The Milky Way is the galaxy we are in. Being immersed in it it is very difficult for us to know its general properties.

Think for example of the total number of stars that make it up: it is impossible to give an exact number, the only thing we can do is to obtain approximate estimates.

In theory, the reasoning for finding the number of stars inside the Milky Way is not complicated. In fact, we can think of obtaining the total mass of the galaxy and of this taking into account only the percentage of mass due to the stars.

Once this value has been obtained, simply divide it by the average mass of a star to obtain the total number of stars.

The practical application of this simple reasoning is, however, much more complex.

First of all, already obtaining the mass of our galaxy can be quite complicated: the final value we will find will in fact be accompanied by a great uncertainty. It is also necessary to estimate what the percentage of mass constituted by stars is and also in this case a further imprecision is introduced.

However, the biggest error is introduced when dividing by the average mass of a star. In fact, not all stars have the same mass: some are tens of times more massive than the Sun while others have just one tenth of its mass. To know the average mass of a star, we need to find a function that links the number of stars to the mass value.

Taking into account all these sources of error, it was found that the number of stars inside the Milky Way is between 100 and 400 billion.

Credit: Raul Jichici.

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