What if our Sun were a different color?

Look at the stars at night. Some look a little red. Some look a little blue.

What if our Sun were red? What if it were blue? What would change?

On a clear night, look carefully at the bright stars. Some have a reddish or orange glow, and others look bluish-white. NASA explains that stars look different colors because they are different temperatures.

So what if our own Sun were a different color? What would that tell us about it, and what might it mean for the plants that live on its light? This is a thought experiment: choose a star and explore what scientists know and what they think might happen.

Stars range from cool red dwarfs to very hot blue stars, and color is the visible signature of temperature. NASA notes that a hotter Sun would shift its peak emission toward blue or violet, and a cooler one toward orange or red.

This thought experiment swaps our Sun for a red star and a blue star in turn. Facts come from NASA's descriptions of stellar types and temperatures; what might happen to plant life is drawn from published NASA-led modeling and is labeled as an idea.

Our Sun looks yellow in the sky. But in space, it would look white!

Sunlight is really all the colors of the rainbow mixed together.

NASA Space Place explains that sunlight looks white but is really made of all the colors of the rainbow; a prism can split them apart. From space, astronauts would see the Sun as white.

From the ground, our air scatters some of the Sun's blue light across the sky, which is why the sky is blue and the Sun can look a little yellow. The Sun's surface is about 5,800 kelvins, and that temperature is what sets the mix of colors it gives off.

NASA Goddard's Lou Mayo explains that the Sun emits energy at all wavelengths, peaking around 500 nm in the blue-green, a peak set by its surface temperature of about 5,800 K. Because it also emits strongly across the whole visible range, our eyes combine the light into white: seen from above the atmosphere, the Sun appears white.

On the ground, the atmosphere scatters shorter blue wavelengths more efficiently, removing some blue from the direct beam, so the Sun is perceived as more yellow.

A red star is cooler than our Sun. A blue star is much, much hotter.

So a red Sun would mean a cooler star. A blue Sun would mean a hotter one.

If our Sun were red, it would be a cooler star: NASA says red stars have surfaces around 3,000 kelvins, about half as hot as the Sun. Most red stars are small ones called red dwarfs, and they are the most common stars in our galaxy.

If our Sun were blue, it would be a much hotter star, with a surface that can be over 30,000 kelvins. Either way, the light reaching Earth would have a different mix of colors. NASA scientists think that could change which colors plants use.

A red Sun would be cooler, around 3,000 K by NASA's figure. Most such stars are red dwarfs: NASA describes them as the smallest and coolest main-sequence stars, a fraction of the Sun's size and mass, making up around 75% of the Milky Way's stars, with low-mass examples possibly lasting up to about 14 trillion years.

A blue Sun would be a far hotter star, over 30,000 K. A planet's surface light depends on the star's spectrum and on how the planet's atmosphere filters it. Kiang and colleagues modeled that combination for Earth-like planets around other stars and concluded that the dominant color of photosynthesis would differ from planet to planet.

Think of a stove that gets hot. First it glows red. Hotter, it glows orange.

Stars are like that. Red ones are cooler. Blue-white ones are hotter.

Anything hot enough glows. NASA uses a stove as an example: when an electric stovetop heats up, its coils glow red at first, then orange, then brighter orange. The center of a candle flame, hotter still, even glows blue.

Stars glow the same way. A cooler star gives off more red light, and a hotter star more blue light. So a star's color works like a thermometer we can read from far away.

Hot objects emit a continuous spectrum whose peak wavelength shortens as temperature rises; NASA's Lou Mayo notes that a hotter Sun would peak in the blue or violet, or even the ultraviolet, and a cooler one in the yellow, orange or red. Astronomers therefore use color as a temperature gauge.

NASA's Ask an Astrophysicist gives the hottest stars as blue-white with surfaces above 50,000 K and the coolest as red, below 4,000 K, while APOD quotes about 3,000 K for red and over 30,000 K for blue: round figures for a continuous range. The Sun sits near the middle.

Plants on Earth are green. They soak up red and blue light.

Scientists think plants on planets around other stars might be other colors, maybe yellow or red!

NASA explains that chlorophyll, the stuff plants use to capture light, absorbs mostly blue and red light and less green, so leaves look green. Plants on Earth are tuned to the light that reaches the ground here.

In 2007, NASA scientist Nancy Kiang and her team worked out what starlight would look like on the ground of planets around other kinds of stars. They found that green, yellow or even red plants might grow on such planets. Nobody has seen alien plants. It is an idea scientists hope future space telescopes can test.

Kiang (NASA Goddard Institute for Space Studies) and Virtual Planetary Laboratory colleagues calculated the stellar light at the surface of Earth-like planets whose atmospheres are consistent with their star types. They concluded that green, yellow or even red-dominant plants may live on extrasolar planets, and that the dominant photosynthesis might even be in the infrared.

Their reasoning starts from Earth: chlorophyll absorbs blue and red more than green, and land plants use red light abundantly reaching the surface. The work is modeling, not observation; NASA's Victoria Meadows said it would help guide designs for future telescopes that study whether such planets are habitable.