2023-12-29 13:29:00
The Sun is the target of countless astronomical research that seeks to understand the behavior of our host star, but a simple question draws the attention of the public interested in the subject: why is space not illuminated by the Sun?
Outer space is always dark, even close to an object as bright as the Sun. Our star is an intense source of light, the result of the intense activity that keeps it “lit”, that is, the unstoppable fusion of atoms that releases a quantity massive amount of energy. Every second, the star fuses around 600 million tons of hydrogen into helium.
This energy is released in the form of electromagnetic radiation emitted in all directions with a spherical shape, and much of the radiation belongs to the visible spectrum, that is, a flow of light that can be perceived by the human eye. There is also, naturally, the emission of infrared and ultraviolet waves (UVA, UVB and UVC).
In short, space is not “illuminated” by sunlight because vacuum does not reflect light. “Vacuum” is empty space, completely devoid of particles. There appears to be nothing between your face and your cell phone screen, but the space between you and your device is filled with particles of carbon dioxide, water vapor, dust, among others.
To help understand this concept, let’s make an analogy. Imagine turning on a lamp in an open space and another in a closed room, painted white. In an open environment, this lamp would only illuminate a small circle around it, but it would be able to illuminate the closed room much more evenly.
Another example is looking at a powerful spotlight. On a humid day, the object would form a beam of light that would extend a considerable distance thanks to the reflection of moisture in the air. Lighting a space requires not just a light source, but also something that reflects it, like the white walls of a room or water droplets hovering in the air.
When we look at the sunny sky on Earth, the blue we see is just the reflection of the particles that make up our atmosphere. Just travel a few hundred kilometers above the surface, beyond the exosphere, and the “sky” will lose color, leaving only a completely black space with small bright spots — stars and planets.
In addition to our atmosphere, there is the vacuum of space. This means that there are practically no particles, therefore nothing that could reflect any light source. The beam of light will still be there, but there is nothing to disperse it and illuminate the space.
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This phenomenon can be observed by looking at the sky from the surface of Mars. During the day, the sky has a grayish red color, which is the reflection of the particles that make up its rarefied atmosphere. The Moon does not have an atmosphere, so its “sky” will be completely black even if the Sun is visible.
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