Aldebaran and Betelgeuse are two of the night sky’s most noticeable stars, instantly recognizable by their distinct reddish-orange coloration and prominent positions in the adjacent winter constellations, Taurus and Orion. For the casual observer, they appear to be stars of similar luminosity, prompting the question: which one shines brighter? The answer is not a single, fixed value, but requires understanding how astronomers measure a star’s light.
Understanding Stellar Magnitude
To accurately compare the brightness of any two stars, astronomers rely on a system of stellar magnitudes, which quantifies light across two distinct scales. The first is Apparent Magnitude, describing how bright a star appears from Earth, influenced by its actual light output and distance. The second scale is Absolute Magnitude, a measure of the star’s intrinsic luminosity. This value standardizes the comparison by calculating how bright the star would appear if it were precisely 10 parsecs (about 32.6 light-years) away from Earth. Crucially, on both scales, a lower numerical value indicates greater brightness, meaning negative numbers represent the most luminous objects.
Aldebaran: The Reliable Giant
Aldebaran, known as the “Eye of Taurus,” is an Orange Giant star classified as K5 III. It is a relatively close neighbor, located approximately 67 light-years from Earth. Aldebaran has evolved off the main sequence, having exhausted the hydrogen fuel in its core, causing it to swell and shine with a consistently bright, steady light. Its apparent magnitude is stable, typically fluctuating only slightly between 0.75 and 0.95, a variation too small for the human eye to perceive.
Betelgeuse: The Variable Supergiant
Betelgeuse, marking the shoulder of Orion, is a Red Supergiant star of the M1-M2 Iab spectral type, residing somewhere between 500 and 700 light-years away. Unlike Aldebaran, Betelgeuse is a semi-regular variable star whose brightness changes noticeably over time, primarily following a cycle of around 400 days. This variability is due to the star’s immense size and pulsating nature as its outer layers expand and contract. The star’s wide range of apparent magnitude, from 0.0 to 1.6, is the widest for any first-magnitude star. This behavior was demonstrated by the “Great Dimming” event of 2019-2020, when its light dropped to a historic minimum of magnitude 1.6, caused by a large dust cloud formed after the star ejected surface material.
The Definitive Brightness Comparison
When comparing the two stars based on how they appear to us, the apparent brightness of Aldebaran and Betelgeuse is often very similar, but Betelgeuse is typically the brighter of the two. Aldebaran’s stable apparent magnitude is usually around 0.85. Betelgeuse, when at its brightest, can reach magnitude 0.0 and usually maintains an average magnitude of about 0.5. This means Betelgeuse is usually the visually brighter star, but during its dimmest phases, such as the 2020 event when it dropped to magnitude 1.6, Aldebaran easily surpassed it.
The true difference in their output becomes clear when comparing their absolute magnitudes, which factors out the distance. Aldebaran has an absolute magnitude of about -0.7, making it hundreds of times more luminous than the Sun. In contrast, Betelgeuse’s absolute magnitude ranges between approximately -5.6 and -6.02, indicating it is tens of thousands of times more luminous than the Sun. Therefore, while Aldebaran is a luminous giant, Betelgeuse is an intrinsically far more powerful supergiant, only appearing slightly brighter or dimmer from Earth because its greater distance significantly diminishes its light.