Have you ever noticed that stars seem to twinkle and shift colors? Sometimes they looked red, then blue, then white. This intriguing phenomenon is one of the many fascinations in stargazing. But why does this happen? Are the stars themselves changing colors? In this post, we’ll go through why stars seem to change colors when you observe them.

Earth’s Atmospheric Refraction

The Earth’s atmosphere is the main reason stars appear to change colors. The atmosphere is not only uniform but also unstable. It’s made up of layers of gases that are constantly in motion. What happens is when starlight travels through these layers, it gets bent/refracted, and scattered in different directions. This process is called atmospheric refraction.

This effect can be seen drastically at the horizon. When the light from a star passes through the thick layers of the atmosphere near the horizon, it gets distorted even more. Different wavelengths of light (which can be seen in colors) bend by different degrees. Therefore, the star may appear to “flicker” between red, blue, and white as the light moves through the air. You also can imagine the atmosphere acts like a moving prism. These prisms break up the star’s light into its component wavelengths/colors.

How Does Light Scattering Create Colors?

As mentioned before, the starlight changing color perception depends on two physics concepts – light refraction and scattering. As the starlight enters the atmosphere, the different air densities with different refractive indices refract light rays at varied angles. Meanwhile, the air molecules, dust, and other particles in the atmosphere scatter the light.

This scattering effect is why the sky appears blue during the daylight with shorter wavelengths (like blue and violet) scattering more than longer wavelengths (like red and orange). Different parts of the star’s light spectrum are bent in other ways. Therefore, the star momentarily appears in various colors as the different wavelengths hit your eyes.  

The Science Behind Specific Colors

When you see a star twinkling with different colors, you observe how atmospheric conditions bend and scatter different parts of its light spectrum. Here’s a quick explanation of what you might notice:

• Red Twinkling: This usually means the starlight passes through more atmosphere, as red wavelengths are less affected by scattering. This is more common near the horizon.
• Blue or White Twinkling: Shorter wavelengths, like blue or white, are scattered more easily by the atmosphere, so a star might appear bluish when there’s less interference from thicker atmospheric layers.
• Flashes of Green or Yellow: These colors are rarer but can sometimes appear when the atmosphere scatters intermediate wavelengths. Brief moments of green or yellow can happen when different air pockets bend the light just right.

Why Stars Near the Horizon Change Colors More?

Stars closer to the horizon tend to twinkle and change colors more dramatically than those high in the sky. It’s simply because when you look toward the horizon, starlight has to pass through a thicker layer of atmosphere. As the light travels through more air, it passes through more atmospheric turbulence and particles. Thus more scattering and refraction happened.

The thicker atmosphere filters out shorter wavelengths (like blue and violet) more effectively, leaving behind the longer, redder wavelengths. This same process is why the sun appears reddish-orange at sunrise or sunset. On the other hand, light from stars high overhead has less atmosphere to pass through. Hence, their light undergoes less refraction and scattering, making them appear steadier and more consistently colored.

Summary

The color-changing stars in our night sky are truly fascinating. While the stars themselves aren’t shifting colors, the atmospheric effects make them appear to do so. This mechanism adds more fun to the stargazing experience. By understanding the science behind it, we will appreciate more the beauty of the night sky.