How far is the sky

How far are the constellations?

Constellations only serve our orientation in the sky - they do not form a real, physical unit. More or less arbitrarily, our ancestors combined groups of bright stars into "images" of animals, heroes and gods. Therefore, no uniform distance can be specified for a constellation: Every star in a constellation has a different distance from us.

Great Bear Star Map

Let us take the most famous constellation in the northern sky, the Great Bear, as an example. Seven bright stars of the Big Bear form a particularly striking shape and are popularly known as the “Big Dipper”. The distance of these seven stars is between 78 light years for Mizar (Zeta Ursae Majoris) at the bend of the drawbar and 124 light years for Dubhe (Alpha Ursae Majoris), the upper right box star.

The situation is even more extreme with the constellation Orion, for example. Of the skyhunter's seven brightest stars, the right shoulder star Bellatrix (Gamma Orionis) is closest to us at 240 light years, and Alnilam (Epsilon Orionis), the middle star of the Belt, is furthest away at 1,300 light years.

Orion star map

In the constellation Orion we can also see very nicely that the brightest stars are not necessarily those that are closest to us: the brightest star Betelgeuse is 600 light years further away than Bellatrix, the third brightest star. Both are giant stars that are considerably larger than our sun, but while Betelgeuse has around 135,000 times the luminosity of the sun, Bellatrix is ​​only around 4000 times brighter than our central star.

Even if the stars of a constellation are all different distances from us, we can say that at least all the brighter stars are all in our immediate cosmic neighborhood with typical distances of a few tens to a few hundred light years. Of the hundred brightest stars, Alpha Centauri is closest to us with a distance of 4.4 light years, the furthest away is Aludra in the Big Dog (Eta Canis Majoris) with around 3000 light years.

The reason for this is that the radiation we receive from a star on Earth decreases with the square of the distance. The stars quickly become dimmer with increasing distance: at ten times the distance, the brightness drops to a hundredth. A star like our sun, for example, could no longer be seen with the naked eye in the sky at a distance of only 60 light years. And even with a bright giant star like Betelgeuse, the limit is around 5500 light years - a stone's throw compared to the diameter of our Milky Way of around 100,000 light years.