The Sun's color has puzzled humanity for ages. Is it white, yellow, or possibly even black? When we ask physicists these seemingly simple questions, their answers often delve into profound complexities.

The color of the sun is one of those seemingly simple questions that any child can ask, yet it becomes more complicated when physicists get involved. Here's the scoop: the sun's light is essentially white, but also a bit yellow, and it is considered to be close to what is known in physics as a black body.

Confused? Let’s begin with the fact that our sun is a giant sphere primarily composed of hydrogen and helium (along with several other types of atoms at very low percentages). Its radius measures 700,000 kilometers, over 100 times larger than Earth. Deep within the sun’s core, temperatures exceed ten million degrees Celsius, which is where hydrogen undergoes nuclear fusion to produce helium, releasing tremendous amounts of energy. This energy then travels outward, through hundreds of thousands of kilometers of hot gas, eventually reaching the sun’s surface, where it is emitted as various types of radiation, including visible light, infrared light, and ultraviolet radiation. The sun’s surface temperature stands at approximately 5,000 degrees Celsius, with the hot gasses on its surface emitting the radiation that we observe.

In light of this, what is the true color of the sun? Broadly speaking, the sun emits white light, that is, light comprising all the colors of the spectrum. This phenomenon is easily observable when we use a triangular prism to disperse sunlight or gaze upon a rainbow. When sunlight passes through a prism or a raindrop, it refracts, causing the different colors to bend at slightly different angles, and thus revealing the full spectrum of visible light.

The sun also emits radiation at wavelengths that are beyond the range of human vision. At longer wavelengths, beyond the color red, we encounter infrared radiation; at shorter wavelengths, beyond the color violet, we find ultraviolet radiation, which can cause burns and cancer when it hits our skin

 

The Black Sun

While the sun's light includes all colors, they don't all come in equal amounts. The sun emits radiation through a process known as black body radiation. In physics, a black body is an object that does not reflect any light but emits radiation solely due to its thermal energy. For instance, at room temperature, a piece of iron does not produce any visible light. However, if it is heated, it will transition from red to yellow, and finally to white. At temperatures reaching thousands of degrees, a piece of iron can emit energy very similar to that emitted by the sun.

In black body radiation, the intensity of each emitted color is determined solely by the body's temperature.The shape, composition, or state of matter does not affect this emission. Both an iron ingot and a giant ball of hydrogen gas emit identical radiation, although with varying intensities. By knowing the temperature, we can accurately calculate the relative intensity of each color emitted from the black body.

In the case of our sun, yellow is the predominant color emitted.  By measuring the intensity of the different colors we can accurately determine the sun’s temperature without the need to land a thermometer-equipped spacecraft on it.

Apart from the light emitted by a black body, various atoms absorb or emit specific colors, in a way unique to each substance, due to electrons transitioning between atomic energy levels. Each type of atom has its unique energy levels, resulting in absorption or emission of light at specific wavelengths. Precise measurement of the emission or absorption of these colors also makes it possible to identify the elements that make up the sun.

When measuring the color of the sun (or more precisely, its spectral composition) from space, it differs slightly from measurements taken on Earth’s surface, within the atmosphere. The ozone layer, for instance, filters out much of the ultraviolet radiation, while the atmosphere (containing dust particles that scatter light) also disperses some visible light.  The air scatters blue light more easily than red light, through a process known as  Rayleigh scattering, giving the sky its blue appearance during the day.

The sun appears slightly redder from Earth than from space, although this difference is hard to discern with the naked eye (and in any case, looking directly at the sun is dangerous). During sunrise and sunset, when the sun is near the horizon, its light travels a longer path through the atmosphere, encountering more air and dust particles. This increases the scattering of blue light, causing violet, blue, and green light to scatter more, leaving the setting or rising sun to appear orange or red. This phenomenon also explains why the moon can appear red when it rises or sets.

בשקיעה ובזריחה כמעט כל הירוק, הכחול והסגול מתפזרים, ואנו רואים בעיקר כתום ואדום | צילום: Shutterstock

At sunset and sunrise most of the green, blue, and purple light is scattered, leaving mainly orange and red hues | Photo: Shutterstock

 

The Color of the Universe

Can the color of the sun tell us something about the colors and properties of other stars (suns)? Absolutely. Measuring the intensity of each color of light emitted by other stars allows astronomers to study our universe. Like our sun, other stars are also spheres of gas that emit black body radiation, and the color of this light reveals their temperature. Red stars tend to be relatively cooler, while blue stars are notably hotter. Stars emitting more yellow light (about halfway between red light and blue light) typically have temperatures similar to our sun.

To measure the intensity of each color, astronomers aim large telescopes at distant stars or galaxies and use prisms or crystals to split the light into its spectral components. The sequence of colors obtained is known as a “spectrum,” and the technique used for this measurement is called “spectroscopy.”

In the past, astronomers observed these light spectra on a screen or by projecting them onto sensitive substances. Today, precise sensors such as those in digital cameras accurately measure the amount of light in each color. Using spectroscopy, scientists analyze the "lines" that appear on the spectrum, caused by absorption or emission from specific atoms on the star's surface, to determine its temperature and composition.