Stars range from roughly 3,000 degrees Fahrenheit on the cool end to over 100,000 degrees Fahrenheit for the hottest known examples. That enormous spread depends on a star’s mass, age, and what stage of life it’s in. A star’s color is the simplest clue to its temperature: red stars are the coolest, white stars fall in the middle, and blue stars are the hottest.
Why Color Tells You a Star’s Temperature
Every hot object emits light, and the hotter it gets, the shorter the wavelength of that light. This means cooler stars radiate most of their energy at longer, redder wavelengths, while hotter stars push their peak output toward shorter, bluer wavelengths. It’s the same reason a heating element on a stove glows red before shifting toward orange and eventually white as it gets hotter.
Astronomers formalized this relationship into a classification system using the letters O, B, A, F, G, K, and M. Each letter corresponds to a temperature range and a visible color. The system runs from hottest to coolest, which feels counterintuitive, but it reflects the history of how stars were first cataloged.
Star Temperatures by Class
Here’s how the seven main spectral classes break down:
- O-type (blue): 30,000 to 60,000 Kelvin (roughly 54,000 to 108,000°F). These are the rarest and most massive stars in the galaxy.
- B-type (blue-white): 10,000 to 30,000 K. Rigel, the brightest star in Orion, falls here with a surface temperature around 11,600°C.
- A-type (white): 7,500 to 10,000 K. Sirius, the brightest star in our night sky, is an A-type star.
- F-type (yellow-white): 6,000 to 7,500 K.
- G-type (yellow): 5,000 to 6,000 K. Our Sun sits here at about 5,500°C (roughly 10,000°F) on its surface.
- K-type (orange): 3,500 to 5,000 K.
- M-type (red): Below 3,500 K. These are the most common stars in the universe by a wide margin.
All of these temperatures refer to the surface, or photosphere. The interior of any star is vastly hotter, because that’s where nuclear fusion actually happens.
The Hottest Stars in the Universe
The most extreme surface temperatures belong to a category called Wolf-Rayet stars. These were once massive O-type stars that have shed more than half their original mass through powerful stellar winds, stripping away their outer layers and exposing their blazing cores. The Wolf-Rayet star in the system WR 140, for example, has an estimated surface temperature of 60,000 Kelvin, or about 110,000°F. That’s more than 10 times the surface temperature of the Sun.
Newly formed white dwarfs can be even hotter. When a Sun-like star dies and sheds its outer layers, the remaining core collapses into a dense, Earth-sized remnant with initial surface temperatures exceeding 100,000 Kelvin. White dwarfs don’t generate new energy through fusion, though, so they gradually cool over billions of years.
The Coolest Stars and Near-Stars
Red dwarf stars, the M-type class, can have surface temperatures as low as about 3,000 K (roughly 5,000°F). That’s cool by stellar standards, but still hot enough to sustain hydrogen fusion in their cores.
Below that threshold sit brown dwarfs, objects that formed like stars but never gathered enough mass to ignite sustained fusion. Their surface temperatures range from a few thousand degrees down to around 250 Kelvin, which is roughly room temperature on Earth. The coolest category, called Y dwarfs, blur the line between failed stars and giant planets. At 250 K, you could technically be “cooler” than some of these objects just by standing in a warm kitchen.
How Hot Is the Sun, Really?
The Sun is a useful benchmark because we know its temperatures in detail. Its visible surface sits at about 5,727 K (around 10,000°F). Its core, where hydrogen atoms fuse into helium under crushing pressure, reaches roughly 15 million K. That’s the minimum ballpark temperature needed to sustain hydrogen fusion in any star.
But the Sun has a strange quirk that still puzzles scientists. Its corona, the wispy outer atmosphere visible during a total eclipse, is hundreds of times hotter than the surface below it, exceeding 1 million K. That’s like walking away from a campfire and finding the air gets hotter the farther you go. One partial explanation comes from NASA’s IRIS mission, which detected packets of superheated material, sometimes called “heat bombs,” traveling outward from the Sun and releasing energy into the corona. Magnetic field activity on the surface likely plays a role too, but the full answer remains one of solar physics’ biggest open questions.
Big Stars Can Be Surprisingly Cool
Size and temperature don’t always move together. Betelgeuse, the famous red supergiant in Orion’s shoulder, is so enormous that if placed where the Sun is, its surface would extend past the orbit of Jupiter. Yet its surface temperature is only about 3,300°C (6,000°F), cooler than the Sun. When a massive star expands into a red supergiant late in its life, its outer layers spread across such a vast volume that they cool significantly, even as the core grows hotter.
Compare that with Rigel, also in Orion but a blue supergiant. Rigel’s surface burns at about 11,600°C, more than twice as hot as the Sun. It’s large, but far more compact than Betelgeuse, which keeps its surface temperature high. The two stars sit in the same constellation but represent opposite ends of what massive stars can look like.
Surface vs. Core Temperature
Every star has a dramatic temperature gradient from its center to its surface. The core must reach at least 13 million K to fuse hydrogen into helium. More massive stars fuse heavier elements at even higher temperatures, with the cores of the largest stars reaching billions of degrees in the final moments before they explode as supernovae.
The surface, by contrast, is the “cool” part of any star. What you see when you look at a star in the night sky, its color and brightness, reflects only this outer layer. Two stars with identical core temperatures can look completely different if one has a bloated, cool envelope (like Betelgeuse) and the other has a compact, hot surface (like Rigel). The surface is the part astronomers can measure directly, which is why stellar temperatures are almost always quoted as surface values unless stated otherwise.