Sunday, February 26, 2012

Protoplanetary disk

Protoplanetary disk HH-30 in Taurus NASA
Red beam of stellar jet 
450 light years from us

Modern theory suggests that in the beginning we have dark cloud of hydrogen molecules. Gravity causes the hydrogen molecules to approach each other and the process accelerates as the mass of the groups of molecules grows. Or in the much more expert words of an anonymous encyclopaedia writer:

Protostars typically form from molecular clouds consisting primarily of molecular hydrogen.

When a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity.

As this collapsing cloud, called a solar nebula, becomes denser, random gas motions originally present in the cloud average out in favor of the direction of the nebula's net angular momentum.

Conservation of angular momentum causes the rotation to increase as the nebula becomes smaller.

This rotation causes the cloud to flatten out—much like forming a flat pizza out of dough—and take the form of a disk.

The initial collapse takes about 100,000 years.

After that time the star reaches a surface temperature similar to that of a main sequence star of the same mass and becomes visible. It is now a T Tauri star.

Accretion of gas onto the star continues for another 10 million years, before the disk disappears, perhaps being blown away by the young star's solar wind, or perhaps simply ceasing to emit radiation after accretion has ended.

The oldest protoplanetary disk ever discovered is 25 million years old.

No comments:

Post a Comment