Stars range in color from deep red to blue-violet, and that color directly reflects their surface temperature. The coolest stars glow red at temperatures below 3,500 K (about 5,800°F), while the hottest shine blue-violet at temperatures exceeding 30,000 K. Between those extremes, stars pass through orange, yellow, white, and blue-white. Astronomers organize this progression into a spectral classification system using the letters O, B, A, F, G, K, and M, running from hottest to coolest.
The Color Sequence From Cool to Hot
Here is the full order of star colors with increasing temperature, along with the spectral class and a familiar example for each:
- Red-orange (Class M): Below 3,500 K. Betelgeuse, the bright shoulder star of Orion, and Proxima Centauri, our nearest stellar neighbor.
- Orange (Class K): 3,500 to 5,000 K. Arcturus, one of the brightest stars in the night sky, and Aldebaran in Taurus.
- Yellow (Class G): 5,000 to 6,000 K. Our Sun, with a precisely measured surface temperature of 5,772 K.
- Yellow-white (Class F): 6,000 to 7,500 K. Polaris, the North Star, and Procyon in Canis Minor.
- White (Class A): 7,500 to 10,000 K. Sirius, the brightest star in Earth’s sky, and Vega.
- Blue-white (Class B): 10,000 to 30,000 K. Rigel, the blue-white supergiant in Orion.
- Blue-violet (Class O): 30,000 to 60,000 K. The stars of Orion’s Belt. Some O-type stars have measured temperatures above 50,000 K.
A common mnemonic for the sequence from hottest to coolest is “Oh Be A Fine Girl/Guy, Kiss Me.” Since the question asks about increasing temperature, just reverse it: M, K, G, F, A, B, O.
Why Temperature Determines Color
Stars are essentially giant balls of hot gas, and like any hot object, they radiate light across a broad spectrum. As temperature increases, the peak wavelength of that radiation shifts toward shorter, bluer wavelengths. This is a fundamental principle of physics: hotter objects radiate more total energy and shift their peak output toward the blue end of the spectrum. A star at 3,000 K emits most of its light in the red and infrared range. A star at 10,000 K peaks in the blue-green range. A star at 40,000 K peaks deep in the ultraviolet.
But stars don’t emit just one color. They produce light across the entire visible spectrum, just in different proportions. A cool red star emits almost entirely red light with very little blue. A hot blue star emits plenty of red light too, but so much more blue that the overall appearance is blue-white. This broad emission is what makes the transition between star colors gradual rather than a sharp jump from one hue to the next.
Why No Stars Look Green
If you scan the sequence, you’ll notice green is conspicuously absent. That’s not because no star peaks in the green part of the spectrum. Our Sun actually peaks in blue-green wavelengths. The reason is human vision. Your eyes have three types of color receptors (cones) tuned to red, green, and blue light. When a star’s temperature puts its peak output around green wavelengths, it’s also emitting substantial amounts of red, yellow, orange, and blue. All three types of cones get stimulated roughly equally, and your brain interprets that combination as white.
This is why the Sun looks white from space, not green, despite its peak emission being in the blue-green range. (It appears yellowish from the ground because the atmosphere scatters away some of the blue light.) As stars get hotter and their peak shifts from green into blue, the blue cones start winning out and the star begins to look blue-white. There’s simply no temperature where the mix of emitted light triggers our cones to produce a green perception. Cameras designed to mimic human vision show the same result.
How Common Each Type Is
The color sequence also reveals something striking about the galaxy’s population. Cool red M-type stars make up roughly 76.5% of all main-sequence stars. They’re small, dim, and long-lived, which is why so many have accumulated over the history of the Milky Way. Orange K-type stars account for about 12%, and yellow G-type stars like our Sun represent around 7.6%. From there the numbers drop sharply: F-types make up 3%, A-types 0.6%, B-types 0.13%, and the hottest O-types a mere 0.00003%. The hottest, most massive stars burn through their fuel fastest, living only a few million years compared to the trillions of years a red dwarf can persist.
Below the Main Sequence: Brown Dwarfs
The M class isn’t quite the end of the temperature scale. Below the threshold for sustained hydrogen fusion lie brown dwarfs, objects that blur the line between stars and giant planets. Astronomers have extended the classification with three additional letters:
- L-type: Roughly 1,300 to 2,400 K. These glow a deep, dim red and are too faint to see with the naked eye.
- T-type: About 700 to 1,300 K. Interestingly, T-type brown dwarfs appear slightly bluer than L-types because clouds of dust and metal clear from their atmospheres, and methane absorption reshapes their emitted light.
- Y-type: Below 700 K. The coolest known examples have surface temperatures comparable to a warm oven, radiating almost entirely in the infrared.
These objects are invisible to the unaided eye and require infrared telescopes to detect, but they complete the temperature scale below the traditional seven-letter sequence.
Reading Star Color in the Night Sky
You can actually see this temperature sequence on any clear night. Betelgeuse in Orion has an obvious reddish-orange tint, marking it as a cool M-type star. Arcturus, high in the spring sky, glows distinctly orange. Sirius blazes a brilliant white, and Rigel, in the opposite corner of Orion from Betelgeuse, shines blue-white. The contrast between Betelgeuse and Rigel in the same constellation is one of the easiest ways to see the relationship between color and temperature for yourself. The redder the star, the cooler its surface. The bluer, the hotter.