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Posted 31 March 2008

The Importance of the Evolution of Stars for the Universe

by Frikkie de Bruyn

"Without stars there would be no life, no planets,
no interstellar gas and dust and no galaxies."

In the beginning

Let’s start at the beginning, not when the big bang happened, but about 380 000 years later, when the cosmic microwave background radiation formed. The Universe consisted of gas, mainly hydrogen, some helium and trace elements of lithium and beryllium. The energy fluctuations in the very early Universe as a result of the uncertainty principle of quantum mechanics that provided the ‘seeds’ for the first stars have been blown up by the exponential inflation of the infant Universe. This ensured that there are slight inhomogeneities in the gas causing it to start fragmenting. The Universe at that time was like a giant star forming area, like the Orion Nebula, where the process of fragmentation and star forming is clearly visible.

The types of stars that formed then were very different from the stars today. This was because the contents of the Universe was mainly hydrogen, some helium and trace elements of lithium and beryllium with no carbon, oxygen, iron or any of the other elements we know today. The first generation of stars were very massive, hundreds of times the mass of the Sun, almost like the super giants we know today. Where we have two or three super giants today it was very different in the early Universe, all stars were extremely massive and hot. The metals formed in later generation stars are needed to cool the star down and the first stars did not have these metals. They were metal poor and known as population III stars.

There is a theoretical limit to how big a star can be because of the balancing act of light pressure and gravitational collapse. Once a star gets too big it generates so much light heat and pressure at the core of the star that it starts blowing off the outer layers of its atmosphere faster than in falling gas due to gravitational collapse. The gas blown off bumped into the surrounding gas and caused the formation of more stars. Until that time the whole Universe was opaque to light, like a very thick fog. When the first stars formed they created light, ionized the gas and made it transparent. It was almost as if a giant switch was flipped and for the first time the Universe was lit up.
The first stars emitted very strong ultraviolet radiation and had powerful stellar winds that were blasting enormous cavities in the surrounding gas clearing out the space around the stars. Can all this be verified observationally? Are there still some of these stars around? The answer is that first generation individual stars cannot be observed. Astronomers did, however, detect a faint infrared glow attributed to these stars. The light detected is infrared due to the expansion of the Universe and the red shifting of the photons.

The second generation stars

The first generation super massive stars had very short life spans of only a few million years. They went through the same process as stars today, fused heavier and heavier elements, exploded as supernovae and, surprisingly, did not create much carbon or oxygen or any of the heavier elements, but rather left behind lots of iron. The second generation stars, also called population II stars, formed from the gas clouds left by the first generation stars when they exploded. The second generation stars formed before all the first generation stars died. So there was an overlap between the two generations. These stars had extremely low abundances of carbon and oxygen abundances compared to their iron contents. As a result the second generation stars are extremely low in metals. The overlap means that there was a interesting mixture of first generation and second generation stars.

Population II stars have different temperatures as a function of their size. This means that they were slightly hotter than the stars we know today and smaller than the first generation stars. There is a mystery. As far as can be established only two second generation stars were found. Astronomers referred to this as the missing G-dwarf star. Spectroscopic analysis revealed that these stars are metal poor. They are mainly to be found in globular clusters, in the halo of the Milky Way. It is, however difficult to classify stars according to their metallic contents alone. Since the population III stars died the metal contents of stars started to increase gradually. When you get a metal rich star, it is difficult to decide if it is a population II or a population I star.

Population I stars

Population I stars are in the disc of the galaxy. They are rich in metals such as carbon, oxygen and iron. These stars can form planets. All the atoms heavier than helium start clumping together to form planets. The Sun is one of the metal rich stars. We go from the metal poor population II stars with 200-300 thousand times less metals than the Sun to those in population II stars that are 10 times less metal-rich than the Sun. The metal contents of stars vary from 300 thousand times less to three and a half time more metal rich than the Sun. No stars fall outside these categories. We find ourselves in a metal rich part of the galaxy. A very interesting area of current research is to determine the possibility that a star may have planets according to its richness in metals.

Frikkie de Bruyn

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