Recycling - as you've never seen

When stars die, their elements don;'t fade away – they help create life itself, writes DICK AHLSTROM

STARS DRIVE A huge recycling system, one that turns old stars into new ones and also helps build planets. Without it we would not have Earth to call home and we probably wouldn’t exist at all.

Only when stars die and get recycled do we get the heavy elements that make planets and support life. And these elements can be seen everywhere across the universe, says artist and science presenter Deirdre Kelleghan.

She is giving a series of talks for students during Science Week entitled: “OMG The Universe is elemental.” The talk describes this massive stellar recycling system and how it builds stars like the sun and supports the life we see on Earth.

How it all works can be watched in detail in the Orion Nebula, a very big star formation area that can easily be seen from Earth without the use of a telescope, says Kelleghan.

It is also known simply as M42 but whatever you call it astronomers are particularly interested in what the nebula can tell us about the births of stars and planets and about life itself.

“It is a huge recycling energy thing. In the Orion Nebula you have the building of stars and baby planets and it is filled with the tiny precursors that can make life,” she says.

We know what elements are up there in the Orion Nebula by splitting up the light that the objects in it give off. Telescopes first capture the light and this is analysed using a spectrograph, a tool that breaks up the light into its “spectral lines”. These series of lines can tell you a great deal about the source of the light.

“It is kind of the DNA of the star or like the bar codes on a packet of sweets. It is the bar code of the star,” says Kelleghan. And this bar code tells you what elements are there.

When you look at young stars you will see mostly hydrogen and helium. That is because the star uses hydrogen as its source of energy, using up hydrogen as a fuel to create helium in a fusion reaction.

That reaction is simple to describe – two hydrogens joining together to deliver one helium – but it is much harder to imagine the massive energy this fusion reaction gives off.

We sense this power in the giant fireball produced when nuclear weapons are tested, but imagine billions of these explosions happening at once in a star, the energy source that all stars including our sun use to produce light.

The spectral lines also produce a unique bar code that is different for every single star, Kelleghan adds. Our sun’s bar code is different from that of any star in the night sky.

But why then do we see so much more than hydrogen and helium in the Orion Nebula? The spectral lines tell us that oxygen is there and iron and the other elements that are all much heavier than the two lightest elements, hydrogen and helium.

These elements would not exist were it not for the big recycling process underway in the Orion Nebula, says Kelleghan. The process depends on the death of old stars which in turn produces the heavy elements needed to build planets and plants and humans.

The Orion Nebula lets astronomers see how a star begins to form, pulling in nearby hydrogen until its gravity builds and it collapses in on itself to kick start the fusion process.

Its dying days are more exciting however with stars going out in one of two ways, she explains. Stars called red supergiants blow up in a spectacular supernova where so much energy is given off the light of it could be seen in the middle of the day.

Most stars – in time including our own star – will end their days as a “planetary nebula”. The core of the ageing star shrinks leaving a hot gaseous shell that eventually expands outwards further and further over thousands of years until it dissipates. The core left behind is known as a white dwarf star.

The important thing is that old stars do not just contain the leftover hydrogen and the helium it produced. Other heavy elements begin to form inside and these are scattered far and wide if the star explodes or are left behind in the white dwarf.

This spread of elements can be seen across the Orion Nebula if you look for their spectral lines, Kelleghan says. And these then can join to form molecules and can compact together by gravity to form both rocky planets Mercury, Venus, Earth and Mars and the gas giants Jupiter, Saturn, Uranus and Neptune.

But the elements in M42 don’t just build planets. “They are precursors of life, the bits and pieces that can gather together to form life,” she says.

“We wouldn’t be here without the stars. They are so important. We might not be here if it wasn’t for our own sun.”

Deirdre Kelleghanis an artist who became involved in science outreach in 2004. She manages to combine both in the work she does by having those attending her talks attempt to draw nebulae and also the moon.

Last August she won a prestigious international award, Science magazine’s online resources in education prize for her “Deadly Moons” schools project.

She does her own illustrations of objects out in space including the Orion Nebula. Examples of her art work and details of the talks she gives are at deirdrekelleghen.net