Final answer:
A protostar takes more than 3 million years to turn into a regular star. Observations show that if a protostar is less than 1 to 3 million years old, its disk extends from the star's surface to tens or hundreds of AU (astronomical units) away. In older protostars, the inner regions of their disks lose most of their dust, resulting in a donut-shaped disk with the protostar centered in its hole.
Step-by-step explanation:
A protostar takes more than 3 million years to turn into a regular star. Observations show that if a protostar is less than 1 to 3 million years old, its disk extends from the star's surface to tens or hundreds of AU (astronomical units) away. In older protostars, the inner regions of their disks lose most of their dust, resulting in a donut-shaped disk with the protostar centered in its hole. The inner, dense parts of most disks disappear by the time the stars are 10 million years old.