Young Star
According to the currently accepted model of star formation, sometimes referred to as the nebular hypothesis, a star is formed by the gravitational collapse of a pocket of matter within a giant molecular cloud. The infalling material possesses some amount of angular momentum, which results in the formation of a gaseous protoplanetary disk around the young, rotating star. The former is a rotating circumstellar disk of dense gas and dust that continues to feed the central star. It may contain a few percent of the mass of the central star, mainly in the form of gas which is itself mainly hydrogen. The accretion disk phase lasts a few to 10 million years. Accretion rates are typically 10−7 to 10−9 solar masses per year but can vary.
The disk gradually cools in what is known as the T tauri star stage. Within this disk, the formation of small dust grains made of rocks and ices can occur, and these can coagulate into planetesimals. If the disk is sufficiently massive, the runaway accretions begin, resulting in the appearance of planetary embryos. The formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form.
Read more about this topic: Circumstellar Disk
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