Rigel and Betelgeuse are two of the most luminous stars in the night sky, anchoring the constellation Orion. These stellar giants appear dramatically different: Rigel shines with a brilliant blue-white light while Betelgeuse glows with a distinct reddish-orange hue. This stark visual contrast is not an atmospheric effect, but a direct result of fundamental physical differences between the two stars. The light we perceive indicates the stars’ current physical states and their distinct evolutionary paths.
Stellar Color is Determined by Temperature
The color a star exhibits is a precise indicator of its surface temperature, a relationship governed by the physics of light emission. Stars approximate a blackbody, an object that absorbs and re-emits electromagnetic radiation based solely on its temperature. As an object heats up, the peak wavelength of the light it emits shifts toward the shorter, higher-energy end of the spectrum. This is the same principle seen when metal is heated: it first glows red, then orange, and eventually turns blindingly blue-white at its hottest.
Stars follow this pattern, with surface temperatures determining the dominant color of the light they radiate. Very hot stars emit the majority of their energy at shorter wavelengths, corresponding to the blue and ultraviolet spectrum. Stars with surface temperatures exceeding 10,000 Kelvin appear blue or blue-white. Cooler stars radiate most strongly at longer wavelengths, falling into the red and infrared spectrum, displaying a distinct red or reddish-orange coloration if their surface temperature is around 3,500 Kelvin or less.
Rigel and Betelgeuse: Applying the Temperature Rule
The color difference between Rigel and Betelgeuse is a direct consequence of their vastly different surface temperatures. Rigel, classified as a Blue Supergiant, is exceptionally hot, with a surface temperature of approximately 12,100 Kelvin. This extreme heat causes its light output to peak strongly in shorter wavelengths, resulting in its brilliant blue-white appearance.
Betelgeuse, known as a Red Supergiant, possesses a much lower surface temperature, hovering at about 3,500 Kelvin. This comparatively low temperature means the star’s peak light emission falls toward the longer, red end of the visible spectrum. The resulting reddish-orange glow is the signature of a much cooler surface, even though Betelgeuse is physically immense in size.
Why Their Temperatures Diverge: Stages of Stellar Life
The reason for the temperature and color divergence lies in the stars’ differing stages of stellar evolution, despite both beginning as massive stars. Betelgeuse has progressed significantly through its life cycle, having exhausted the hydrogen fuel in its core. Without the outward pressure from core hydrogen fusion, gravity caused the core to contract and heat up, igniting hydrogen burning in a shell surrounding the core.
This shell burning generated immense energy, causing the star’s outer layers to swell dramatically, sometimes reaching a diameter hundreds of times larger than the Sun. This massive expansion spreads the star’s total energy output over an enormous surface area, leading to a drop in surface temperature and the resulting red color. Betelgeuse is currently burning heavier elements like helium and carbon, placing it in the final phase before it is expected to explode as a supernova.
Rigel is at an earlier evolutionary stage than Betelgeuse. It has exhausted the hydrogen in its core and is likely fusing helium into carbon, making it a Blue Supergiant. Rigel’s outer envelope has not yet expanded to the immense size of Betelgeuse, allowing it to maintain an extremely high surface temperature. This hotter, blue phase is the evolutionary step massive stars experience before they swell into the cooler, red supergiant phase that Betelgeuse has already entered.