Ken Tapping, May 23, 2007

A few days ago people across Southern British Columbia saw a fireball, an extremely bright starlike object that shot across the sky southwards, leaving a glowing, pinkish trail. There was a report, at present unconfirmed, that the object hit the ground near Osoyoos, just north of the US Border. Events like this are often regarded as unique. In fact they are part of the ongoing process of world building.

On any clear night, a telescope or binoculars will show dark lanes and blobs against the Milky Way. These dark features are not holes; they are huge clouds of gas and dust. Between five and ten billion years ago, one of these clouds became unstable and started to collapse under its own gravity. As the collapse proceeded, and the cloud got more condensed, the gravitational attraction increased and the collapse accelerated.

As the disc became smaller and denser, collisions between particles became more frequent. Some of them stuck together, forming increasingly large lumps. Eventually the material settled into a huge, rotating disc, with the dust and growing lumps spiralling slowly inward. The biggest lump formed in the centre, and under the weight of the inflowing material, it became increasingly compressed, and hotter and hotter. About five billion years ago the temperature in the middle hit between 12 and 15 million degrees, and nuclear fusion started. The Sun was born.

The heat and radiation from the young star started to push away and evaporate the material in the middle of the disc, stopping the inward collapse. This caused small discs to form in the big disc. Each of these small discs formed one of the planets. However, this is not a process that happened and is now over; it is still going on. As the new worlds formed, more and more of the cloud material was swept up and incorporated into the planets. Gradually the remnant material got less and less. Even today, about 4.5 billion years since our planet first formed, there is still a lot of material out there, and planet building goes on. This material ranges from extremely fine dust to objects metres or even kilometres in diameter. On average our world is sweeping up about 100 tonnes a day. Most of it is in the form of dust, which is so fine that when it ploughs into our atmosphere at tens of kilometres a second, it just slows down and falls slowly to the surface, as micrometeorites. Larger pieces, maybe around the size of sand grains, are heated and vaporized by friction; causing the transient streaks of light in the sky we call shooting stars, or meteors. Bodies roughly the size of baseballs produce a more dramatic spectacle, like the one above southern BC not long ago. Then of course there still are much bigger lumps wandering around out there. Fortunately they are now very rare, but the large craters we see on the Canadian Shield and in Arizona show that our world has been hit many times. It will be hit again; it's only a matter of time. Even today, 4.5 billion years after the formation of the Solar System, world building is still in progress, and that includes our world too. We are still living on a construction site.

Venus dominates the western sky until about 11pm. Mercury lies very low in the sunset twilight; Saturn shines high in the south overnight. Jupiter rises around midnight, and Mars shortly before dawn. The Moon will reach First Quarter on May 23.

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Ken Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca