If we imagine the Sun we almost always think of a large and bright yellow disk in the sky, which illuminates our days from dawn to dusk. It is a rather deep-rooted mental image, which begins to make its way into our imagination as children, after being exposed to drawings and representations of the Sun that prompt us to look for pencils or yellow markers to insert it in our drawings; as we grow, we receive further confirmation. Books, comics, paintings, cartoons, weather icons and many objects use the same color to represent our only star: yellow. To many it is so natural to think of it in this way that they do not ask themselves: what color is the Sun really?
The most honest answer to this question is: it depends, on who observes it and how. Here on Earth its color changes according to the times of the day and the seasons, because its light is filtered by the atmosphere. To astronauts on the International Space Station, in orbit around the Earth, it always appears more or less the same color and with a less diffused brightness, precisely because of the absence of the atmosphere. To particular telescopes, which see a part of the light that we cannot see with the naked eye, it still appears different and as NASA images show, depending on the instruments we can observe a Sun with false colors, for example mint green, very different from our common experience .
Before getting to color, let’s start with the fundamentals. The Sun is the only star in our solar system, around which the Earth, seven other major planets, the dwarf planets, their satellites and a gigantic amount of minor celestial bodies, debris and dust scattered in the space between the planets orbit ( interplanetary space). The Sun is huge and rather dense: it has a diameter of almost 1.4 million kilometers (109 times that of the Earth) and a mass that alone equates to 99.86 percent of the entire mass of the solar system. Every second, 600 million tons of hydrogen melt into helium in its core.
The Sun is about 4.6 billion years old and has before it a long phase of equilibrium that will last another 5 billion years, in which it will continue to fuse hydrogen into helium releasing large amounts of energy, essential for our existence.
At the end of this phase, our star will gradually become more unstable (red giant) and larger and then shrink, exhausting every thermonuclear process. The outermost parts will create a planetary nebula, while the innermost parts will collapse on themselves forming a white dwarf, which will continue to cool for hundreds of billions of years, eventually becoming a black dwarf.
Our star is classified as a “yellow dwarf” (star of spectral type G and luminosity class V), and this could mislead some about its actual color and how it may appear depending on how it is observed.
The color of the Sun
If we are based on the surface temperature (over 5,500 ° C), we can say that the Sun has a very intense and cold white color (it has a peak of brightness in the green, which however our eyes would not catch). It is so bright that it cannot be observed with the naked eye without the risk of damage to the retina. Astronauts in orbit observe it with filters and viewers, while here on Earth we can see it with the naked eye only when it appears low and less bright on the horizon, and it is partly for this reason that in the collective imagination it is yellow if not orange.
In astronomical terms, the Earth is enveloped in a very thin layer of dust and gas which we call the atmosphere. When the Sun appears high in the sky, its rays have to travel less to overcome this thickness than when the Sun is low on the horizon: in the first case they cross it almost perpendicularly, while in the second they have to do it diagonally overcoming the equivalent of more thicknesses (it reaches ten times as much), therefore taking longer.
As Isaac Newton demonstrated, white light can be scattered through a prism, which separates it into the colors that make up the visible spectrum, that is, the portion between red and violet of the electromagnetic spectrum that our eyes can perceive. When sunlight reaches the Earth, it encounters particles dispersed in the atmosphere that are the same size as the wavelength of the part of the spectrum that includes blue, indigo and violet. More or less like Newton’s prism, when the Sun is high these particles disperse a very small portion of those colors (which we call “cold”) giving the sky its typical blue color.
When the Sun moves in its apparent motion becoming gradually lower on the horizon, its rays have to deal with the crossing of a greater quantity of atmosphere, which increases the encounters with the particles that disperse the cold light; for this reason, towards sunrise and sunset the sky becomes a very intense blue-blue (in the clearest mornings and evenings).
Deprived of much of its colder light, the Sun at this point appears yellow, orange and in some cases reddish tending to purple. It is also the only time we can observe it directly, and this has probably always contributed to making us perceive it as yellow, even though we now know that it is essentially white.
However, there are some cases in which the Sun can be observed through a natural filter: the clouds. When it appears high in the sky and it is slightly cloudy, we can observe the solar disk without being dazzled. In that case, it clearly appears pale white, closer to its true color than around sunrise or sunset. In these cases, many tend to think that it appears this way because of the clouds, on days that are often gloomy and in which the light is colder and which hides the classic yellow that they associate with the Sun.
There is a more intuitive way to think about the true color of the Sun, or what comes closest to us in our Earth experience.
A sheet appears white to us when it is illuminated by a lamp that emits white light, because it reflects the entire spectrum of the visible. If we apply a green filter to the lamp, the sheet appears green to us, because it is reflecting only a portion of the visible spectrum. Now let’s imagine observing a snow-covered mountain at noon, when the Sun is high in the sky: if it were yellow, we would also see yellow snow.
(Vittorio Zunino Celotto / Getty Images)
The fact that snow looks white to us comes from its being illuminated by a white light source, and not just any, but the largest we have. At other times of the day, snow can appear yellowish, pink or blue for the same reasons that sunlight appears differently colored due to its passage through the atmosphere.
The white light of the Sun is so intense that it is difficult to observe our star in the visible spectrum. For this reason, researchers use special tools to observe it at wavelengths different from those of the visible. Depending on the filters and systems used, the Sun can appear in different colors, which help to focus on particular features of its surface.