# Practical Exercise 6:

Galaxy GN-z11 in the constellation of Ursa Major, discovered in 2016 by Hubble Space Telescope, is one of the remotest observed objects in the Universe. Its actual distance is estimated to 9 800 Mpc. How many light years is that? How many times is this distance larger that the Galaxy diameter?

Answer: Using the equation 1 pc ≈ 3.26 ly , and so 1 Mpc ≈ 3 260 000 ly we get:

d = 9 800x3 260 000 ly ≐ 32 000 000 000 ly = 32 Gly

In comparison with the Galaxy diameter (100 000 ly) we find out that the Milky Way fits 320 000 times in this particular distance. It is a huge number – if we imagined whole our Galaxy in the size of a pinhead (approx. 1mm), then the GN-z11 Galaxy would lie in a distance of 320 m, i.e. more than three lengths of a football field.

## Objective of Practical Exercise:

Objective of practical exercise is to remind of the Mpcs unit and its conversion to ly, with which we work the most frequently in the activity sheets in this chapter. Megaparsec is also a very frequently used unit indicating the distance between the galaxies, even though this unit is too big for the distances within our local group, e.g. M31 Galaxy in Andromeda constellation is situated in the distance of 2 500 000 ly ≐ 0.77 Mpc. For „foreign“ galaxies, we get the values up to thousands of Mpc, e.g. for the group of galaxies in Virgo constellation it is 65 000 000 ly ≐ 20 Mpc, hence „reasonably big“ numbers.

Curious students might ask how it is possible to observe an object in the distance of 32 billion ly if the age of the Universe is estimated to be less than 14 billion years and the light could have travelled the distance of maximum 14 billion ly since its existence. The expansion of Universe enters the game here – in the very moment when the galaxy emitted the observed signal it was in the distance of 13.4 billion ly (and this journey to us was really made by the signal), but the galaxy itself has, in the meantime, doubled the distance from us thanks to the expansion of the Universe. This effect needs not to be taken into account in the Solar system or within the Galaxy, but we must calculate with it for the very remote objects. It is the objective of the following activity sheet.