Before the Solar System

The formation of the Sun and stars

The Sun is somewhat a late-comer in our Galaxy, the Milky Way. It was born 4.6 Gyr ago (4.5685 Gyr ± 0.5 Myr, to be precise, from the decay of specific radioactive heavy elements in the most primitive meteorites – see below). This is to be compared with the age of the Universe, constrained by the best theoretical fits to the observed spatial fluctuations of the ‘cosmic background radiation’ to be 13.7 Gyr after the Big Bang, within 2%. When galaxies form is less certain, but current estimates give a time lapse of less than 1 Gyr after the Big Bang – implying that our own Galaxy has an age of over 12.7 Gyr and that the Sun was born over 8.1 Gyr later. So at the time the Sun formed, our Galaxy was already sufficiently evolved by successive generations of stars that it presented no major differences with the one we observe today. Therefore, we can safely derive conclusions about the distant birth of the Sun from observations of contemporary young stars.

In a nutshell, from various observations we know that bright nebulae, including some famous ones like Orion, the Eagle or Carina nebulae, are ‘stellar nurseries’ (Figure 8.1), where stars like the Sun form in clusters of thousands of low- to intermediate-mass stars. A few massive stars, like the Orion Trapezium, for which the highest mass is of order 45 Mʘ also form in these stellar nurseries.