Brightness determines how far we can see; not distance
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
03 April 2022
For the longest time
in human history, people assumed that the process of seeing involves
something being emitted by the eyes to illuminate the object that you
are looking at. Indeed, as recent as the late 20th century,
this false notion was behind the “X-ray vision” abilities of the
fictional character, Superman. If Superman can produce x-rays, he would
never, ever see through walls because the x-rays would not come back to
his eyes!
Another false notion
that is quite prevalent is that there is a limit to how far we can see.
This is not true! We can see anything from any distance as long as it is
bright enough. Brightness is the only determining factor of what we can
see. Think about it: when it is dark, we are not able to see things that
are just a few dozen centimetres away.
So, instead of asking
what is the farthest distance we can see, the right question should be
what is the farthest object that we can see? The second version is not
about the distance, but, about the object; that is, whether it is bright
enough.
Of course, we can see
the bright sun and it is 150 million kilometres away. We can also see
the stars that are even farther: many trillions of kilometres from us.
Indeed, the nearest star after the sun is over 40 trillion km away – and
it is visible without using a telescope.
Now, 40 trillion
kilometres is a very, very long distance. Just pause for a moment and
think about it. The sun is 150 million km from us; this is called the
Astronomical Unit (AU). So, how many AU is this star? What is 40
trillion divided by 150 million? Do the math!
But that is still not
the farthest object we can see. All the other thousands of stars in the
night sky are much, much farther than that! Because these distances are
mind-bogglingly large, astronomers came up with another unit of
measurement – the light year (LY). This is equal to about 9.5 trillion
km, which it the distance that a beam of light will cover in one year.
Thus, the nearest
star is about 4LY from earth. Most of the stars we see at night are
hundreds and even thousands of LY away. With the unaided eyes, the most
distant thing visible is the Andromeda galaxy which is about 2.5 million
LY! How many km is that?
 |
 |