14-06-2008 We are stardust (Article 141)

Moderator: SAC Committee

Post Reply
Simon Kenny
Posts: 527
Joined: Tue Feb 12, 2008 9:17 pm
Location: Shannon, Co. Clare, Ireland

14-06-2008 We are stardust (Article 141)

Post by Simon Kenny » Wed Dec 31, 2008 10:40 am

We are stardust (Article 141)

From studies of star clusters a curious difference is revealed between the youngest and the oldest stars in the Milky Way, our home galaxy. Stars in the youngest clusters are considered to be metal rich (this is known by studying their spectra). Astronomers use the term metal to describe any element heavier than the two lightest elements of Hydrogen and Helium. Stellar evolution has produced two distinct populations of stars and these metal rich stars are known as Population I stars. For example our star, which we call the Sun, is a relatively young Population I star.

The other distinct population of stars is called Population II stars and by contrast the spectra of these stars, in the oldest of star clusters, reveal weak lines of heavy elements. These very old stars are regarded as metal poor stars. Globular clusters of stars can contain several hundred thousand stars and these clusters exist above and below the plane of our galaxy. The individual stars in these globular clusters are metal poor Population II stars.

To understand why we have these two distinct star populations we have to look back to the Big Bang of 10 to 15 billion years ago when the universe is thought to have come into existence. The early universe almost exclusively consisted of Hydrogen and Helium, the lightest elements, with virtually no heavy elements i.e. metals. So, when the first stars formed they were metal poor. The least massive of these stars have survived to the present day and are now the ancient stars of Population II.

The more massive of the original stars evolved rapidly and no longer shine. Such massive stars evolved to have Helium burning in their cores and hence producing metals such as carbon and oxygen and the even more massive stars went on to produce even heavier elements. Very massive stars lead relatively short lives and die in an unimaginable explosion called a Supernova and in this process they expel their metal rich gases into space which in turn joins the interstellar medium. These gases then condense to form new second generation stars which start their lives already metal rich, these are Population I stars. Our Sun is a great example of this process. Just to recap here: Population I stars are members of a second generation of stars, whereas Population II stars belong to an older first generation.

Because our Sun has such a high level of heavy elements means that the Solar Nebula from which our Sun and planets formed must have been metal rich. The Earth and indeed our bodies are almost entirely composed of heavy elements. It is unlikely that Earth could have formed from the metal poor gases of Population II stars.

In summary we can see that Helium burning in massive stars produced the same carbon and oxygen atoms found most commonly on Earth. These reactions happened billions of years ago in an earlier generation of stars that died and surrendered their atoms to the interstellar medium. These same atoms later became part of our Solar System, our home planet Earth and our bodies. The world famous astronomer Carl Sagan once commented that ‘we are made of the stuff of stars’.

Post Reply