The very earliest stars, that started to form about 100 million years after the birth of the Universe, are called Population 3. They comprised only hydrogen and helium, as these were the only elements produced during the big bang, apart from faint traces of deuterium and lithium. They would have been much larger than stars that form today, massing up to several hundred, and possibly even several thousand, times the mass of the sun. They would have been very hot, generating huge amounts of radiation, particularly ultra-violet. They would also have been very short lived; less than a million years for the largest. None has ever been observed directly, though there is some indirect evidence of them in a gravitationally lensed galaxy that is a very great distance away. The image, to the right, probably represents the glow of Population III stars, and was taken by NASA's Spitzer Space Telescope.
As the early Population 3 stars died, they ejected large amounts of heavier elements, up to iron, into interstellar space. Stars that became supernovae also added the heavier elements beyond iron. These heavier elements then combined with the prevailing hydrogen and helium clouds. The enriched clouds coalesced into new stars, but this time they would have a higher proportion of heavier elements. This produced the Population 2 stars. These were not as large, due to the heavier elements, massing, perhaps, up to 20 to 100 times solar masses.
The final population of stars, that includes our own sun, is Population I. As some of the Population 2 stars also died, they ejected ever larger amounts of heavy elements. These heavier elements further enriched the hydrogen and helium clouds that went on to produce the Population 1 stars. This process continues today, with virtually every galaxy having some new star forming regions. Supernova explosions disrupt clouds of dust and gas, causing compression that can lead to star formation, and close encounters between galaxies also has the affect of whipping up gas and dust clouds to start star formation.
The diagram to the right shows the distribution of Population I and II stars in the Milky Way Galaxy, which is typical of the distribution in spiral type galaxies. Generally, the closer to the center, the younger the stars. Most of the stars in the Globular Clusters are older Population II stars.
Astronomy & Cosmology -
Stars - Life & Death of Stars