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Monday, 19 July 2010

An alien beginning:

Above: The Orion nebula, one of the most active star forming regions in this part of the galaxy. our solar system was born in a similar nursery. Image courtesy of NASA.
It may be a strange thought, but 5 billion years ago the world wasn't here. There was no Sun, no planets, asteroids, no Earth, no rock water or wind. If you were taken back to that time you would find yourself in  a place that scientists today call ‘the pre-solar nebula’. It's a giant molecular cloud, made of incredibly thin gas, mainly hydrogen, helium and dust particles as fine as smoke. It may have stretched for hundreds of light-years in every direction before.

This vast near-void contains the matter that will build our entire 6,070,000,000,000,000,000,000 metric ton solar system, and many more besides, but it's almost invisible. The gas is so rarefied that on Earth it would be a high quality vacuum. 
So there is not much to block the view of the stars: The milky way stretches unbroken across the field of view, there are no recognizable constellations and the stars have not yet moved into their familiar positions. The temperature is far below freezing, only tens of degrees above absolute zero.

As your eyes adjust to the dark you might begin to make out some more details*, things that would mark the view as different from that seen from clear space: Denser knots of gas known as Bok globules, visible as dark ‘holes’ against the background mark the spots where new stars are forming. If your eyes could see into the infra red these would apear to glow an ultra-deep maroon. A few intense points of blue-white light are scattered amongst the dimmer stars. These are the biggest, fastest burning stars to be birthed by the nebula; Blue supergiants, many times larger, heavier, brighter, and hotter than our Sun. Their intense radiation excites the gasses of the nebula, causing it to glow. Inside the nebula these glows are too diffuse to be visible but if you were to navigate away from the cloud to a distance of a few hundred light years the view would become spectacular.

Above: The Tarantula Nebula - an aweseome sight from afar. Courtesy of NASA.

Pay attention to the dust...... it's going to be you one day!

Pluck one of the grains of dust from its place in the cloud and examine it with modern instruments and it would tell you its own unique story: It may have been born gently in the atmosphere of an elderly red giant star, or forged violently in the death throes of a dying supergiant. It may be an ancient grain, that has wandered for many billions of years, or it might be almost new, and buzzing with radiation from unstable short lived isotopes.

The grains are the only solid material in the pre-solar nebula - and on the chemical level interesting things are happening with them. Although the temperature may be as low as 10 degrees Kelvin (- 263 degrees Celsius) the grains are exposed to high energy cosmic rays, and ultraviolet light and other radiation from nearby stars in the nebula. They pick up gasses from the surrounding cloud, and are bombarded by stellar winds. These provide enough energy and material to power chemical reactions. Carbon rich grains are especially interesting- the chemical precursors to the chemistry behind life are forming on them. Laboratory experiments have shown that amino acids can form under ultra cold space conditions.

How do we know this?
How do we know about this vanished place, which by our solid standards can barely be said to have been there at all?
There are remnants of the nebula itself. Pre-solar grains have been found in primitive meteorites and in micrometeorites.  There have been dust detectors flown on many space missions - and missions such as Stardust have flown with aerogel collectors to capture grains from comets, thought to contain pre-solar material.

In fact Earth gains almost 40 tons of extraterrestrial material a day. There are also pristine remnants of the pre-solar nebula still in interstellar space, and at the outer edge of our solar system- in near absolute zero deep freeze conditions well beyond the orbits of Neptune and Uranus. These can be studied using techniques such as infra red spectroscopy. By these methods we can build up a picture of the composition of grains still floating between stars. Still more examples of pre-solar material are believed to be incorporated into comets.

From these clues we've pieced together a picture of what was here before the Sun shone or the planets circled it....

*The clear view may seem odd if you have followed some of the stunning images produced by telescopes. That's because nebula images often seen in the press are the result of many hours of exposing very sensitive detectors to the nebula light, allowing extremely faint colours to be enhanced.

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