Protoplanetary disks glow by the reflected light of their central stars

Most young stars are surrounded by a protoplanetary disk consisting of gas and dust. The cloud surrounding an infant star begins as an approximately spherical cloud that begins to collapse under its own weight. Dust initially makes up approximately one percent, by mass, of the collapsing globule.

As the globule collapses, the conservation of angular momentum ensures that the globule flattens forming a disk. Viscous and turbulent transfer of angular momentum transfers large quantities of angular momentum to small quantities of mass that then escape from the system. Gas and dust also fall into the young stellar object at the centre of the system.

Proto-planetary disks are quasi-stable structures that typically exist for tens of millions of years before finally dissipating. They have masses of the order of one percent of their central star and glow by the reflected and reprocessed light emitted by their central star. There is, in addition ,a certain amount of thermal energy generated by viscous dissipation but this is comparatively small for all but the most massive disks.

Proto-planetary disks around very young stellar objects (those less than a few million years in age), can be very dynamic - material inflow through the disk to the stellar surface being relatively chaotic with pile-ups of material in the disk being followed by intense episodes of ultraviolet emission as the mass is funnelled to the surface of the young star by strong magnetic fields anchored in the stellar photosphere.

A visualisation of the inner region of a protoplanetary disk around a young stellar object. The puffed inner-rim of the disk casts a shadow on the region surrounding it. The disk glows from the reflected/reprocessed light of the central star. The temperature of the disk material drops and appears more red the further from the  central star it is located.

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