelengths.
Current Evolutionary Status and Future
At an age of only about 10 million years, Betelgeuse is considered an old star due to its initial high mass, which forced it to burn through its fuel at an accelerated rate. It has exhausted the hydrogen in its core and is currently in the process of fusing helium into heavier elements like carbon and oxygen. This internal change caused the star to swell into its current Red Supergiant form.
The star’s instability was illustrated by the “Great Dimming” of 2019-2020, when its brightness dropped significantly. This dimming was the result of a massive ejection of plasma from the star’s surface. This ejected material cooled to form a cloud of obscuring dust that temporarily blocked the light from reaching Earth.
Betelgeuse is now in one of the final stages of its life cycle. When the helium fusion in its core eventually ceases, the core will collapse under its own gravity. This will lead to a catastrophic core-collapse supernova, known as a Type II supernova. The star’s high mass dictates that this explosive end is inevitable.
While this event is certain to occur, current models predict the supernova will happen within the next 100,000 years. This time frame is considered “astronomically soon” but does not suggest an immediate explosion. When it does occur, the resulting supernova will be visible from Earth, potentially shining as brightly as the full Moon for several weeks.