The evolution of stars in the universe


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COSMOLOGY: The evolution of stars in the universe

THE lunar eclipse the other night got me in awe of space again. It’s amazing to think that without stars there would be no planets, no galaxies, no light and no life. Our exploding balls of gas in the sky can be thought of as giant factories or furnaces that manufacture the elements and scatter them across the universe. All the base elements that together make up planet Earth are thanks to the stars. Even the heavier elements, such as iron, have been cooked within stars and rained down on Earth. All inhabitants, including you are me, are made up of these elements. So essentially, we are all stardust. Here’s how it all began.

In the beginning…

Image: NASSAAbout 380 000 years after the Big Bang, the universe consisted of gas — mainly hydrogen and helium. The universe at that time was like a giant star- forming area, where the process of fragmentation and star-forming was clearly visible. The types of stars that formed then were very different from the stars today. This was because the contents of the early universe still lacked carbon, oxygen, iron or any of the other elements that we are familiar with today.

The first generation of stars were massive — hundreds of times the mass of the sun. However, they were too hot to manufacture any metals and needed to cool in order to do so.

According to Frikkie de Bruyn, the director of the Cosmology Section of the Astronomical Society of Southern Africa­, “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 blowing off in the surrounding space has the affect of forming more stars. De Bruyn explains that “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”.

These first generation stars emitted strong ultraviolet radiation and very powerful stellar winds that blasted enormous cavities in the surrounding gas. Unfortunately, we are unable to observe these first generation stars, but De Bruyn says that astronomers have been able to detect a faint, infrared glow attributed to these early stars in the universe.

The second generation stars

The super massive first generation stars were only around for a few million years, which is a relatively short life span in cosmological terms. Like the stars we stare at today, they fused together heavier elements and eventually went supernova. They did not create much carbon or oxygen at this point, but did leave behind lots of iron, explains De Bruyn.

The second generation stars, also known as population II stars, formed before the first generation stars died — formed from the gas clouds left behind when the first generation stars exploded. These population II stars were also extremely low in metals, but the overlap between their formation and the death of their ancestors, resulted in an interesting mixture of first and second generation stars.

Population II stars were smaller but slightly hotter than the stars we know today. Their metal contents gradually started to increase. However, there is a mystery, explains de Bruyn. “As far as can be established only two second generation stars were found. Astronomers refer to this as the missing G-dwarf star. Spectroscopic analysis reveal that these stars are metal poor. They are mainly to be found in globular clusters, in the halo of the Milky Way.”

Population I stars

In the disc of the Milky Way galaxy, we find metal-rich Population I stars. These beauties contain carbon, hydrogen and iron, and are capable of forming planets. This occurs when all the atoms heavier than helium start clumping together.

So we find ourselves in a metal-rich part of the galaxy. The metal contents of the earliest stars were between 200 000 and 300 000 times less than our metal-rich sun. As these stars developed and reproduced, we now have stars that are three and a half times more metal-rich than the sun. De Bruyn­ says that a very interesting area of current research in cosmology is to determine the possibility of stars having planets according to their richness in metals.

Source: Frikkie de Bruyn, director of the Cosmology
Section
of the Astronomical Society of Southern Africa.

“You are a child of the universe no less than the trees and the stars; you have a right to be here. And whether or not it is clear to you, no doubt the universe is unfolding as it should” – Max Ehrmann, Desiderata

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