Friday, July 30, 2021

Magnitude of the Sun

The Sun is by far the brightest star in the Earth’s sky. It has an apparent magnitude of -26,74, compared to the -1,46 of Sirius, the brightest star in the night sky. 

As you can imagine, the exceptional brightness of the Sun is not due to its particular characteristics, but only to its extreme proximity to Earth. The Sun is in fact a star like many others, with size and mass neither too big nor too small.

It is therefore obvious that the further we move away from the Sun, the less its brightness decreases. Imagine that we are on the surface of a moon of Jupiter: from here the Sun has an apparent magnitude of -23.

Moving towards the boundaries of the Solar System the apparent magnitude of our star is still growing and becomes -18,20 if observed from the aphelion of Pluto’s orbit. Even from this position, however, the Sun is by far the brightest star in the sky.

So let’s move further away from Earth and suppose we are in the Alpha Centauri system.

From here the Sun has become a star like many others, with nothing in particular that can distinguish it. To find it in the sky we should observe in the constellation of Cassiopeia, where it would appear as a star of magnitude +0,5. The Sun would then be as bright as Raccoon seen from Earth (the brightest star of the Dog Minor and the brightest octave of the whole sky).

Let us now imagine moving further away from Earth: the Sun becomes weaker and weaker. Remembering that the limit of visibility to the naked eye is around the sixth/seventh magnitude, we would find that at about 58 light years away from Earth the Sun would become too weak to be observed with the naked eye.

Credit: NASA/SDO.

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