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In the night sky: Orion
In the night sky: Orion

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4.2.2 Collapse of a nebula

In the video, a fraction of the nebula collapses under gravity to produce a rotating disc of gas and dust.

Note: this video has no sound.

Download this video clip.Video player: ou_futurelearn_orion_vid_1102.mp4
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Parts of the disc clump together to form bodies which eventually become planets. The central region of the disc is the hottest area, and eventually emerges as a star.

Most very young stars – less than about 10 million years old – have these discs of dust and gas around them. The discs are seen less and less around older stars, suggesting that, over time, they dissipate or form planets. Around 15% of nearby stars have discs around them. These discs have proportionally high amounts of dust, and low amounts of gas. The dust is a mix of ice, silicates and carbonaceous (containing carbon) material and the gas is mostly hydrogen and helium.

The dust grains are initially less than a thousandth of a millimetre across. Aided by heating events that allow the dust to melt and stick together, the dust coagulates. Eventually, metre to kilometre-sized boulders, called planetesimals, form.

Next, you will find out more about planetesimals.