Why are able to see the stars yet they are so far away?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

28 December 2025

Ben Jarabi asks a seemingly simple question: “…how come man is able to see stars with a naked eye?” He is wondering about it because stars are very far away. The nearest one to our sun being about 40 trillion kilometres away – a beam of light, travelling at one billion km/h, takes over four years to reach us from this nearest star!

There are two reasons why we are able to see stars with unaided eyes. First is that our eyes are very sensitive light and secondly is that stars are very, very bright.

The intensity of visible light coming from a source is measured in lux. (side note: the bath soap called Lux was probably so named to give the impression that is brightens the skin!). To get an idea of the magnitude of these units, a normal house with curtains open during the day may have about 50 to 100 lux of light.

A 7W LED light bulb (equivalent to about 60W filament type), placed about 3m away will be about 5 to 10 lux of light intensity. Now compare that to the sun: on a clear, cloudless day, its brightness can reach over 100,000 lux when viewed directly. But when you go under a shaded area, the figure drops by a factor of ten to about 10,000 lux – still much brighter than the LED.

Clearly, stars are extremely bright. Even at 150 million kilometres away, our sun is 1,000 times brighter that an LED at close quarters. Still, we haven’t looked at the sensitivity of the human eye to light…

Our eyes can see all the way down to about one millionth of a lux. Below that and it will be pitch darkness. So dark that there is no difference when you close your eyes with a blindfold. In my language, the call it “darkness that can bite”!

On the other extreme, our eyes can withstand up to about 10,000 lux but it will be very uncomfortable and even painful. Generally, it is recommended that we keep the brightness below 1,000 lux.

With this in mind, it is not surprising then that we are able to see stars that are great distances away. Indeed, we are able to see even the galaxy in the Andromeda constellation which is 24,000,000,000,000,000,000 km away. The light from there takes 2.5 million years to get here. But at about 0.1 lux, it’s still bright enough for our eyes.