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Skywatching

Dating stars through waste elements detected through light

In search of the first stars

On a clear, dark night, our unaided eyes can see maybe two thousand stars.

Our galaxy, the Milky Way, contains around a billion stars. Considering the universe contains billions of galaxies, the universe as we observe it today contains a huge number of stars and planets. However, there was a point, billions of years ago when the universe contained no stars or planets. The universe was dark, lit only by the fading glow of the Big Bang.

At the time of its birth—the Big Bang—just under 14 billion years ago, the universe was small, unbelievably dense and incredibly hot. The matter making up our world and the observable universe today could not exist. However, as the universe expanded, it cooled, until it reached a point suitable for hydrogen and helium atoms to form.

By the time conditions were suitable for the formation of other atoms, there was nothing left from which to make them. The young universe contained mostly hydrogen, with some helium. There were none of the other elements, which would be needed to make future planets, and life. However, hydrogen and gravity are all that is needed to make stars, and about 100 million years after the Big Bang, clouds of hydrogen and helium collapsed to form the first ones.

Stars make energy through nuclear fusion, converting hydrogen into other elements. These first stars shone brightly, burned their fuel profligately, collapsed and exploded. All those "waste elements" formed by the stars' energy production were scattered out into space, where they mixed with the cosmic hydrogen clouds.

When subsequent generations of stars formed, they collected a contribution of those waste elements. As generations of stars were born and ended their lives in huge explosions, the concentration of those waste elements increased.

The Sun is a fairly average yellow dwarf star, powered by nuclear fusion in its core and about halfway through its life. When we analyze its light, we see signatures of hydrogen, helium, and small amounts of other elements, such as carbon, iron, calcium and so on. Because during the stable phases of a star's life the waste materials remain in the core, the elements we detect in the Sun's surface layers were not produced in the Sun. It picked them up from its birth cloud.

These waste products make up around 1% of the Sun's material. The Sun was born about 4.5 billion years ago, so we can assume the concentration back then of waste elements in hydrogen clouds in our galaxy, was somewhere around 1%.

We think our galaxy was born around 13.5 billion years ago, something like 200 million years after birth of the first stars in the universe. It is likely the oldest stars in our galaxy formed close to the time our galaxy did, and as expected, they contain less of those waste elements, around a tenth of the concentration in the Sun.

That suggests that 10% of the concentration of waste elements in the Sun were generated in just 200 million years and the other 90% took several billion years.

The high energy outputs, and short lives of those first stars are believed to be due to the absence of waste elements.

Even in tiny amounts, they affect the workings of a star quite dramatically. As the concentration of waste products inherited from previous generations rose, stars shone less brightly but did so for longer, providing more opportunities for life to develop on their planets.

In addition, over millions and billions of years the elements in the cosmic clouds interacted with each other, forming a vast range of chemical compounds, some of which form the basis of life.

The earlier these materials became available in the universe, the sooner the formation of planets and the development of life could get started.

•••

• After sunset, Venus lies close to the SW horizon.

• Jupiter and Saturn are low in the southern sky.

• Venus shines brightest and Saturn the faintest.

• The Moon will reach last quarter on Nov. 27



More Skywatching articles

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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