PHY 1060 Lecture Notes - Lecture 3: Protoplanetary Disk, Escape Velocity, Orbital Inclination
Astronomy Module 3
Chapter 7: The Birth and Evolution of Planetary Systems
• Our Solar System is only a tiny part of the universe. There are many more like it
o Theories of its origin must explain its contents: planets, moons, asteroids, etc.
• Young stars are surrounded by disks of gas and dust
o The infant Sun would also have been surrounded by such
o The rest of the solar system formed from that rotating disk
• Protostar: large, hot ball of gas; not a star yet
o Forms in a collapsing cloud of gas and dust
o Forms at the center, where it is densest
o When the right conditions are met it becomes a star
• The rest of the mass is the protoplanetary disk.
o The planets and other objects in the solar system will form from it
o The flattened disk is a result of angular momentum conservation
• The cloud begins as a diffuse spherical collection of material
o Parts of it are going in the same direction
o The angular momentum of the system is conserved
o The result is a spinning sphere will become a flattened, rotating disk.
• Angular momentum depends on the rotational speed of an object, its mass, and how its
mass is distributed
o A spinning uniform sphere’s angular momentum:
o Lspin = 4pimR2 / 5P
o The spinning angular momentum of a collapsing sphere in space stays constant
o As it collapses, it must speed up. Speed is inversely related to the rotational
period.
• The collapse is slowed perpendicular to the rotation axis, but not parallel to it
o It is easier for the parts along the rotation axis to fall in.
o Most of the gas lands on an accretion disk, which continues the rotation.
o Accretion = growth by infall
• Within the disk, small particles will collide and stick
o Small particles are blown into larger ones by gas motions
o This leads to larger particles, 1 km or larger, called planetesimals
• Once they reach this size, planetesimals will pull more particles onto them by gravity,
leading to planets
o Today’s remaining planetesimals are asteroids and comets
• The inner disk is hot
o The gravitational energy of the infalling material is converted into heat
o Material that lands on the inner part of the disk has fallen farther and has more
energy to convert into heat.
• Particles in the outer disk do not have as far to fall
o The protostar at the center is contracting and heating up
o This also heats the inner disk more than the outer disk
• Important differences in composition due to this
o Inner disk: only materials that do not melt at high temperatures can form or
remain
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